Electrical System for High Rise Building

ELECTRICAL SYSTEM DESIGN for HIGH-RISE BUILDING High – rise buildings normally refer to occupancy for: • general offices • commercial establishments • hotels / condominiums or their combinations. The definitions as to number of floors and areas vary from one party to another. These buildings distinctively differ from industrial buildings for manufacturing with regards to electric utilization equipment installed and the number of floors. The latter are mostly single or two-storey structure due to operational requirements and constraints. Exceptions are the taller silos for stockpiling of materials or finished products.

1 – HIGH RISE BUILDING POWER SUPPLY REQUIREMENTS: 1. General Lighting & Power • light for general illumination, seeing tasks, decorative features, hallways and stairways, others • power for appliances and office machines 2. Heating, Ventilation & Air-conditioning (HVAC) System •air-conditioning for temperature control •blowers and fans ventilation •heaters for humidity control 3. Transport System •elevators and escalators •conveyors, dumbwaiters, others 4. Water Pumps •potable and non-potable water supply •water sprinkle (fire suppression) •pumps / drainage •sewage ejectors

5. Communication System • PABX telephone system • Intercom 6. Automatic Doors • entrance and exit for pedestrians • garage and freight 7. Central Computer System • CPU and peripherals • Terminals 8. Auxiliaries • intrusion and hold-up control system • fire suppression and alarm system • background music and paging • sound reinforcement and video facilities • noise masking and acoustics, others

Note: The latter items could be integrated into the building automation system as may be provided in the design.

2 – HIGH-RISE BUILDING SYSTEM COMPONENTS A high-rise building electrical system is composed of hundreds of components, designed and assembled into a safe, functional power-delivery system. In figure 2.1 shows a typical building electrical system riser diagram, where the building’s electrical system is connected to the utility system. Here, it is a pad-mounted transformer, but in other cases it might be a bank of transformers mounted overhead on a utility pole (for a demand less than 1,000 kVA). The underground service connects the utility system to building’s main distribution panel (MDP). Located within the MDP is the main building over-current device, or main disconnect, as well as individual over-current devices for the system components connected to the MDP. The MDP may also contain provisions for utility metering, as well as instrumentation for the measurement of system voltage and current.

The main disconnect device can be either a circuit breaker or a fused switch. This main device often contains special circuitry for sensing low-level faults (i.e. ground faults for more than 1,000 Amp main), which otherwise might escape detection. The MDP might be thought of as the electrical nerve center of the building. It is normally located near building exterior wall and as close as possible to the utility transformer to minimize the cost of main service feeders. Thus, all components of the system must be chosen carefully based on design requirements and must function safely, under normal operating conditions and also under abnormal conditions, such as short circuits.

3 – POWER SUPPLY SYSTEM The franchise utility power company serves at nominal level of 230/115-volt, single- or three-phase, two-, three or four wires depending on the type of load and as long as it does not exceed 1,000 kVA. For extremely large service entrance current, multiple conductors may be used. Likewise, multiple protective/disconnect devices not exceeding six (6) may be connected in parallel for the service entrance (as stated by P.E.C.). For establishment of greater than 1,000 kVA load, as most commercial and industrial consumers, the power company requires a load center unit sub-station and serves power at primary line distribution level of 13.8 or 34.5k Volts or whatever potential level available in the vicinity. The size of the load center depends on the proposed connected load and allowances for future growth of the establishment; its configuration, on the other hand, depends on the requirements and available facilities of the utility company.

The major components of the load center are: (1) (2) (3) (4)

High-voltage switchgear; primary side Power transformer section Low-voltage switchgear; secondary side Metering equipment

3.1 – Utilizing Voltage Usually in large installations with private load centers, the practice is to use 208/120-volt for general lighting and power, and 460-volt for motors. This appears to be the more economical and practical arrangements. Three-phase electric motors are normally dual-voltage, i.e. 460/230 volts and using the higher 460-volt rating will result in half-as-much ampere draw, hence smaller wires, lower circuit breaker rating (although higher voltage) and smaller starter unit. For lighting and appliances, 460-volt line can likewise be used but availability of fixtures for such potential may not be easily procured, i.e. 265-volt ballasts for fluorescent and convenience outlet with built-in unit transformers of 50 to 100 VA, 460-230/115-volt ratings.

For total load of 1,000 kVA or less, the power supply is 208/120-volt or 230/115-volt only. In some cases, and for temporary construction power, the power company would serve 460-volt for use of construction equipment, subject to their requirements, rules and regulations.

3.2 – Configuration of Load Centers

Should the customer enterprise be required to provide its own load center unit substation, several options are available, again subject to approval of the power utility company.

OPTION – 1 : High-voltage supply line from power company transformed to lighting and separate power

utilization voltage of 208/120-volt for general power, and 460-volt for motors using two transformers as shown in fig. 3.2(a).

