Wire, Cable & Conduit

Engineering Encyclopedia Saudi Aramco DeskTop Standards

EVALUATING ABOVE-GRADE WIRE, CABLE, AND CONDUIT INSTALLATIONS

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: EEX-206.02

For additional information on this subject, contact PEDD Coordinator on 874-6556

Engineering Encyclopedia

Wire and Cable Systems Evaluating Above-Grade Wire, Cable, and Conduit Installations

CONTENT

PAGE

INTRODUCTION............................................................................................................. 4 ABOVE-GRADE INSTALLATION TECHNIQUES: CONDUIT, CABLE TRAYS, AND EXPOSED CABLE ....................................................................................... 5 Conduit ................................................................................................................. 5 Rigid Steel ................................................................................................. 6 EMT ........................................................................................................... 7 Flexible Liquid-Tight................................................................................... 8 Cable Trays: Design, Construction, and Usage Requirements........................... 8 Aluminum................................................................................................... 9 Fiberglass ................................................................................................ 10 Exposed Cable: Uses and Routing Requirements............................................. 11 Metal-Clad/Armored................................................................................. 11 Routing Requirements ............................................................................. 12 DETERMINING CABLE TRAY INSTALLATION REQUIREMENTS.............................. 13 Loading............................................................................................................... 14 Magnetic Heating Effects.................................................................................... 18 Circuit Separation ............................................................................................... 19 Grounding and Bonding Requirements and Methods........................................ 22 Tray Separation .................................................................................................. 23 Supports/Fastenings........................................................................................... 24 Tray Routing/Protection Covers.......................................................................... 26 Fittings, Bends, and Drops ................................................................................. 27 DETERMINING CONDUIT INSTALLATION REQUIREMENTS.................................... 28 Conduit Types and Applications ......................................................................... 28 Conduit Sizing and Routing ................................................................................ 30 Conduit Fill............................................................................................... 30

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Jam Ratio................................................................................................. 32 Cable Clearance Within the Conduit ........................................................ 33 Magnetic Heating Effects ......................................................................... 34 Conduit Clearances ................................................................................. 34 Fire Proofing ............................................................................................ 34 Conduit Bending ................................................................................................. 35 Minimum Bending Radii ........................................................................... 35 Conduit Threading .............................................................................................. 36 Indoor and Outdoor Conduit Terminations ......................................................... 38 Fittings ..................................................................................................... 38 Seals (Explosion Proof) ........................................................................... 39 Expansion Joints...................................................................................... 40 Conduit Supports................................................................................................ 40 DETERMINING CABLE PULLING REQUIREMENTS .................................................. 43 Rigging Procedures ............................................................................................ 43 Pulling Grips ............................................................................................ 47 Pulling Lines ............................................................................................ 48 Duct Lubricating....................................................................................... 49 Cable Pulling Parameters ................................................................................... 50 Maximum Pulling Tensions ...................................................................... 50 Sidewall Pressure .................................................................................... 64 Rigging Method Effects Calculation ......................................................... 68 DETERMINING HAZARDOUS AREA WIRING AND SEALING REQUIREMENTS ...... 73 Wiring ................................................................................................................. 74 Conduit Sealing .................................................................................................. 77 Cable Sealing ..................................................................................................... 80 List of Figures

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Figure 1: Aluminum Cable Tray Load/Span Class Designation (from NEMA VE-1) ..... 15 Figure 2: Fiberglass Cable Tray Load/Span Data (from NEMA FG-1) ......................... 16 Figure 3: Fiberglass Cable Tray Temperature Correction for Allowable Working Load (from NEMA FG-1).................................................... 17 Figure 4: Minimum Circuit Separation Distances for Signal Cabling in Cable Tray (from SAES-J-902) ............................................. 21 Figure 5: Conduit Sizing Requirements........................................................................ 31 Figure 6: Allowable Percentage of Conduit Fill (from NEC, Chapter 9) ........................ 32 Figure 7: Required Dimensions of Conduit Threads (from UL 6) ................................. 37 Figure 8: Maximum Distance Between Rigid-Metal Conduit Supports ......................... 42 Figure 9: Dynamometer Used to Measure Pulling Tension .......................................... 45 Figure 10: Basket Grip on Cable .................................................................................. 47 Figure 11: Pulling Eye on Cable ................................................................................... 48 Figure 12: Cable Configurations................................................................................... 53 Figure 13: Vertical Conduit Bends................................................................................ 59 Figure 14: Example Pulling Tension Calculation............................................................. 61 Figure 15: Sidewall Pressure on Cable During a Pull...................................................... 65 Figure 16: Inside Radius of Standard Conduit Elbows.................................................... 67 Figure 17: Sample Rigging Methods Effects Calculation ................................................ 68

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Wire and Cable Systems Evaluating Above-Grade Wire, Cable, and Conduit Installations

INTRODUCTION In order to evaluate various types of approved Saudi Aramco above-grade wire, cable, and conduit installations for applicability of use, the Participant must have a thorough understanding of the types of installation techniques that are available, the minimum requirements of the various governing documents for each type of installation, and the methods that are used to determine the installation requirements for each type of installation. The cable installation methods that are described in this Module (e.g., cable trays and above-ground conduit) are all used for the same reason: to install cable so that the cable will function safely and adequately throughout its projected operating life. As such, the optimal cable installation method should be selected for the facility installation. The optimal cable installation method is selected through an evaluation of the specific cable installation requirements, installation topography, and the installation method cost. In addition to the above-grade cable installation methods that are outlined in this Module, the below-ground cable installation methods that are covered in EEX 206.03 should also be considered. This Module provides information on the following topics that are pertinent to evaluate above-grade wire, cable, and conduit installations for applicable use: o

Above-grade Installation Techniques: Conduit, Cable Trays, and Exposed Cable.

o

Determining Cable Tray Installation Requirements

o

Determining Conduit Installation Requirements

o

Determining Cable Pulling Requirements

o

Determining Hazardous Area Wiring and Sealing Requirements

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ABOVE-GRADE INSTALLATION TECHNIQUES: CONDUIT, CABLE TRAYS, AND EXPOSED CABLE There are several different installation techniques that can be used to install above-grade cable in Saudi Aramco facilities. Cable can be installed in an enclosed channel (raceway), an open channel (cable tray), or simply exposed to the elements. Rigid metal (bus ducts) can also be used to conduct electricity over short distances, but this method is limited and is not covered in this course. The selected installation method is to a large degree a matter of the Proponent's preference, however, the installation must comply with the applicable Saudi Aramco Engineering Standard (SAES). Standards, in the title Saudi Aramco Engineering Standards (SAESs), is a term that refers to the minimum mandatory requirements for the design, construction, maintenance, and repair of equipment and facilities for Saudi Aramco. This section of the Module describes the following above-grade installation techniques: o

Conduit

o

Cable Trays: Design, Construction, and Usage Requirements

o

Exposed Cable: Uses and Routing Requirements

Conduit A conduit is defined as a metallic or nonmetallic tube that is used to protect electric wires and cables. Although there are various types of nonmetallic conduit systems that are available for use, Saudi Aramco allows only metallic-type conduit systems to be used for above-grade conduit installations. The types of conduit that are used in Saudi Aramco above-grade installations are rigid steel, electrical metallic tubing (EMT), and flexible liquid-tight. Due to the nature of Saudi Aramco cable installations (e.g., the cable use and installation environment), intermediate metallic conduit (IMC) is prohibited in all areas. Saudi Aramco considers the additional cost of rigid steel conduit to be worth the added protection that rigid steel offers over IMC.

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Rigid Steel

Rigid-steel conduit is medium-thickness water pipe that has been reamed out to eliminate burrs and rough edges. Hot-dipped, galvanized, rigid-steel conduit is specified for all Saudi Aramco installations in which rigid-steel conduit is used; hot-dipped, galvanized, rigid-steel conduit is rigid-steel conduit that has been dipped in molten zinc during the forming process. Rigid-steel conduit is manufactured in standard lengths of approximately three meters (ten feet) and is required to be threaded on both ends. Additional details on the construction of rigid-steel conduit systems that are used in Saudi Aramco above-grade installations are provided in the section of this Module that is titled "Determining Conduit Installation Requirements." The requirements for the use of conduit and other equipment (e.g., cables and cable trays) are determined to some extent by the possibility of fire or explosive hazards. The specific classes of hazardous locations are described in the section of this Module that is titled "Determining Hazardous Area Wiring and Sealing Requirements," and they are briefly described here for requirement clarification. A Class I classification describes a location in which flammable gases or vapors could be present. Class II and Class III are locations where combustible dusts or fibers respectively exist. Each of these three classifications are, in turn, broken down further into a Division 1 location in which danger is imminent at any or all times, or a Division 2 location in which danger is not present under normal conditions but is likely to arise. The following are the requirements for the use of rigid-steel conduit in Saudi Aramco above-grade installations: o

Rigid-steel conduit should be used when conduit is to be installed in Class I, Division 1 (hazardous) areas.

o

Rigid-steel conduit should be used when exceptional mechanical protection is required.

o

Rigid-steel conduit should be used when conduit is installed above ground in outdoor industrial facilities.

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o

Rigid-steel conduit should be used when conduit is installed in severe corrosive environments; the conduit should be PVC-coated.

o

Rigid-steel conduit should be PVC-coated when it is installed in offshore locations or when it is installed within one kilometer (3500 feet) from the shoreline of the Arabian Gulf or thee kilometers (10,500 feet) from the shore line of the Red Sea.

EMT

EMT is similar in construction to rigid-steel conduit except that EMT is constructed of a much thinner material. EMT can also be referred to as thin-walled conduit. EMT is manufactured in standard lengths of approximately 3 meters (10 feet). Unlike rigidsteel conduit, EMT is not threaded (due to its thin wall), and it is joined by threadless couplings. Additional details on the construction of EMT systems that are used in Saudi Aramco above-grade installations are provided in the section of this Module that is titled "Determining Conduit Installation Requirements." EMT does not offer the same degree of mechanical strength as rigid-steel conduit; therefore, the applications of EMT are limited when compared to rigid-steel conduit. The following are the requirements for the use of EMT in Saudi Aramco above-grade installations: o

EMT is acceptable only in non-hazardous (classified), indoor locations.

o

EMT should not be used where corrosion can cause damage.

o

EMT should not be used where it will be subjected to severe physical damage.

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Flexible Liquid-Tight

Flexible liquid-tight conduit is constructed of a single strip of aluminum or galvanized steel that is spirally wound and interlocked. An outer jacket is used to make the flexible conduit assembly liquid-tight. The interlocked construction of flexible liquid-tight conduit provides a round cross-section that has a high degree of mechanical strength and great flexibility. Additional details on the use of systems that include flexible liquid-tight conduit in Saudi Aramco above-grade installations are provided in the section of this Module that is titled "Determining Conduit Installation Requirements." The following are the requirements for the use of flexible liquidtight conduit in Saudi Aramco above-grade installations: o

Flexible liquid-tight conduit should be used in all areas (except those areas that are classified as Class I, Division 1) for connections where vibration, movement, or adjustments will occur.

o

Explosion-proof flexible couplings should be used instead of flexible liquid-tight conduit in Class I, Division 1 (hazardous) locations. When explosion-proof flexible fittings are necessary, a Crouse-Hinds, EC Series, or equivalent flexible conduit should be used.

Cable Trays: Design, Construction, and Usage Requirements Cable tray is defined as a unit or an assembly of units or sections (and associated fittings) that is made of metal or some other noncombustible material and that forms a continuous rigid structure. Cable trays are used to support cables and raceways, and they can be found in the form of ladders, troughs, and channels. Although Saudi Aramco standards specify the use of only copper-free, aluminum, ladder-type cable tray for Saudi Aramco above-grade installations, fiberglass, ladder-type cable tray is authorized for use in special applications with the approval of the Proponent Operating Department Manager. This section describes the design and construction of both aluminum and fiberglass cable trays.

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Aluminum

Aluminum ladder-type cable tray is a prefabricated metal structure that consists of two longitudinal side rails that are connected by individual transverse members at regularly spaced distances. Only ladder-type cable tray is authorized for Saudi Aramco abovegrade installations. Because the aluminum cable trays could potentially become energized, they are grounded. To facilitate the transmission of fault current, and because cable trays are hung or mounted in specific lengths, the cable tray lengths must be bonded together. Bonding is the method of joining together the cable tray lengths to ensure electrical continuity. Ventilated, louvered, cable tray protective covers are required to allow for mechanical protection and solar radiation deflection for all outside cable tray installations. Cable tray covers should be made of the same material that is used for the cable tray, and they should not have a black or dark surface that is exposed to the sun. Covers for aluminum cable tray should be fastened to the cable tray with stainless steel banding. The maximum distance between the stainless steel bands is one band for every 1.5 m (5 feet) of cover length. There should always be at least two bands per length of cable tray. Additional details on the construction of aluminum ladder-type cable tray systems that are used in Saudi Aramco above-grade installations are provided in the section of this Module that is titled "Determining Cable Tray Installation Requirements." The following are the requirements for the use of aluminum ladder-type cable tray in Saudi Aramco above-grade installations: o

Cable tray is the preferred method of power distribution in Class I, Division 2 (hazardous) areas.

o

Cable tray should not be used where it will be subjected to severe physical damage.

o

If an outdoor cable tray installation contains only I&C cables, the cable tray covers that are used can be of the solid (unventilated) type.

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Fiberglass

Fiberglass ladder-type cable tray is an assembly of fiberglassreinforced plastic tray sections and accessories that form a rigid structural system to support cable. The fiberglass ladder-type cable tray is a prefabricated, sunlight (UV)-resistant, fiberglass structure that consists of two longitudinal side rails that are connected by individual transverse members at regularly spaced distances. Only ladder-type cable trays are authorized for Saudi Aramco above-grade installations. Outdoor, fiberglass cable tray installations should use covers that are made of the same material as the cable tray and that have provisions for ventilation. Additional details on the construction of fiberglass, ladder-type cable tray systems are provided later in this Module. The following are the requirements for the use of fiberglass laddertype cable tray in Saudi Aramco above-grade installations: o

Cable tray is the preferred method of power distribution in Class I, Division 2 (hazardous) areas and in unclassified areas.

o

Cable tray should not be used where it will be subjected to severe physical damage.

o

If an outdoor cable tray installation contains only instrument and control cables, the cable tray covers that are used can be of the solid (unventilated) type.

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Exposed Cable: Uses and Routing Requirements For some Saudi Aramco cable installations, the routing or design of the cable installation may require the cable to be exposed (i.e., not enclosed in a raceway). When the cable will be exposed, metal-clad (Type MC) or armored (in accordance with IEC 60502) cable will be used. Type MC cable and armored cable are permitted to be installed exposed only when the cable will not be subject to damage by vehicular traffic or similar hazards. If a cable type other than MC or armored cable is installed, the cable should not be installed so that it is exposed above ground. Cable types other than MC or armored cable should only be installed in cable trays, conduit, or flexible liquid-tight conduit. This section of the Module describes the construction, use, and routing requirements of metal-clad and armored cable that is used in Saudi Aramco installations. Metal-Clad/Armored

Type MC cable is a factory assembly of one or more conductors that are individually insulated. The assembly is enclosed in a metallic sheath of interlocking tape or in a smooth, corrugated tube. Type MC cable that is used for Saudi Aramco applications should be supplied with a PVC-jacketed aluminum sheath that meets UL 4 (0 to 2000 V) or UL 1072 (2001 to 35 kV) specifications. According to the NEC, the uses of Type MC cable include the following applications: o o o o o o o o o o o

Saudi Aramco DeskTop Standards

Services, feeders, and branch circuits. Power, lighting, control, and signal circuits. Indoors or outdoors. Where exposed or concealed. Direct buried where identified for such use. In cable tray. In any approved raceway. As an open run of cable. As aerial cable on a messenger. In hazardous locations as permitted by NEC (NFPA 70), articles 501 through 504. In dry locations.

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Type MC cable is always specified with a PVC jacket, and is therefore suitable for installation in wet locations. Type AC (Armored Cable) cable that is described in NEC Article 333 is light-duty cable that is seldom used in Saudi Aramco installations. The armored cable that is widely used in Saudi Aramco installations is galvanized steel tape or steel wire-armored cable that is manufactured in accordance with IEC Article 502. The IEC-armored cable is heavy duty, and it is considered to be equivalent to NEC Type MC cable. Suitable armored cable terminators should be used to terminate and ground the armor, and the armor should be mechanically joined (bonded) through the installation so that is forms a continuous electric conductor. The cable armor should be connected to all boxes, fittings, and cabinets to provide effective electrical continuity throughout the installation. Routing Requirements

Specific installation routing requirements for Type MC cable are listed below: o

Type MC cable should be supported and secured at intervals not greater than 1.83 m (6 feet) unless the cable is fished. If Type MC cable is installed as a branch circuit in a dwelling unit, the cable should be secured within 305 mm (12 inches) of every outlet box, junction box, cabinet, or fitting.

o

Type MC cable that is installed in a cable tray should comply with the installation requirements for cable tray. Routing requirements for cable tray are discussed in the section of this Module that is titled "Determining Cable Tray Installation Requirements."

o

The requirements for Type MC cable that is directly buried are discussed in Module EEX 206.03, Evaluating Underground Wire, Cable, and Conduit Installations.

Specific installation routing requirements for armored cable per IEC 502 are the same as for Type MC cables.

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DETERMINING CABLE TRAY INSTALLATION REQUIREMENTS Generally, the only type of cable tray that is authorized for use in Saudi Aramco installations is copper-free, aluminum ladder-type. However, with the approval of the Proponent Operating Department Manager, fiberglass ladder-type cable tray can be used in special applications. When a cable tray is chosen by the Electrical Engineer for use in a Saudi Aramco installation, there are many factors that should be taken into consideration in the selection of the type and size of the cable tray. The cable tray that is selected should be able to adequately hold the cable (or group of cables) in the installation for the maximum operating life of the installation. The cable tray should be large enough to account for future system growth, but it should not be too large that the tray purchase becomes economically restrictive. There are also requirements that involve the tray installation support structure, grounding, bonding, and placement. This section of the Module provides information on the following topics that are pertinent to determining cable tray installation requirements: o

Loading

o

Size/Fill

o

Magnetic Heating Effects

o

Circuit Separation

o

Grounding and Bonding Requirements and Methods

o

Tray Separation

o

Supports/Fastenings

o

Tray Routing/Protection Covers

o

Fittings, Bends, and Drops

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Loading Cable trays are classified in accordance with their allowable, mechanical, working-load capacity per unit of span length; span is the term that is used to describe the distance between the cable tray supports. The allowable, mechanical, working-load capacity of aluminum and fiberglass cable tray is determined through division of the destruction load capacity of the cable tray (as determined by testing) by a unitless 1.5 safety factor. The mechanical loading requirements for aluminum cable tray, as defined in NEMA VE-1, are classified in accordance with several load/span class designations. There are three working load categories and four support span categories for aluminum cable tray systems. The working load categories that are specified for aluminum cable tray are as follows: o

Class A - 74.4 kg/m (50 pounds per linear foot)

o

Class B - 111.6 kg/m (75 pounds per linear foot)

o

Class C - 148.8 kg/m (100 pounds per linear foot)

The support span categories that are specified for aluminum cable tray are as follows: o

2.44 m (8 feet)

o

3.66 m (12 feet)

o

4.87 m (16 feet)

o

6.09 m (20 feet)

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The table that is shown in Figure 1 lists the class designations that are used to select aluminum, ladder-type, cable tray systems. The class designation is selected through determination of the amount of working load and the length of the support span.

Load/Span Class Designations Working Load

Support Span

Class

Lbs./ft

kg/m

Feet

m

Designation

50 75 100

74.4 111.6 148.8

8 8 8

2.44 2.44 2.44

8A 8B 8V

50 75 100

74.4 111.6 148.8

12 12 12

3.66 3.66 3.66

12A 12B 12V

50 75 100

74.4 111.6 148.8

16 16 16

4.87 4.87 4.87

16A 16B 16V

50 75 100

74.4 111.6 148.8

20 20 20

6.09 6.09 6.09

20A 20B 20V

Figure 1: Aluminum Cable Tray Load/Span Class Designation (from NEMA VE-1) The mechanical loading requirements for fiberglass cable tray, as defined in NEMA FG-1, are classified in accordance with three working load class designations that are based on a support span of 6.09 m (20 feet). The working load categories that are specified for fiberglass cable tray are as follows: o

Class A - 74.4 kg/m (50 pounds per linear foot)

o

Class B - 111.6 kg/m (75 pounds per linear foot)

o

Class C - 148.8 kg/m (100 pounds per linear foot)

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Unlike the load/span class designations that are used for aluminum cable tray, the mechanical loading (working load) requirements for fiberglass cable tray vary as the support span distance varies, as shown in Figure 2. The class of fiberglass cable tray that should be used for a given installation is based on the mechanical load that the cable tray must support and the length of the support span that will be used.

Support Span

Working Load in Lbs./Linear Foot

In Feet

Class A

Class B

Class C

20 18 16 14 12 10

50 61 78 100 139 200

75 92 117 150 208

100 123 156 200

Figure 2: Fiberglass Cable Tray Load/Span Data (from NEMA FG-1) The amount of mechanical load that a given cable tray will be required to support is determined by the sum of the weight of the cables that will be installed in the cable tray (in pounds per foot). To account for future circuit growth, a 20 percent correction factor is applied to the combined cable weight. Finally, an equivalent weight is added to the corrected combined cable weight to account for the effect of a 200-pound person standing on the cable tray at the center of the span. Details on the procedure to determine the amount of mechanical load that will be present on a given span of cable tray are provided in Work Aid 1.

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Because the strength properties of reinforced plastics are reduced when they are continuously exposed to elevated temperatures, the allowable working load of a fiberglass cable tray should be reduced for Saudi Aramco installations in which the cable tray will be exposed to an average ambient temperature of 50 degrees C. The table in Figure 3 shows the approximate percent of strength that the fiberglass cable tray will possess at various temperatures.