OPTION – 2 : High-voltage supply line from power company transformed to 460-volts; general lighting and power fed by the 460-volt line through a unit dry-type transformer, 460-208 / 120-volt, as shown in fig. 3.2(b).

OPTION – 3 : Similar to Option – 2 “except” several units of smaller units of smaller dry-type transformers are distributed in the areas or floors for general lighting and power system; these unit transformers are fed by 460-volt line or lines from the load center as shown in fig 3.2 (c).

Any of these configurations will serve the purpose of transforming the incoming high-voltage line from the utility company to acceptable utilization equipment level. The final choice of the desired system is normally dictated by costs and equipment availability. Power transformers are either dry-type or oil-immersed. The common disadvantage of all of the above load center configurations is its inflexibility. In cases of breakdown of any of the main components, i.e. high or low-voltage switchgear mains, or the transformer itself will result in total system shutdown.

3.3 – Load Center Flexibility & Reliability

While a “fail-safe” system could not be adopted due to its prohibitive cost, still some degree of flexibility and reliability of the system can be reasonably reached. The load center can be split into two (2) equal or identical units to serve the likewise equally, as far as practicable, divided electrical loads. In cases of failure of any of the major components of either unit, the remaining half is still operational. System selectivity can be attained, either on the primary or secondary sides or both, by using “tie-breaker”.

Properly coordinated interlocking system should be provided between the tie and main breakers to prevent accidents. The load centers described in the proceeding paragraph will be served on two (2) separate ends and thus termed “double-ended” unit. Customarily, the power company serves this type of load center from two (2) separate distribution feeder lines to further enhance the system’s selectivity. Figure 3.3 shows the one line diagram of a typical “double-ended” system as adopted from Option 3. The same can be done for both Option 1 and 2.

Note: The interlock, mechanical/electrical, will prevent putting “ON” the tie-breaker until either of the main breaker is “OFF”; metering CT’s and PT’s are to be installed in both the high-voltage incoming lines 1 and 2.

4 – EMERGENCY POWER SYSTEM The power requirements of the building can be sufficiently supplied by the power company at acceptable level, continuity and characteristics. There are, however, instances when the power may be interrupted due to the system fault or deficiencies, some of which are inherent in the power transmission and distribution. Longer interruptions will greatly inconvenience the building occupants and may even be dangerous to life and limbs. Losses in terms of unproductive manhours and business opportunity may also than substantial. The suggested solution, other the self-contained battery-powered emergency lights, is a stand-by diesel enginegenerator set or sets. It is not economically sound, and probably poor engineering practice, to provide 100% back-up or stand-by power because of the tremendous costs of the generator set or sets to be used in relatively short time of main power interruptions.

The more essential loads of the building are to be supplied with emergency power in cases of main power failure. Normally, these are the following: • Stairways’ and hallways’ lighting for safety purposes • Counter areas for public transactions • Water pumps and fire pumps • One or two elevators to be used by physically handicapped • Computer system • Rooms or suites of top executives Power transfer to stand-by generator can be done manually by double-throw transfer-switch or automatically by automatic-transfer-switch (ATS). For the latter, it is necessary that the feeder/s or line/s serving the essential loads should not include the non-essential facilities. Separate emergency lines and panel boards will be provided exclusively for the purpose. A typical one-line diagram is shown in fig. 4.1 as adopted from fig. 3.3.

The system operates as follows: • When main power voltage dips to 70 to 80% of nominal value, the ATS automatically starts the generator and buildup same to its rated output voltage; after 20 seconds of such power condition, the ATS automatically transfer the emergency feeder mains to generator; • When main power is restored to its rated level, the ATS instantly transfer the load back to the main power feeder; after 1 or 2 minutes of main power stabilized conditions, the generator set automatically stops; • The ATS could also be programmed to automatically “exercise” or operate the generator at no-load for 15minute, twice-a-week periods in order to keep the set and auxiliaries in good running conditions. Voltage level and time setting as mentioned may be adjusted to the desired level of the user, but instant transfer from main to stand-by power is not possible since it will require sometime for the generator voltage to build-up.

For uninterruptible power supply as may be required by computer hardwares and the like, a different equipment configuration is necessary. It Power Supply for Computer System is discussed in the succeeding paragraphs. Computer hardwares and operations requires controlled environment as to temperature, humidity and dust for satisfactory performance.The same is true for its electric power supply. Power hits and dips which is normal occurrences in AC power system are sometimes beyond the tolerable limits of the computer. Some hardwares can not tolerate power disturbance of more than 1/5 of a cycle of the normal 60 Hz power. While specially designed automatic-voltage-regulators (AVR) may serve the purpose, the problem will be in the response time to correct the abnormalities, not to mention actual power interruptions. On power interruptions, the time-lag for the emergency generator to build-up and supply power through the ATS is a way beyond the recovery time of the computer. Hence, the computer will shut-down, will have to be reset and re-started. If the computer is on a long-batch run schedules, it may be necessary to re-run the batch (viz. program) from the start. There is also the possible errors and damages to the computer hardwares and softwares which may prove very costly.