Temperature in Degrees C

Temperature in Degrees F

Approximate Percent of Strength

24 38 52 66 79 93

75 100 125 150 175 200

100 90 78 68 60 52

Figure 3: Fiberglass Cable Tray Temperature Correction for Allowable Working Load (from NEMA FG-1) In addition to the mechanical loading requirements that have been previously discussed, a completed cable tray system should be able to withstand a horizontal wind force of 1.4 kPa (30 lbf/ft2), which is approximately equivalent to a wind speed of 140 km/hr, or 87 mph. To determine the cable tray sizes that should be used for a given installation, the Electrical Engineer must evaluate the cable tray fill requirements. The fill requirements differ depending on whether the installation includes multiple-conductor cables that are rated 2000 V or less, single conductor cables that are rated 2000 V or less, or single- and multiple-conductor Type MV or Type MC cables that are greater than 2000 V.

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The fill requirements for multiple-conductor cables that are rated 2000 V or less vary with the type of cable that is used (power, lighting, control, and/or signal type cables) and the size of the cables. The fill requirements for single conductor cables that are rated 2000 V or less vary only with the size of the cables. The fill requirements for single- and multiple-conductor Type MV or Type MC cables that are rated for greater than 2000 V vary only with the diameter of the cables that are installed in the cable tray. To determine the correct cable tray size, the cable dimensions (e.g., diameter or cross-sectional area) are added and the sum is multiplied by a growth correction factor. For Saudi Aramco cable tray installations, the growth correction factor recommended is 20 percent. The 20 percent growth correction factor ensures that the cable tray can be used for an increase in load as a result of future expansions. Work Aid 1 describes the procedure that is used to determine the size of a cable tray based on the fill requirements. Magnetic Heating Effects Metallic raceways are susceptible to magnetic heating effects, which include hysteresis heating and "induced current" heating. Hysteresis heating is caused by the opposition that ferrous raceways offer to a changing magnetic field. The heating occurs due to energy losses within the raceway as the elementary particles (each containing a magnetic field) that exist within the raceway seek to align themselves to the changing magnetic field. Because only aluminum and fiberglass cable tray is authorized for use in Saudi Aramco installations, and because neither material is ferrous, there will be no hysteresis heating effects. Although magnetic hysteresis will not occur in a nonferrous material, induced currents can exist in the nonferrous material if the material is also an electrical conductor (such as aluminum). Induced current heating caused by alternating magnetic field that exists around the conductors in aluminum cable trays are not significant enough to be a problem.

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There are design requirements that can minimize magnetic heating effects. If the phase conductors, neutral conductor (if any), and equipment grounding conductor are grouped within the same raceway, induced current heating can be minimized. When a single conductor that carries an alternating current passes through metal that has magnetic properties, the following actions can be taken to minimize the magnetic heating effects: o

Slots can be cut in the metal between the individual holes through which the conductors pass.

o

All of the conductors can be passed through an insulating wall that is large enough for all of the conductors of the circuit.

Circuit Separation Circuit separation requirements are established for safety and to minimize the effects of induced currents in adjacent instrument and control cables. Based on the voltages of the cables and the type of cable that is installed in the cable tray (e.g., power, lighting, control, and/or signal-type cables), the circuit separation requirements affect cable routing. When the cable tray systems that contain cables from different systems converge or use the same route, the circuit separation and cable placement requirements that must be observed are described below: Cables for light and power systems that are rated 600 V or less are permitted to occupy the same cable tray as long as all of the conductors are insulated for the maximum voltage that will exist for any of the cables that are within the cable tray. Cables for light and power systems that are rated above 600 V are not permitted to occupy the same cable tray as cables that are rated 600 V and below unless one of the following conditions is satisfied: o

The cables that are rated above 600 V must be separated from the cables that are rated 600 V or below by a solid, noncombustible, fixed barrier.

o

All of the cables must be Type MC or armored.

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Conductors that are used for signaling, instrumentation, or communication systems should not occupy the same cable tray as the conductors of lighting, power, 120 V control, or 24 V dc and above relay control systems. If all of the cables are insulated for 600 V or more, power systems control, metering, alarm, and relaying circuits that are associated with one major piece of electrical equipment (such as a motor or a transformer) can be run within a single cable tray. Inter-tripping circuits that run between substations can also be run within one cable tray with the following exceptions: o

Circuits that are associated with alternate power sources for primary selective, secondary selective, or spot network substations should be kept separate.

o

Differential relay circuits should be kept separate from all other circuits.

Circuit separation and placement requirements for instrument cables in Saudi Aramco installations are shown in Figure 4. To determine the minimum circuit separation distance that should be maintained between two systems, the first system in the first column and the second system in the first row should be located; the intersection of the two systems in the table is the minimum circuit separation distance. For example, the minimum circuit separation distance that should exist between 125 V dc systems and RTD systems in cable tray is 6 inches (150 mm). If the insulation of the cables that are installed in the raceway is rated for at least 450 to 750 V, there are no minimum circuit separation requirements between power and/or control conductors for dc or for ac circuits that carry power at voltages that are less than 1000 V.

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RTD

Thermocouple

Milli- Volt Pulse

RTD

0

0

0

Thermo Couple

0

0

Milli-Volt Pulse

0

4-20 mA Analog (24 VDC)

4-20 mA Analog (24 VDC)

24 VDC 48 VDC

125 VDC

120 VAC

>120 VAC

0

1 (25)

6 (150)

12 (300)

24 (600)

0

0

1 (25)

6 (150)

12 (300)

24 (600)

0

0

0

1 (25)

6 (150)

12 (300)

24 (600)

0

0

0

0

1 (25)

6 (150)

12 (300)

24 (600)

24 VDC 48 VDC

1 (25)

1 (25)

1 (25)

1 (25)

0

6 (150)

6 (150)

18 (450)

125 VDC

6 (150)

6 (150)

6 (150)

6 (150)

6 (150)

0

0

12 (300)

120 VAC

12 (300)

12 (300)

12 (300)

12 (300)

6 (150)

0

0

12 (300)

>120 VAC

24 (600)

24 (600)

24 (600)

24 (600)

18 (450)

12 (300)

12 (300)

0

Note that all values are shown in inches (millimeters)

Figure 4: Minimum Circuit Separation Distances for Signal Cabling in Cable Tray (from SAES-J-902) Cables that carry different signal types should also be routed so that they cross each other only at right angles. Also, when dc instrumentation and control signal cabling is routed past a source of strong electromagnetic fields (such as transformers, motors, and generators that are rated greater than 100 kVA), a minimum spacing of 2 m (6 feet) should be maintained between the signal cabling and the source of the electromagnetic field. When trays that contain different systems converge or use the same route, they should preferably be placed in the following order (from top to bottom in different trays as required): o

Power cables.

o

Control cables.

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o

Alarm circuits.

o

Dc electronic cables and pneumatic tubing (the pneumatic tubing should be separated from the dc circuit by a barrier).

o

Thermocouple cables.

Grounding and Bonding Requirements and Methods In most high-voltage power distribution cables, there are various metallic support systems (e.g., shields and cable trays) that could carry fault or induced currents. Equipment grounding describes the manner of grounding the support system equipment. Bonding describes the manner of electrically interconnecting the various segments of the support systems. An equipment grounding conductor is not the same as a grounded conductor in that a grounding conductor carries only current during fault conditions while the grounded conductor may carry current under normal conditions. Cable trays must be grounded and bonded so that freedom from dangerous electric shock voltages is ensured and that sufficient current-carrying capability is provided to accept the ground-fault current that is required by the overcurrent protection system. For Saudi Aramco installations, the entire cable tray system is required to be mechanically and electrically connected to ensure that there is a path for electric fault current. The acceptable methods that are used to meet the grounding requirements for aluminum cable trays are listed as follows: o

Enclosures of MCCs, motor controllers, switchgear, and other electrical devices that are fed from a cable tray system should be structurally and mechanically connected and bonded to the cable tray system.

o

With some exceptions, a conduit, cable tray, cable armor, or cable shield should not be used as the sole means of grounding equipment. For safety and reliability, a grounding conductor should be installed in the same cable tray as the power conductors.

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o

A metallic cable tray should be grounded at both of its end points.

The acceptable methods that are used to meet the bonding requirements for aluminum cable trays are listed as follows: o

To prevent any faults that may occur within the cable tray system from arcing to ground, metallic cable trays should be bonded to the plant grounding system at maximum intervals of 25 m (84 feet).

o

Bonding can be accomplished through use of metallic connections to the building or structural columns that support the cable trays.

o

Bonding jumpers should be provided whenever a cable tray is insulated from its metallic supporting structure or whenever a cable tray expansion joint is used. Expansion fittings (or joints) are required to accommodate expansion and contraction due to ambient temperature changes. The gap at expansion points depends on the spacing between these joints.

Tray Separation NEC Article 318 requires sufficient space around cable trays to permit adequate access for installation and maintaining the cables. Saudi Aramco standards no longer specify distances (tray separation). The following separation distances were specified before 1984 and can be used as guidelines if possible. For separation between multiple horizontal cable tray systems, a minimum of 50 mm (2 inches) of separation should be provided between the cable tray side rails. A minimum of 25 mm (1 inch) of separation should be provided between any vertical support and a cable tray side rail.

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No more than four 750 mm (30 inch) cable trays or four 600 mm (24 inch) cable trays should be located adjacent to each other on one horizontal tier. If more than four cable trays are required, a 450 mm (18 inch) spacing should be provided between each group of four cable tray units so that access is provided to each of the cable tray groups. The separation between vertical-tiered cable trays should be at least 450 mm (18 inches) with a minimum clear space of 300 mm (12 inches); if the total combined width of a given cable tray tier exceeds 900 mm (36 inches), the vertical space should be increased by 150 mm (6 inches). If the total width of the cable tray system exceeds 900 mm (36 inches), the vertical clearance should be increased to 450 mm (18 inches). The lower voltage cables are usually placed in the bottom cable trays, and the higher voltage cables in the upper cable trays. The vertical space between a cable tray and a ceiling, beam, or other obstruction should be a minimum of 300 mm (12 inches). When cable tray is located over any electrical gear, 600 to 900 mm (24 to 36 inches) of vertical separation should be maintained from the top of the electrical gear to the bottom of the cable tray. Supports/Fastenings To ensure adequate support, cable tray supports should be constructed from hot-dip galvanized steel. Cable tray supports that are installed in severe corrosive environments should be protected through use of one of the following methods: o

PVC-coated at the factory.

o

Coated in the field prior to installation.

The following were requirements of the standards prior to 1984 and can be used as guidelines if possible.