The recommended conditioned power system for computer is the “uninterruptible power supply” system or UPS. In the country most UPS are static type. A typical block diagram is show in fig. 4.2.

The rectifier-charger, fed from emergency feeder thru “RCB” breaker, supplies the D-C bus. The battery is charged and at the same time supplies power to the inverter where D-C power is inverter to A-C output for the computer load. Charging of the battery is appropriately controlled. The output feeder line to the computer is protected by the inverter circuit breaker “ICB”. In cases of power supply and system infirmities: • Hits and dips will not be reflected in the A-C output lines as this is absorbed in the rectifier / charger only; the UPS in effect, filters the power to the computer. • When power is interrupted, the floating battery will supply D-C bus such that the inverter will not suffer any power stoppage; the battery bank is normally rated to supply power for 10 to 15 minutes, time enough to build-up and put on the line the emergency generator. The battery bank composed of 100 to 150 industrial type-heavy duty units, each with a rated terminal voltage of 2 to 2 ½ Volts for higher capacity units.

•

When the UPS itself fails, the static-transfer switch “SS” will automatically transfer the output to the by-pass line; the transfer is of the make-before-break operation that the A-C output will not detect the switch made; Manual transfer to the by-pass line can also be made thru the commercial circuit breaker “CCB”. There are several supplies of imported and locally manufactured UPS who can assists the users in selecting the configuration most suited to their respective purposes. Several options of Redundancy features are available to enhance the reliability the system. A ofrotary type combination of an A-C motor-drivenalternator and stand-by diesel engine prime mover with “flywheel” is another configuration of a UPS system as shown in fig. 4.3, this is sometimes termed as “dynamic UPS” system. The flywheel stores and supplies the rotating power (i.e. kinetic energy) for the alternator before the stand-by prime mover assumes the A-C motor drive functions in cases of power failure. This is similar with the principle of rotating regulators in maintaining the speed of a D-C generator.

5 – FEEDER : NUMBER & SIZES Feeder line can either be bus way (bus bar trunking) or insulated conductors or combination of both. The former is more versatile, neat in appearance but decidedly more expensive Bus ways are very popular especially for high ampere capacity lines. It can carry up to 7,000 Amp as compared against wires of 540 Amp maximum per set. Bus ways, however, should not be used in highly corrosive atmospheres as in battery rooms, in concealed locations, and where it may be subjected to serve mechanical injury as in hoist ways. For these cases, only insulated conductors in rigid steel conduit will suffice. All feeder runs will terminate in the low-voltage switchgear and will be protected with appropriately rated circuit breakers or fuses. (see fig. 5.0) There is no limit placed in determining the number of feeders, its maximum load and hence its corresponding circuit protection. This is decided by the individual’s perception as regards to flexibility, functionality and economy.

5.1 – Consideration in the Design of Feeders and

Protections Flexibility • While a single feeder may sufficiently supply several areas or floors or loads, so the scope of it affects in cases of For breakdown. • the general lighting and power system of a highrise building, some designer distribute the loads among several feeders, example: Feeder I - to serve Ground, 3rd, 5th floors Feeder II - to serve Basement, 2nd, 4th Feeder III - to serve 6th, 8th, 10th … and so forth; if the floor is sufficiently large, it may even be divided into zones and fed from different feeders. • The idea is to minimize areas affected by a single feeder breakdown; • The same principle could be adopted for air conditioning, elevators, pumps and other motor loads

Conveniently Conductors

Available

Sizes

of

Bus

ways

or

• While multiple bus ways or conductors can be used to meet desired current-carrying capacity of the line, there should be some limit to this multiplicity for practical purposes in handling and where building spaces are restricted. for example, three or more sets of large sized conductors can not be conveniently terminated in a single receiving or originating lugs or breaker terminal. Minimized Number of Replacement Breakers in Stock to Reduce Inventory Carrying Costs • A set of feeder breakers, say 10 units of 1,000 Amperes each, another set of 10 units of 500 amperes each and so forth will require lesser number of replacement units in stocks as compared against numerous feeders of substantially different sizes of breakers.