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Indoor ceiling hangars that are used for cable tray supports should be made from 12 mm (1/2 inch) galvanized-steel rods. All supports that are used for cable trays should provide a minimum weight-bearing surface of 45 mm (1 and 3/4 inches) as well as provisions for hold-down clamps and fasteners. The hold-down clamps and fasteners should be used at all cable tray support points. Vertical cable tray fasteners should not rely on friction to secure the cable tray to its supports. Cable trays should be supported from the structural steel of pipeways and buildings with noncombustible racks or hangers. Cable tray supports should to be spaced a maximum of 6 m (20 feet) on horizontal runs or 2.4 m (8 feet) on vertical runs. When ceiling hangers are used to support the cable tray, the hangers should be spaced no more than 3 m (10 feet) apart. Cantilever cable tray sections should be limited to a length of 900 mm (3 feet); additional support should be provided for cantilever cable tray sections that are greater than 900 mm in length. Cable tray splice points should not be located directly over the cable tray supports, and they should not be located at mid-span. The ideal location for a cable tray splice point is within the one quarter points of the span as measured from the cable tray supports. For example, if there are 4 meters between cable supports, the splice point should be within 1 meter of either cable support. Splice plates, expansion joints, and connectors should join the cable tray sections so that the rated vertical and horizontal load of the cable tray is not diminished. All cables should be fastened to the cable tray every 1.8 m (6 feet) on horizontal cable tray runs, every 450 mm (18 inches) on vertical cable tray runs, and every 450 mm (18 inches) on cable tray bends (horizontal and vertical). Vertical cable tray systems should provide suitable methods of cable support through the use of cable hangers or metal clamps. Nylon cable ties can be used for most fastening applications. Cable ties in outdoor locations should be black, and they should also be resistant to UV radiation. When circuits in cable trays are paralleled, single conductor cables should be fastened in groups that include one conductor per phase or neutral to prevent current imbalance.

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Tray Routing/Protection Covers The following can be used as guidelines, where possible. For Saudi Aramco installations, there are certain requirements for cable tray routing and cable tray protective covers. Considerations that are taken into account when routing cable tray systems are utilization of building walls as structural support and the proximity to hazardous equipment, process equipment, and grid-type walkways. Cable tray should be run parallel to the building structure or the building walls as applicable. Also, cable tray should not be routed any closer than 7.5 m (25 feet) horizontally to any fire hazardous equipment or other types of equipment that can produce temperatures that could damage the cables that are installed in the cable tray, such as steam lines. Cable trays should be located above all process piping and other process facilities. When cable trays are routed under grid-type walkways, barriers should be used to add additional protection to the cable tray. When installed, these barriers should not hinder cable tray ventilation. For cable tray locations to which future cable installations will be added, extra space should be provided in the cable tray. The extra space should allow for the future installation of the same basic type of cable tray and should be arranged so that the spare space will be unused and available for the future installation. The following circumstances would require the use of protective covers on a cable tray system: o

When cable trays pass through walls.

o

When cable trays are near areas that could be damaged by workers or by nearby equipment.

o

When cable trays are routed outdoors.

When cable trays pass through a combustible partition or wall, the cable tray should be completely enclosed in metal with a bushed steel conduit sleeve or similar device. When cable tray entry occurs in switch and control houses or when the cable tray passes through a fire wall, the cables in the cable tray should be sealed with Nelson Electric MCT or equivalent cable transits.

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To allow for drainage, cable trays that enter a building should be sloped away from the building at a minimum rate of ten millimeters per meter of cable tray (1/8 inch per foot). A concrete curb or metal kick plate should be provided for cable tray that passes vertically through floors or platforms. To prevent contact with or damage to the exposed cables, the tray should also be covered on all sides to a distance of 1.8 m (6 feet) above the floor or platform. Although the cable trays will be provided with covers, cables that are installed in outdoor cable trays should have sunlight-resistant (UV radiation-resistant) jackets. Ventilated, louvered cable tray covers are required to provide mechanical protection and solar radiation deflection for all outside cable tray installations. Covers for aluminum cable tray should be fastened to the cable tray with stainless steel banding at a rate of one band per 1.5 m (5 feet) of cover length and at least two bands per length of tray. Cable tray covers should not have a black or dark surface that is exposed to the sun. Fittings, Bends, and Drops For Saudi Aramco cable tray installations, the cable trays require the use of specific fittings, elbows, bends, and drops. Recommendations: vertical and horizontal elbows should have a minimum radius of 300 mm (12 inches), but they should not be less than the minimum cable bending radii. For vertical drops that are greater than 1.5 m (5 feet), outside vertical elbows and drop out fittings should be used at the higher elevation. For vertical drops that are greater than 4.5 m (15 feet), inside vertical elbows should be used at the lower elevation. Horizontal elbows should be used for changes of direction that occur at the same elevation.

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DETERMINING CONDUIT INSTALLATION REQUIREMENTS Prior to the installation of an above-grade conduit system, the Electrical Engineer should examine the various factors that will affect the installation. These factors include the type of equipment that is to be installed, the installation method that will be used, and the classification of the area in which the equipment will be installed. The requirements of SAES-P-104 (Wiring Methods and Materials) should be followed for Saudi Aramco above-grade conduit installations. The conduit installation method that is used (rigid-steel conduit, EMT, or flexible liquid-tight conduit) will affect the routing requirements of the installation. Other determinations, such as the correct size of the conduit for the installation, must also be made with respect to routing the conduit and cabling. The hazardous classification of the installation location determine the sealing and termination requirements. This section of the Module describes the following aspects of determining conduit installation requirements: o

Conduit Types and Applications

o

Conduit Sizing and Routing

o

Conduit Bending

o

Conduit Threading

o

Indoor and Outdoor Conduit Termination

o

Conduit Supports

Conduit Types and Applications The types of conduit that are available for use by Saudi Aramco in above-grade conduit systems are rigid-steel conduit, EMT, and flexible liquid-tight conduit. IMC is prohibited for use in Saudi Aramco installations. The additional cost of rigid-steel conduit is considered to be worth the added protection that it offers over IMC.

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Hot-dipped, galvanized, rigid-steel conduit is specified for all Saudi Aramco installations in which rigid-steel conduit is used. The application requirements of rigid-steel conduit systems have been described previously in this module and are listed briefly as follows: o

Used for Class I, Division 1 areas.

o

Used when exceptional mechanical protection is required.

o

Used when conduit is installed above ground in outdoor industrial facilities.

o

Used when conduit is installed in severe corrosive environments

o

If installed in severe corrosive environments, the rigid-steel conduit should be PVC-coated.

EMT does not offer the same degree of mechanical strength that is offered by rigid-steel conduit, and it should not be used where it is subjected to severe physical damage. EMT is only acceptable in nonhazardous, indoor locations, and it should not be used where corrosion can cause damage. The applications of flexible liquid-tight conduit are limited to connections in which vibration, movement, or adjustments will occur. Flexible liquid-tight conduit is allowed for use in all areas except Class I, Division 1 hazardous locations.

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Conduit Sizing and Routing Two critical aspects of a conduit installation are conduit sizing and conduit routing. When the conduit size is chosen, the conduit inside diameter should be large enough to install all of the cables that were selected to be installed in that conduit without damage to any of the cables. The conduit should also be large enough to minimize any adverse heating effects on the conduit or on the cables that are contained within the conduit. When the conduit is installed, there are also routing and placement requirements that should be met. The routing requirements are important to minimize inter-conduit heating and conduit heating that results when a conduit is routed near process facility equipment that radiates heat. Conduit Fill

Conduit fill is expressed as a percentage of the cross-sectional area of the conduit that the cables are allowed to occupy, and it depends on the number of conductors that are to be installed in the conduit. The allowable percentage of conduit fill is based on the combined heating effects of all of the cables that are installed in the conduit. Knowledge of the allowable percentage of conduit fill helps the Electrical Engineer to select the proper size of conduit for a particular installation.

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The table in Figure 5 describes the various size requirements that are specified for rigid-steel conduit, EMT, and flexible liquid-tight conduit; the references for each requirement are also listed. The size requirements include the minimum size of the conduit, the maximum size of the conduit, and the allowable conduit fill. Conduit Sizing Requirements

Rigid-Steel Conduit

EMT

Flexible Liquid Tight Conduit

Minimum Size

¾” except instrument panels, inside buildings, prefabriacted skids, or non-industrial areas

Size requirements are the same as those specified for rigid-steel conduit.

Size requirements are the same as those specified for rigid-steel conduit.

Maximum Size

N/A

4”

4”

Allowable Conduit Fill

Refer to Figure 6

Allowable conduit fill requirements are the same as those for rigidsteel conduit.

All conduit fill requirements are the same as those specified for rigid-steel conduit.

Figure 5: Conduit Sizing Requirements To determine the allowable fill, the Electrical Engineer should first choose an applicable duct from the tables for selecting conduit size that are shown in Work Aid 2. Once the size of the conduit is selected, the total cross sectional area of all of the cables that will be contained in the conduit should be determined through use of the table of cable dimensions that is shown in Work Aid 2. Now that the cross-sectional area of the cables has been determined and the conduit has been chosen, the percentage fill of the conduit can be determined. Work Aid 2 provides the tables and the details on the procedure that is used to size conduit for Saudi Aramco, above-grade installations. The allowable percentage of conduit fill, based on the number of conductors that are to be installed in the conduit, is shown in Figure 6. See NEC Chapter 9 for additional information. Saudi Aramco DeskTop Standards

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When conduit sealing fittings are used (type EYS or similar), the wire fill of the conduit sealing must not exceed 25% based on the conduit size (i.e., the ratio of the sum of the cross-sectional areas of wires and multi-conductor cables to the internal cross-sectional area of a conduit of the same trade size must not exceed 25%). If the percentage of fill of the conduit sealing exceeds 25%, oversized sealing fittings with reducers may be used in order to use the highest permissible conduit wire fill. Percent of Cross Section of Conduit and Tubing for Conductors Number of Conductors

1

2

Over 2

All conductor types

5 3

31

40

Note. A multi-conductor cable of two or more conductors shall be treated as a single conductor cable for calculating percentage conduit fill area. For cables that have elliptical cross section, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter. Figure 6: Allowable Percentage of Conduit Fill (from NEC, Chapter 9) Jam Ratio

The natural weight of the cables that are contained in the conduit will cause them to settle to the lowest part of the conduit that the conduit space will allow. Depending on the size, configuration, and number of cables, the cables could get jammed in the conduit during installation. A useful unitless value that is used when cables are installed in conduit is called the "jam ratio." The jam ratio is used primarily during cable pulling tension calculations, and it will be explained in detail in that section of this Module; but it is also used in the conduit selection process, and, so, it will be described briefly here. The jam ratio is the ratio of the conduit's inside diameter to the diameter of the largest cable that will be installed in the conduit. The jam ratio provides a factor that describes the probability that the cable will jam during its installation in the conduit. The equation below is used to calculate the jam ratio: Jam Ratio =

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1.05 D d

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where: "D" is the conduit inside diameter. "d" is the diameter of the largest cable that is in the conduit. "1.05" includes a correction factor of 5% that accounts for the oval cross-section of conduit bends. Cable Clearance Within the Conduit

If the jam ratio is greater than 3.0, jamming is not likely to occur, and cable clearance can be ignored. If the jam ratio is between 2.5 and 2.8, jamming is probable; if the jam ratio is between 2.8 and 3.0, serious jamming is probable. If jamming is probable, the Electrical Engineer should evaluate the need to increase the size of the conduit. Cable clearance is the distance between the uppermost cable in a conduit and the inside top of the conduit. A gap should be present between the uppermost cable in a conduit and the top of the conduit to prevent rubbing during pulls and to allow for expansion and contraction. For a single cable installation, the cable clearance is calculated through use of the following equation: Clearance = D - d where: "D" is the conduit inside diameter. "d" is the diameter of the largest cable that is in the conduit. For a three cable installation (or three tripled conductors), the cable clearance is calculated using the following equation: D (D - d)  d  Clearance = - 1.366(d) + ⋅ 1-   2 2  D- d 

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The cable clearance should be maintained within a band of 6 to 25 mm (1/4 to 1 inch). If the cable clearance is less than 6 mm (1/4 inch), the cable clearance is not satisfactory, and the Electrical Engineer should evaluate the need to increase the size of the conduit. Magnetic Heating Effects

Metallic raceways are susceptible to magnetic heating effects, which include induced current heating and hysteresis heating. In order to avoid magnetic heating effects, all phase and neutral conductors of a three phase system must be in one conduit, or if there are parallel conductors, each conduit must have all phases and neutral. Conduit Clearances

The conduit clearance is defined as the distance between the outside surface of a conduit and walls, other conduit, or other equipment. When routing conduit for above-grade installations, proper conduit clearances should be established. The conduit clearance is required to ensure that the conduit is not routed too close to process facility equipment that radiates heat. Conduit runs should be symmetrical and should be routed vertically, horizontally, or parallel to structure lines. Conduit should not be installed near ladder rungs or at platform levels so that the conduit restricts passage or interferes with existing steps. As a guideline, the minimum clearance for conduits that cross or run parallel to process lines should be 150 mm (6 inches) for uninsulated process lines and 100 mm (4 inches) for insulated process lines. Fire Proofing

Fireproofing is required for critical power and control cables that are located above ground in a fire-hazardous zone, e.g., within 7.5 m (25 feet) horizontally of fire-hazardous equipment. Critical power and control cables are cables whose loss would render emergency shutdown, fire protection, or alarm systems inoperative. Fire-hazardous equipment is defined as equipment that processes, stores, or produces flammable materials. Fireproofing requirements are contained in SAES-B-006. Saudi Aramco DeskTop Standards

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Fireproofing must provide a minimum of 15 minutes of protection to the integrity of the circuit against temperatures of 1100°C (2000°F) in accordance with UL 1709. Conduit Bending Conduit bending requirements for a given installation are designed so that the conduit is not injured during the installation and so that the internal diameter of the bent conduit is not effectively reduced. No more than four quarter conduit bends (360 degrees total) should be made in one run of conduit between pull points. Minimum conduit bending radii requirements are based on the minimum cable training radii for the cable that is to be installed in the conduit and the physical size of the conduit. Minimum Bending Radii

If a wire or cable is bent too much, the act of bending may cause damage to the wire or cable that results in subsequent cable failure. To prevent cable failure, a minimum bending radius (curvature of bend) limits cable and wire bending. With large power distribution cables, the construction of the cables (e.g., insulation and shielding) places additional restrictions on the minimum bending radius that a cable can withstand before damage to the cable will occur. As a cable passes, enters, or exits a conduit, the cable will usually have to be bent. The act of bending a wire or cable during the installation process is called "training." The minimum bending radius of any cable should not be exceeded when the cable is trained in a conduit. To prevent damage to the cables during the installation process, the minimum bending radii of the cables must also be considered during cable installation. The minimum bending radii of the inner surface of a given cable is determined through use of a calculation in which a specific multiplication factor is multiplied by the overall diameter of the cable.

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Before the minimum bending radii of the conduit is specified, the minimum cable training radii of the cables should be determined. To ensure that damage will not occur to the cables when they are trained, the minimum conduit bending radii should not be less than the minimum cable training radii. The procedure that is used to determine the cable minimum bending radii is provided in Work Aid 2. The minimum conduit bending radii is selected from a table based on the size of the conduit that is used for the installation. A hand bender can be used to make field bends for conduit that is sized at 1-1/2 inches or less. A bending machine should be used for conduit that is larger than 1-1/2 inches. Bends that are accomplished with a hand bender are measured from the inner surface of the conduit; bends that are accomplished with a machine bender are measured from the center line of the conduit. Work Aid 2 provides the procedure that is used to determine conduit bending requirements for Saudi Aramco above-grade installations. Conduit Threading Rigid-steel conduit is required to be threaded on both ends for Saudi Aramco conduit installations. Because EMT has a thin wall, individual sections of EMT are only permitted to be joined through use of threadless couplings. Rigid-steel conduit is manufactured in standard lengths of 10 feet (3 m). During an installation, the conduit must be cut into proper lengths. The proper length to which the rigid-steel is cut is dependent on the location and conditions of the conduit installation. After the conduit has been cut to the proper length, it must be field-threaded and then chamfered (reamed) to remove the burrs and sharp edges that are formed during cutting. All field threads for rigid-steel conduit are required to be full and continuous. A minimum thread engagement of five full threads should be made at all fittings. Field threads should be cut with a standard conduit cutting die that has a 3/4 inch taper per foot. All conduit threads must be tapered; running threads are not permitted for any application. Raw threads should be protected from corrosion with CRC "Zinc-It" or an equivalent coating.

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Rigid-metal conduit is available in sizes of 1/2 to 6 inches. Conduit trade sizes are referred to as the approximate inside diameter of the raceway. All fittings and knockouts on boxes are identified by the trade size of the raceway for which the device is intended. Figure 7 describes the required dimensions of conduit threads for the various trade sizes of rigid-steel conduit. Critical measurements include the effective length of the threads (L2), the total length of the threads (L4), the pitch diameter at the end of the conduit (E0), and the required number of threads per inch of conduit. L4 total length of threadsa Trade size of conduit in inches

Number of threads per inch

3/8

a b

L2 effective length of threads

E0 pitch diameter at end of conduit

inches

mm

inches

mm

inchesb

mmb

18

0.60

15.2

0.41

10.4

0.612

15.5

½

14

0.78

19.8

0.53

13.5

0.758

19.3

¾

14

0.79

20.1

0.55

14.0

0.968

24.6

1

11 ½

0.98

24.9

0.68

17.3

1.214

30.8

1¼

11 ½

1.01

25.7

0.71

18.0

1.557

39.5

1½

11 ½

1.03

26.2

0.72

18.3

1.796

45.6

2

11 ½

1.06

26.9

0.76

19.3

2.269

57.6

2½

8

1.57

39.9

1.14

29.0

2.720

69.1

3

8

1.63

41.4

1.20

30.5

3.341

84.9

3½

8

1.68

42.7

1.25

31.8

3.838

97.5

4

8

1.73

43.9

1.30

33.0

4.334

110.1

5

8

1.84

46.7

1.41

35.8

5.391

136.9

6

8

1.95

49.5

1.51

38.4

6.446

163.7

A minus tolerance of one thread applies to the total length of threads L4 Plus and minus tolerances of one turn apply to the pitch diameter E0 Figure 7: Required Dimensions of Conduit Threads (from UL 6)

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Indoor and Outdoor Conduit Terminations Metal raceways should be mechanically joined to form a continuous electric conductor; raceways should also be connected to all boxes, fittings, and cabinets for effective electrical continuity. Conduit terminations are used to complete the conduit system through connection of the metal raceway (i.e., rigid-steel conduit or EMT) to the boxes, fittings, and/or cabinets that are used in the conduit system. Conduit systems can terminate at service entrance fittings, panels, pull boxes, or access fittings, and they can include the use of insulated bushings and conduit seals. For example, indoor conduit runs that terminate in the open should be equipped with an insulating bushing. Outdoor conduit that terminates in the open should be equipped with a service entrance fitting. Also, insulating grounding bushings should be installed on conduit that is inside of all boxes except where a threaded hub is provided as part of the conduit thread connection. Fittings

A fitting is an accessory that is provided for a conduit system. Fittings are used to perform mechanical connections to conduit and associated conduit support equipment. Items, such as lock nuts, bushings, conduit couplings, EMT connectors and couplings, and threadless connectors, are considered to be fittings. Conduit fittings should be made of cast or forged steel, cast iron, or malleable iron that is either hot-dip galvanized or zinc electroplated (as supplied by the manufacturer). Aluminum fittings are not allowed for use in Saudi Aramco conduit installations. Only malleable iron sealing fittings are to be used for new installations, However, for repair purposes, gray, cast iron, splittype retro-fit sealing fittings are allowed. For the connection of conduit, EMT, or other raceways (except cable trays), a box or fitting should be installed at each conductor splice connection point, outlet, switch, junction point, or pull point. Conduit bodies are considered to be fittings and are allowed to contain splices if they have adequate volume.

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All conduit fittings should be accessible from a platform, ladder, or stairwell. Cover openings should not be blocked by any structural steel or pipe that would prevent access to the interior of the fitting for maintenance. For indoor and outdoor conduit terminations, there are certain requirements that should be met. Indoor conduit runs that terminate in the open should be equipped with an insulating bushing. Outdoor conduit that terminates in the open should be equipped with a service entrance fitting. Insulated grounding bushings should be installed on conduits inside of all boxes except where a threaded hub is provided as part of the conduit thread connection. Seals (Explosion Proof)

Explosion proof seals in a conduit system should only be provided where required by the NEC. Non-required sealing is expensive and an operational problem since for any future circuit modifications the seal fitting must be cut out and, the conductors spliced or removed. In additon, each conduit entering a process unit control house should be sealed outside the point of entry for above grade runs and inside at the point of entry for below grade runs. Seals, when required, should be located within 450 mm (18 in.) of an enclosure. Vertical or horizontal conduit runs which require sealing should be sealed with combination vertical/horizontal seals, EYS or equal. Explosion proof seals should be filled as follows: (a) (b) (c) (d)

Saudi Aramco DeskTop Standards

A dam of fiber (Chico “X” or equal) should be made around and between the wires to prevent the sealing compound from entering a conduit run. After fixing the dam, the sealing compound (Chico “A” or equal) equal to the diameter of the conduit (but not less than 5/8 inch) should be poured into the seal. All sealing compound should be mixed with clean fresh water. Do not pour sealing compound into draining chambers of EYD seals.

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Wire fill of sealing fittings should not exceed 25% based on the size of the conduit. Expansion Joints

An expansion joint is a mechanical device that is used to allow for the thermal expansion and contraction of a run of conduit. Because a given run of conduit that is placed in an above-grade conduit installation will be exposed to considerable temperature fluctuations over a year, expansion joints should be considered. However, expansion joints should not be used for long, vertical conduit runs; the conduit should be offset whenever possible to allow for the expansion and contraction of the conduit. An expansion joint should be installed in any run of conduit that is over 60 m (197 feet) in length. Additional expansion joints are required at intervals of 120 m (394 feet) unless the conduit run is supported by a steel structure such as a pipeway. When only one expansion joint is used for a run of conduit, the expansion joint should be located at the midpoint of the straight run. Multiple expansion joints should be equally spaced in the straight run. When a run of conduit is supported by a steel structure, the conduit expansion joints should be provided at the same location as the expansion joints that are provided for that steel structure. Conduit Supports A conduit support is a mechanical device that provides structural strength for a vertical or horizontal conduit system. Conduit supports should be constructed of cast or forged steel, cast iron, or malleable iron, and they should be either hot-dip galvanized or zinc electroplated as supplied by the manufacturer.

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Conduit supports can be used for single conduit runs or for grouped conduit runs. For single conduit runs on bare steel, Ubolts or one-hole malleable clamps (bolted to steel) should be used. U-bolts should also be used for the conduit support whenever the structure to which the conduit is mounted is subject to vibration. Single conduit runs on concrete or masonry should be supported with one-hole malleable clamps with expansion bolts. If the conduit run is on hollow tile, one-hole malleable clamps with toggle bolts should be used. Grouped conduit runs should be supported with suitable field fabricated hangars or Unistrut-type supports (or equivalent). Except when expansion joints are included in a straight run of conduit, the clamps and straps that are used for conduit supports should be made specifically for the trade size of the conduit. When expansion joints are used, one normal size conduit clamp should be firmly attached at each midpoint between adjacent expansion joints. Also, one normal size conduit clamp should be firmly attached at the midpoints that exist between both ends of the conduit run and the adjacent expansion joints. The clamp is used to equalize the expansion and contraction that occur at each expansion joint. All other conduit supports that are used when an expansion joint is present should be oversized conduit clamps that allow the conduit to move axially (along the axis). Clamps that rely on friction for their support on the base structure (such as Korns clamps) should not be used for the oversized clamps. In straight conduit runs that include an expansion joint, U-bolt-type clamps that are securely bolted to the base structure are acceptable for use. Conduit runs should not be supported from process lines or other pipelines unless no other practical method is available. The minimum clearance for conduit that crosses or runs parallel to process lines is 150 mm (6 inches) for uninsulated process lines and 100 mm (4 inches) for insulated process lines. The maximum distance that is allowed between rigid-metal conduit supports is dependent on the size of the conduit. The table in Figure 8 contains the maximum distance both in meters (m) and in feet (ft) for various conduit sizes.

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To ensure that the conduit system remains rigid and vibration-free, additional conduit supports may be required at bends, fittings, and fixtures. Conduit supports should be located at a maximum of 1 m (3 feet) from each outlet box, junction box, or fitting. In order to allow the conduit to flex when a long horizontal run of conduit ends in an angle or a bend, the next clamp around the angle or bend should not be placed adjacent to the angle or bend.

Max Distance Conduit Size (in.) ½ and ¾ 1 1 ¼ and 1 ½ 2 and 2 ½ 3 and larger

M 3 3.6 4.2 4.8 6.0

ft. 10 12 14 16 20

Figure 8: Maximum Distance Between Rigid-Metal Conduit Supports

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DETERMINING CABLE PULLING REQUIREMENTS To install a cable into a conduit, it must be pulled from one end of the conduit to the other with a strong wire. When a cable is pulled into a conduit, there are maximum pulling tensions that the cable can withstand without damage. There are various types of pulling equipment that can be used to pull a cable into a conduit. Each different type of pulling equipment has a maximum pulling tension (or pulling force) that it can withstand. The configuration of the conduit, the type of cable that is to be installed, and the types of pulling equipment that are chosen for the installation should all be evaluated so that damage to the cable or to any installation equipment does not occur. Calculations to determine the maximum pulling tensions that could occur with various conduit configuration and pulling equipment combinations are performed during the design phase of an installation. These calculations are evaluated to ensure that maximum pulling tensions are not exceeded during a cable installation pull. There are various ways to reduce the pulling tension for a given cable installation: the rigging equipment can be varied, the size of the conduit can be increased, the conduit configuration (e.g., turns or angles) can be altered, or the pull point frequency can be changed. This section of the Module provides information on the following topics that are pertinent to determining cable pulling requirements: o

Rigging Procedures

o

Cable Pulling Parameters

Rigging Procedures During the design phase of the wire or cable installation, once the installation type (e.g., conduit) and the cable route have been chosen, the Electrical Engineer selects a rigging method and then performs a pulling tension calculation. If the pulling tension calculation indicates that maximum tensions could be exceeded by the cable pull, design changes are made. Before the cable pulling parameters and pulling tension calculations are described, a description of the cable rigging equipment and methods is necessary.

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The method that is used to rig the cables for pulling in abovegrade conduit and cable tray systems depends on the length of the pull and the size of the conductors to be installed. Smaller conductors that are installed in a short run can most likely be pulled in by hand. Pulling equipment will probably be required to install larger conductors or to install conductors in long runs so that a constant pulling tension can be maintained on the cables. Suitable pulling equipment that is in good working condition should be on hand for the pulling operation. Hydraulic pulling equipment that has smooth, variable-speed control is a good choice for cable installations in above-grade conduit installations. To ensure that the maximum allowable pulling tension for the installation is not accidentally exceeded, a steady pulling rate should be maintained, whenever possible, during the pull. Cable pulling speed should never exceed 15 m/min (50 feet/min) and, if at all possible, the cable should not be pulled slower than 4.5 m/min (15 feet/min). If the cables will be installed in a grouped conduit run, the conduit that will be used for the installation of a single cable should be identified throughout the entire length of the run to avoid cable crossovers during the installation process. As much as possible, the same relative position in the group should be maintained throughout the run. In general, the longer cables should be installed in the lower raceways, and the shorter cables should be installed in the upper raceways to facilitate the ease of installation. Before the pulling operation begins, the direction of the pull should be checked to ensure that it is the direction that results in the minimum pulling tensions and sidewall pressures. Care must also be taken when moving the cable reels into their proper positions. If the pull and cut method of installation is used, cable damage can occur during the setup phase of a cable pull. The pull and cut method is most often used for pulls at several different locations. During cable installation, once the protective covering is removed from the reel, the cable is particularly vulnerable to mechanical damage.

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The cable reels must be supported by an axle at the installation location so that the cable can be pulled with minimum friction. The pulling tension equations should take into account any friction that results from the setup at the feed end of the conduit so that the maximum allowable installation tension is not exceeded. To measure the pulling tension that is applied to the cables, a dynamometer is often used during the pulling operation. The dynamometer can be connected at the feed end of the conduit or at the discharge end of the conduit. An idler is attached to the dynamometer at the feed or discharge end of the conduit, and the pulling line and/or cable is routed over the idler to allow the tension that is applied during the pulling operation to be measured. Figure 9 shows a method that could be used to attach the dynamometer at the discharge end of the conduit. The equation that is used to determine the tension on the cable in the conduit is also shown in Figure 9. If the dynamometer is not zeroed with the idler attached, the weight of the idler must be subtracted from the meter reading.

Figure 9: Dynamometer Used to Measure Pulling Tension

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When several single-conductor cables are to be pulled into a conduit or duct, the cable reels should be set up in tandem, and all of the cables should be simultaneously pulled into the conduit or duct. The cables should be continuously trained into the conduit in such a manner that the cable will not drag on the edge of the conduit. The cables should be fed into the conduit by hand or, for large conductors, by a large diameter sheave (pronounced "shiv"). In addition to reducing friction, selection of the correct diameter sheave for the job ensures that the minimum-bending radius of the cable is maintained. Although quality sheaves are generally treated as frictionless (no effect on pulling tension), extreme care and good judgement should be exercised in their use. Short cable bends, sharp edges at the feed to the raceway, and cable crossovers should be prevented at the point where the cable enters the raceway. The Electrical Engineer should ensure that the diameter of the sheave that is used for the installation is large enough that the sidewall pressure that is exerted on the cable does not exceed limits. Sidewall pressure (SP) is defined as the crushing force that is exerted on a cable as it is pulled around a bend section of conduit or duct. Supports should be used to alleviate stress on the cables where they enter the conduit or cable tray. Cables should be laid out or pulled into cable tray runs. When cable is pulled through a cable tray, cable rollers, pulleys, and sheaves should be used to prevent cable damage. If the pulleys, sheaves, and rollers that are used for cable installation in cable trays are undersized, the cable can be damaged if it bends below its minimum allowable bending radius. If the rollers that are used for the installation are too widely spaced, the cables can be damaged from abrasion against the cable tray rungs. Rollers should be separated by a maximum distance of 3 m (10 feet) during cable pulls in cable trays. Sharp points in the cable tray, such as bent or burred metal, dropped tools in the cable tray, and reversed bolts (heads on the outside of the cable tray rather than on the inside of the cable tray), can cause extensive cable damage during the cable pull, and they can lead to early failure of the installed cables. The maximum pulling tension equations that are listed in the following sections should be followed for both conduit and cable tray systems.

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The remainder of this section of the Module provides information on the following topics that are pertinent to cable rigging procedures: o

Pulling Grips

o

Pulling Lines

o

Duct Lubricating

Pulling Grips

Pulling grips are used to fasten the pulling line to the cable that is to be pulled into the raceway. There are two basic types of pulling grips that are frequently used for cable installations: basket grips and pulling eyes. A basket grip is a flexible metal device that slips over the end of the conductor that is to be pulled; a pulling line is attached to the basket grip so the cable can be pulled through the raceway. The basket grip has a web-like grip that tightens as tension is applied to the pulling line. Long basket grips should be used to pull type MC cables. To use the basket grip, the armor is removed for a short distance, tape is applied over the armor and onto the conductors, and the basket grip is placed onto the cable to allow the grip to squeeze both the armor and the conductors. Figure 10 shows how a typical metal basket grip is installed onto a cable. To avoid cable twist during the pulling operation, a swivel has been installed onto the pulling line where it attaches to the basket grip.

Figure 10: Basket Grip on Cable

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Because the conductor and the cable insulation are stressed during the pulling operation, the parts of the cable that are directly affected by the basket grip should be removed before a splice or termination is installed. Sufficient slack must be present at the pulling end so that the last 600 mm (2 feet) of cable beyond the basket grip can be removed before the cable is spliced or terminated. Pulling eyes are sometimes used to pull larger sizes and long sections of cable; pulling eyes allow for a higher pulling tension than basket grips. A pulling eye is a steel eye that is usually fastened directly to the cable conductors. Many manufacturers supply a pulling eye or pulling bolt to the leading end of the cable that is on the cable reel. Once the pulling operation is complete, the pulling eye should be removed from the cable. Figure 11 shows how the cable conductors are securely fastened and solder-wiped to the shank of the pulling eye. In Figure 11, a swivel is used to connect the pulling line to the pulling eye to avoid cable twist during the pulling operation.

Figure 11: Pulling Eye on Cable Pulling Lines

A pulling line is used to pull the cable through the conduit or cable tray during the cable installation. Pulling lines can be made of rope or wire, and they are provided in various sizes. The type of pulling line that is used for a given installation depends on the size of the conductor to be pulled, the type of pulling grip to be used, and the length of the pull.

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The pulling line is typically drawn into a conduit with a steel fishing wire or "snake." Fishing wire is a tempered-steel wire that has a rectangular cross-section. Although the flat rectangular fishing wire is preferred because of the ease with which it can be run through a completed conduit system, galvanized-steel wire can also be used for fishing. Any size wire from #14 up to #6 can be used for fishing. When the fishing wire is drawn through the conduit, the pulling line is attached to the fishing wire. The fishing wire is then withdrawn from the conduit so that the pulling line is completely drawn through the conduit system. Fishing wire is not required to pull cables through cable tray systems. After the pulling line is completely drawn through the affected raceway, the pulling line is firmly attached to the pulling grip and to the pulling device through an idler. The idler is attached to a dynamometer to allow the tension on the pulling line to be evaluated throughout the pull. Duct Lubricating

To reduce friction during the pulling operation, lubrication should be liberally and continuously applied to conduit. Increased friction during pulling can also cause an increase in the pulling tension that is applied to a cable during its installation. The use of minerallac #100 or equal lubricant is recommended for duct lubrication in Saudi Aramco above-grade conduit installations. Duct lubricant should not be applied to the first and last 15 meters (50 feet) of the cable for reasons of convenience and cleanliness in splicing operations. Duct lubrication is not required during cable tray installations.

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Cable Pulling Parameters Cable pulling parameters include maximum pulling tension and sidewall pressure calculations. The tension calculations take into account the maximum allowable tension that can be withstood by the pulling device and the conductors. The sidewall pressure calculation takes into account the stress effects that a bend in the conduit or cable tray has on the cable during the pulling operation. The general procedure that is used to determine the cable pulling tensions and parameters is to calculate the pulling tensions for the entire length of the pull and then to determine whether the sidewall pressure is too great at the conduit bends. When the pulling tensions are calculated for the pull, the calculations are performed twice: once for a pull in one direction and again for the pull in the opposite direction. This section of the Module describes cablepulling parameters in terms the following topics: o

Maximum Pulling Tension

o

Sidewall Pressure

o

Rigging Method Effects Calculations

Maximum Pulling Tensions

After the size of the raceway has been determined (based on allowable fill, the jam ratio, and cable clearance), the maximum tension for the pulling device and the maximum tension that can be safely applied to the conductors should be calculated. The most limiting maximum tension is used as the maximum allowable pulling tension (Tm). Next, the pulling tension (T) that is actually required to pull the cable through the raceway is calculated and compared to the maximum allowable pulling tension. If the actual pulling tension that is calculated for the pull exceeds the maximum allowable pulling tension, the conditions of the pull should be changed. When the actual pulling tension exceeds the maximum allowable pulling tension for the installation, the following actions should be considered: o

Increase the bending radii that will be used for the conduit installation.

o

Decrease the allowable conduit fill.

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o

Reduce the number of bends in the conduit.

o

Perform a reverse pull.

o

Perform the pull in stages.

o

Decrease the length of the pull.

The maximum allowable tension for the pulling device (Tdevice) is dependent on whether the pulling device is a pulling eye or a basket grip: o

The maximum pulling tension that can be applied when a pulling eye is used should not exceed 22 kN (5000 lbf) per cable (assuming that each cable has its own pulling eye) for single-conductor cables or 27 kN (6000 lbf) per cable for multiple-conductor cables.

o

The maximum pulling tension that can be applied when a basket grip is used over the outer jacket of a cable should not exceed 4450 N (1000 lbf) per cable (assuming that each cable has its own basket grip) in any case.

The maximum allowable tension for the cable (Tcable) is dependent on whether single conductor cables or multiple conductor cables are to be used in the installation. Different conductor sizes can be pulled at the same time, but a simultaneous pull is not recommended if the dimensions of the conductors are significantly different. If different sizes of conductors are to be pulled into the same run of conduit, care must be taken not to exceed the maximum pulling tension of any one cable during the pull. Because a pulling rope under tension could possibly cut previously existing cables in conduit systems, pulling additional cables into an existing conduit is generally not recommended. However, consideration should still be given to whether conductors that are of different sizes should be pulled at the same time for a given installation. The equations that are used to determine the maximum allowable cable tension are as follows:

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o

The maximum pulling tension that can be applied to a single-conductor (copper conductor) cable is calculated through use of the following equation: T cable = 0.008 = 71

o

lbf • AREA (in cmil) cmil

N mm

2

• AREA (in mm 2)

The maximum pulling tension that can be applied to multiple conductors when there are three or less conductors (for cables that are in parallel, multiplexed, or are multipleconductor cables) is calculated through use of the following equation:

T cable = Σ T c Where Σ Tc is the summation of the maximum pulling tension for each individual cable. o

The maximum pulling tension that can be applied to multiple conductors when there are more than three conductors (for cables that are in parallel, multiplexed, or are multipleconductor cables) is calculated through use of the following equation:

T cable = 0.8 • Σ T c Where Σ Tc is the summation of the maximum pulling tension for each individual cable. Once the maximum allowable tension on the pulling device and the maximum allowable tension on the cable have been determined, the two values are compared, and the lowest of the two values is designated as the maximum allowable pulling tension for the installation (Tm). Next, the actual pulling tension for the installation is calculated and compared to the maximum allowable pulling tension.

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The configuration of the cables that are being pulled into a conduit system affects the pulling tension calculations; the weight correction factor (w) is a calculated value that is used in the tension equations to account for the effect of cable configuration. Figure 12 shows the various cable configurations that can occur for a single-cable installation, a dual-cable installation, a threecable installation (cradled or triangular configuration), and a cable installation for more than three cables (complex configuration). The three-cable installation configurations are of special interest. Based on the result of the jam ratio (recall that the jam ratio is equal to 1.05 times the D/d ratio), a three-cable installation will have a cradled configuration or a triangular configuration. If the jam ratio is less than 2.5 or if assembled cables (triplexed cables) are to be pulled, the installation will result in a triangular configuration.

Figure 12: Cable Configurations

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The equations that are used to calculate the weight correction factor are selected based on the configuration of the cables for the installation. The weight correction factor equations are as follows: o

The weight correction factor for a single-cable configuration (including the case of a multiple-conductor cable) is as follows: w=1

o

The weight correction factor for a dual-cable configuration is as follows: w=

o

 d  1-   D-d

The weight correction factor for three cables with a triangular configuration is as follows: w=

o

1

1  d  1-   D - d

The weight correction factor for three cables with a cradled configuration is as follows: w =1+

o

4  d  ⋅  3  D- d 

2

The weight correction factor for complex cable configurations is as follows: ww = 1.4

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Another factor that is of critical importance to the tension equations is the cable weight (W). Cable weight is the weight per unit length of the cable that will be pulled through the raceway. When cables are to be installed in parallel (at the same time), the cable weight factor that is used in the pulling tension equations should be the sum of the individual cable weight factors. If the conductors are cabled (triplexed or quadraplexed), one percent per conductor weight should be added. Because weight is a measure of the force that is exerted on an object in the direction of the earth's gravitational pull, the units that are used for the cable weight factor are Newtons per meter (N/m) or pounds-force per foot (lbf/ft or lb/ft); the correct units must be used in the tension calculations. When values are given that are not in the same units as the pulling tension equations, unit conversion factors are used to convert the given cable weight for a single cable to the correct units. The following examples show how the cable weight factor (W) for a single 5 AWG copper conductor is determined: o

If the cable weight that is specified by the manufacturer is 102 pounds per 1000 feet, the cable weight factor (W) (in pounds-force per foot) should be calculated. To convert pounds per 1000 feet to pounds per foot, divide by 1000.

W=

o

If the cable weight that is specified by the manufacturer is 102 pounds per 1000 feet, the cable weight factor (W) (in newtons per meter) should be calculated. To convert pounds per 1000 feet to newtons per meter, divide by 1000 and multiply by 14.6.

W=

Saudi Aramco DeskTop Standards

102 lb 0.102 lb = 1000 ft ft

1.489 N 102 lb ⋅ 14.6 = 1000 ft m

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If the cable weight that is specified by the manufacturer is 152 grams per meter, the cable weight factor (W) (in pounds-force per foot) should be calculated. To convert grams per meter to pounds per foot, multiply by 0.0098 and divide by 14.6.

o

W=

152 g 0.0098 0.102 lb ⋅ = m ft 14.6

If the cable weight that is specified by the manufacturer is 152 grams per meter, the cable weight factor (W) (in newtons per meter) should be calculated. To convert grams per meter to newtons per meter, multiply by 0.0098.

o

W=

152 g 1.489 N ⋅ 0.0098 = m m

The pulling tension that will exist for a given above-grade conduit or cable tray installation is determined through use of various equations whose use is dependent on the type and direction of the pull; the pulling tension is evaluated along segments of the raceway through use of the information from a layout drawing. The variables that are used in the pulling tension equations are defined as follows: Tin

=

tension into a section (newtons or pound-force).

Tout =

tension out of a section (newtons or pound-force).

Tm

=

maximum allowable pulling tension for the installation (newtons or pound-force).

w

=

weight correction factor (dimensionless).

µ

=

coefficient of dynamic friction = 0.5 in all cases (dimensionless).

W

=

total assembly weight (newtons per meter or poundforce per foot; 1 lbf = 4.45 N).

L

=

straight-section length (meters or feet).

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Lm

=

maximum length of straight-section pull (meters or feet).

θ

=

straight-section angle from horizontal (radians).

φ

=

bend-section angle (radians).

R

=

bend-section radius (meters or feet).

e

=

natural logarithm base.

The equations that are used to calculate the pulling tension that will exist for the various conduit and cable tray configurations are listed as follows: o

Maximum Length of a Straight Section Pull: This equation is used to calculate the maximum length for a straight section pull that will not exceed the maximum allowable pulling tension for the installation.

Lm =

o

Tm wµW

Horizontal Straight Section: This equation is used to calculate the pulling tension that will be felt at the end of a horizontal straight section pull.

T out = w µ WL + T in o

Pulling Up a Vertical Straight Section: This equation is used to calculate the pulling tension that will be felt at the end of a vertical or incline straight section pull (pulling up).

T out = WL(sin θ + w µ cos θ ) + T in

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o

Pulling Down a Vertical Straight Section: This equation is used to calculate the pulling tension that will be felt at the end of a vertical or decline straight section pull (pulling down).

T out = - WL(sin θ - w µ cos θ ) + T in o

Horizontal Bend Section: This equation is used to calculate the pulling tension that will be felt at the end of a horizontal bend; this equation does not represent the sidewall pressure that is exerted on the cable.

Tout = Tin (cosh wµ ) + (sinh wµ ) ⋅ Tin + (WR) 2 2

o

Horizontal Bend Approximations: It is common practice to use the following approximation for a horizontal bend provided that (Tin > 10 WR).

T out ≈ T in ⋅ e

w µφ

It is common practice to use the following approximation for a horizontal bend when Tin > 10 WR and w ≈ 1.

T out ≈ w T in ⋅ e

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µφ

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Figure 13 shows the difference between a vertical up-bend and a vertical down-bend in conduit.

Figure 13: Vertical Conduit Bends The following equations are used for vertical concave up-bends and down-bends: Pulling Up Through a Vertical Concave Up-Bend: This equation is used to calculate the pulling tension that will be felt at the end of a vertical concave up-bend when pulling up through the bend. This equation does not represent the sidewall pressure that is exerted on the cable.

o

T out = (T in e

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w µφ

 WR   ⋅ [2 w µ sin φ - (1 - (w µ ) 2) ⋅ (e w µφ - cos φ)] -  2   1 = (w µ ) 

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Pulling Down Through a Vertical Concave Up-Bend: This equation is used to calculate the pulling tension that will be felt at the end of a vertical concave up-bend when pulling down through the bend. This equation does not represent the sidewall pressure that is exerted on the cable.

o

T out = (T in e

w µφ

 WR  ⋅ [2 w µ e w µφ sin φ + (1 - (w µ ) 2) ⋅ (1 - e w µφ - cos φ)] -  2   1 = (w µ ) 

Pulling Up Through a Vertical Concave Down-Bend: This equation is used to calculate the pulling tension that will be felt at the end of a vertical concave down-bend when pulling up through the bend. This equation does not represent the sidewall pressure that is exerted on the cable.

o

T out = (T in e

w µφ

 WR  ⋅ [2 w µ e w µφ sin φ + (1 - (w µ ) 2) ⋅ (1 - e w µφ - cos φ)] +  2   1 = (w µ ) 

Pulling Down Through a Vertical Concave Down-Bend: This equation is used to calculate the pulling tension that will be felt at the end of a vertical concave down-bend when pulling down through the bend. This equation does not represent the sidewall pressure that is exerted on the cable.

o

T out = (T in e

o

w µφ

 WR   ⋅ [2 w µ sin φ - (1 - (w µ ) 2) ⋅ (1 - e w µφ - cos φ)] +  2   1 = (w µ ) 

Vertical Bend Approximations: (It is common practice to use the following approximation for a vertical bend when Tin > 10 WR and µ = 0.5.)

T out ≈ T in ⋅ e

w µφ

It is common practice to use the following approximation for a vertical bend when Tin >10 WR, µ = 0.5, and w ≈ 1.

T out ≈ w T in ⋅ e

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Wire and Cable Systems Evaluating Above-Grade Wire, Cable, and Conduit Installations

When the tension equations are examined, it can be seen that a bend in a conduit or cable tray system multiplies the incoming tension by some exponential factor. A 90-degree bend will have a larger effect on the pulling tension than a 45-degree bend. When pulling cable in conduit, the NEC allows no more than four quarter conduit bends (360 degrees total) to be made in one run of conduit between pull points. Also, the tension that is required to pull in either direction through a bend must be calculated and evaluated. The cable should be pulled through the bend in the direction that exerts the least amount of tension. The example in Figure 14 shows how the tension is calculated for a pull in either direction through a conduit system. The tension is calculated for both directions to determine the direction of pull that causes the least amount of tension.

Figure 14: Example Pulling Tension Calculation

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The initial conditions of the pull are as follows: Tension at starting point = 0 N Friction coefficient (u) = 0.5 Three THHN copper (1/C), size 4/0 cables are being pulled through a 2-inch conduit Conduit inside diameter (D) = 2.067 inches Single cable outside diameter (d) = 0.626 inches Weight of the cable is specified by the manufacturer as 711 lbs. per 1000 feet 1.

Determine the weight correction factor (w). Since the weight correction factor is based on the configuration of the cables in the conduit, the type of configuration must be determined from the jam ratio.

Jam Ratio =

1.05 D (1.05)(2.067) = = 3.47 d (0.626)

Since the jam ratio is greater than 2.5, the three cables will be a cradled configuration, and the weight correction factor is determined with the following equation:

4  d  w = 1+ ⋅  3 D−d

2

4  0.626  w = 1+ ⋅  3  2.067 − 0.626  w = 1.25 2.

2

Determine the cable weight (W) for the installation. The units N/m should be used because the length of the conduit is specified in meters.  711 lb  31.14 N (14.6)(3 cables) = W =  m  1000 ft 

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3.

Determine the pulling tension from A to D. a.

Because A is the starting point, TA = 0

b.

TB is calculated with the equation for a horizontal straight section. TB = w µ W L + TA = (1.25)(0.5)(31.14)(3) + 0 = 58.39 N

c.

TC is calculated with the equation for a horizontal bend section. To determine if the approximation equation can be used, TB is compared to (10WR). If TB is greater than (10WR), the approximation can be used: 10WR = (10)(31.14)(1) = 311.4

TC = TB (cosh wµφ) + (sinh wµφ) ⋅ TB + (WR)2 2

TC = 58.39(cosh 0.9817) + (sinh 0.9817) ⋅ (58.39) 2 + (31.14 ⋅ 1) 2 TC = 164.78 N TC = 37.05 lb d.

TD is calculated with the equation for a horizontal straight section:

TD = wµWL + TD = (1.25)(0.5)(31.14)(1) + 164.78 = 184.24 N = 42.42 lb 4.

Determine the pulling tension from D to A a.

Because D is the starting point, TD = 0.

b.

TC is calculated with the following equation for a horizontal straight line:

TC = wµWL + TD = (1.25)(0.5)(31.14)(1) + 0 = 19.46 N = 4.37 lb

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c.

TB is calculated with the equation for a horizontal bend section. Because TC is less than (10WR), the approximation equation cannot be used:

TB = TC (cosh wµφ) + (sinh wµφ) ⋅ TC + (WR) 2 2

TB = (19.46)(cosh 0.9817) + (sinh 0.9817) ⋅ (19.46) 2 + (31.14) 2 TB = 71.74 N TB = 16.13 lb

5.

Compare the tension of the pull from A to D to the tension of the pull from D to A. Because the tension for the pull from D to A is the lesser of the two values, the cable should be pulled from D to A.

Sidewall Pressure

The radius of a conduit or cable tray bend is limited by two factors: the minimum training radii of the cable under static load and the sidewall pressure that will be exerted on the cable during its installation. Sidewall pressure (SP) is defined as the crushing force (in Newtons or pounds-force) that is exerted on a cable as it is pulled around a bend section of conduit or duct. For Saudi Aramco cable pulling operations, the maximum SP for any conduit installation should not exceed 4350 N/m times the radius of the conduit bend in meters (300 lbf/ft time the radius of the conduit bend in feet).

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Figure 15 illustrates the SP that is exerted on a given cable as it is pulled through a bend with some tension (Tout) and some bend radius (R).

Figure 15: Sidewall Pressure on Cable During a Pull

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SP is directly proportional to the pulling tension out of the bend and is inversely proportional to the radius of the bend; that is, as the pulling tension increases, the sidewall pressure also increases, and as the bending radius decreases, the SP increases. The equation that is used to calculate the SP that is exerted on a cable is dependent on the configuration of the cables that are in the conduit. The equations are as follows: o

For a single conductor installation, the following equation should be used to calculate the SP:

SP =

T out R

For three cables that are in a cradled configuration, the following equation should be used:

SP = o

(3 w - 2) ⋅ T out 3R

For three cables that are in a triangular configuration, the following equation should be used:

SP = o

w ⋅ T out 2R

For more than three conductors, the more limiting case of a triangular cable configuration to calculate the SP should be used.

After the pulling tensions have been evaluated throughout the raceway, SPs are calculated for each bend and are compared to the maximum allowable SP limit. If the SP of any bend exceeds the allowable limit, the radius of that bend should be increased. The table in Figure 16 shows the inside radius of various sizes of standard conduit elbows; the inside radius of the elbow is used as the radius of the conduit bend (R) in the SP calculations. The inside radius of standard aluminum and fiberglass cable tray elbows are 305, 610, and 914 mm (12, 24, and 36 inches).

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Standard Conduit Elbow Size in mm (ft.) Conduit Size (in.)

Std.

300 mm 12 in.

380 mm 15 in.

460 mm 18 in.

610 mm 24 in.

760 mm 30 in

915 mm 36 in.

1065 mm 42 in.)

1220 mm 48 in.

½

100 (0.33)

¾

105 (0.34)

1

135 (0.44)

295 (0.96)

360 (1.21)

445 (1.46)

600 (1.96)

750 (2.46)

900 (2.96)

1055 (3.46)

1210 (3.96)

1¼

160 (0.55)

280 (0.94)

365 (1.19)

440 (1.44)

595 (1.94)

745 (2.44)

897 (2.94)

1050 (3.44)

1202 (3.94)

1½

180 (0.62

285 (0.93)

360 (1.18)

435 (1.43)

590 (1.93)

740 (2.43)

895 (2.93)

1045 (3.43)

1200 (3.93)

2

215 (0.71)

275 (0.91)

355 (1.16)

430 (1.41)

585 (1.91)

735 (2.41)

890 (2.91)

1040 (3.41)

1195 (3.91)

2½

235 (0.77)

350 (1.15)

425 (1.40)

580 (1.90)

730 (2.40)

885 (2.90)

1035 (3.40)

1190 (3.90)

3

295 (0.96)

420 (1.37)

570 (1.87)

720 (2.37)

875 (2.87)

1025 (3.37)

1180 (3.87)

3½

335 (1.10)

410 (1.35)

565 (1.85)

715 (2.35

870 (2.85)

1020 (3.35)

1175 (3.85)

4

355 (1.17)

560 (1.83)

710 (2.33)

865 (2.83)

1015 (3.33)

1165 (3.83)

5

545 (1.79)

700 (2.29)

850 (2.79)

1005 (3.29)

1155 (3.79)

6

685 (2.25)

840 (2.75)

990 (3.25)

1145 (3.75)

Figure 16: Inside Radius of Standard Conduit Elbows

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Rigging Method Effects Calculation

This section of the Module shows how to use the previously mentioned calculations for a given installation to determine the type of pulling device that can be used, the maximum pulling tensions that can safely be applied to the cable, the actual pulling tension that will be applied to the cable, and the SP that will be felt by the cable during its installation. Figure 17 provides a sample above-grade conduit installation and shows the calculations that must be made to determine whether the installation is within the various limits. A procedure is provided in Work Aid 3 to perform the rigging method effects calculation. Three THHN (1/C) 4/0 copper conductor cables are being pulled throu a 2-inch conduit. µ = 0.5 Weight of cable is 711 lb per 1000 ft. The distance from A to B is 2 m.

D = 2.067 inches

The distance from C to D is 30 m

d = 0.626 inches

The distance from E to F is 3 m

Cable Area = 199 m2

Figure 17: Sample Rigging Methods Effects Calculation

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All conduit bends are 90-degree bends that use a 36-inch sweep elbow (inside radius of the elbow = 2.91 ft = 0.89 m) Tension at starting point is assumed to be 200 N. Basket grips will be sued to pull the cables through the conduit. The conduit installation has already been satisfactorily evaluated against the minimum cable training radius. 1.

The configuration of the cable is determined with the jam ratio:

Jam Ratio =

1.05 D (1.05)(2.067) = = 3.47 d (0.626)

Since the jam ratio is greater than 2.5, the cables will be a cradled configuration 2.

The weight correction factor (w) is determined with the following equation:

4  d  w = 1+ ⋅  3 D−d

2

4  0.626  w = 1+ ⋅  3  2.067 − 0.626  w = 1.25 3.

2

Determine the cable weight (W) for the installation. The units N/m should be used because the length of the conduit is specified in meters.  711 lb  31.14 N  (14.6) (3 cables) = W =  m  1000 ft 

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4.

The maximum allowable pulling tension is determined as follows: Tdevice = 4450 N per cable for a basket grip x 3 cables = 13,350 N Tcable = ∑TC

TC =

71 N ⋅ Area = (71)(199) = 14,129 N mm 2

Tcable = (3 cables)(14,129) = 42,387 N 5.

The tension for pulling from A to point F is calculated as follows: TA = 200 N The equation for pulling down a vertical section must be used to calculate TB. TB = − WL(sin θ − wµ cos θ) + TA TB = −3(31.14)(2) sin π / 2 − (1.25)(0.5) cos π / 2) + 200 N) TB = 137.72 N

10WR = (10)(31.14)(.89) = 277.15 N Since TB