Allowable Voltage Drops in the Conductors • for branch circuits the allowable voltage drop not exceeding 3% at the farthest outlet of power, heating and lighting loads. While for both feeder and branch circuits to the farthest outlet the allowable voltage drop not exceeding 5% to provide reasonable efficiency of operation. Note: The sizes of feeder and branch circuit wires are based on the connected load, allowances for future growth, demand factor and diversity factors. The following can be used as reference this purpose: • for branch circuit wires for lighting, heating, and similar loads should have a capacity of not less than 125% of the load supplied with over current protection not exceeding the capacity of the conductors or 150% of the rating of the load; it is considered good engineering practice if the capacity of the conductor is not less than 150% of the load supplied. • for motors, owing to its starting current and occasional overload runs, the sizes shall be computed as follows.

Single motor load Size of conductor = 125% of motor full-load ampere (FLA)

Size of circuit breaker protection = M% times motor fullload ampere Where the multiplier M% value depends on the type or class of motor as well as the starter or controller to be used, Approximately it will be: • 250 to 300% for smaller motors, less than 7 ½ hp with full-voltage or across-the –line starters. • 150 to 200% for bigger motors with reduce voltage, wye-delta or autotransformer starters.

Example: A 150 hp, 3-phase 440-Volt squirrel cage induction moto with auto-transformer starter and full load ampere draw o 180 Amp. Size of conductor = 125% of 180 A = 225 A Use: 3 - 125 mm² or 250 MCM THW Cu wire @ 225 Amp capacity in 65 mm ø or 2 ½ inch ø rigid metal conduit (RMC)

Size of circuit breaker = 150% of 180 A = 270 A Use: 300 AT, 400 AF, 3P, 500V circuit breaker From the tables, it shows the approximate sizes of conductors and circuit protection for different sizes of electric motors. Note that the equivalent rating for safety switch is slightly higher than those of the circuit breaker. Group of motor and other loads Example : Given a group of motors with their corresponding full-load ampere (FLA) and other loads being supplied by several feeders referring to fig. 5.1, provide the appropriate size of main feeder conductor and rating of circuit breaker for protection. Assume 3phase, 460volt, 60 Hz supply.

Size of conductor = (125% full-load ampere of highest rated motor plus full load current of other loads) x demand factor= [125% of 180 Amp + 27+14+65+27+27+ (75,000/ 3 x 460)] x 80% (assume an 80% demand factor refer to P.E.C. for the provision this item) = 383 Amperes Use: 3 - 400 mm² or 800 MCM THW Cu wires @ 485 AMP capacity in 100 mm ø or 5 inch ø rigid metal conduit (RMC) Note: 325 mm² (700 MCM) wire with 425 Amp capacity may be sufficient, but the next bigger wire is chosen to allow for future growth. Voltage drops should also be computed to determine the propriety of the selected size of conductors; Where computed load exceeds the maximum available wire size, multiple or parallel runs can be used.

Size of circuit breaker protection = (highest motor breaker rating plus full-load ampere other loads) x demand = [300A + 27 + 14 + 65 + 27+ 27+ of (75,000/ 3 x 460)] x 80% factor = 443 Amperes Use: 500 AT, 600 AF, 3P, 500V ACB The size of feeder conductor should be maintained through-out. It should not be reduced, say at point “x” regardless of the reduced current beyond the said point-of-tap. Such reduction in conductor size may be allowed if appropriately sized breaker is installed to serve as protection for the reduced line. The exception are for runs not exceeding 25 feet and within sight, provided that the reduce line capacity is not less than 1/3 of the main run capacity.

6 – LOAD CENTER, PANEL BOARDS or SWITCHGEARS

Ideally, load centers and panel boards should be located on the center of the loads to be served to save on wire runs and to minimize line voltage drops. However most of the time the designing architect, for aesthetic purposes, has the final decision on the matter.

The electrical engineer should, however, strive to locate the panel boards at point where the farthest load to be served is within 30 meters. Otherwise, larger sized wires may be necessary to compensate for the voltage drop. Panel boards, Switchboards & Switchgears In general panel boards and switchgear are used as control protection points for groups of feeder or branch circuits serving the elctrical loads in building area, usually a floor or a section of the floor. A panel board consists of a metal enclosure containing bus bars to which circuit breakers or fused switches are attached. The interior space of the housing provides sufficient physical space for safe installation of the circuit conductors to their respective over current devices (see fig. 6.1). They are generally classified into two categories:

a) Lighting & appliance panels b) Power distribution panels Panel board mounting of motor starter units may also be involve

A switchboard & switchgear, on the other hand are free standing assemblies of switches, fuses and circuit breakers, which serve as locations for larger over current devices, or as main distribution panels for an entire building. Switchboards are physically larger than panel boards, due to the size of the over current devices involved, and are design to provide the necessary space for installation of larger cables (see fig. 6.2). There is no clear distinction made between the terms “switchboard” and “switchgear”, although often highvoltage equipment (above 600 Volts) is referred to as switchgear. When molded case circuit breakers are utilized in a switchboard it is often known as building type switchboard. Main metal-enclosed switchgear for commercial, industrial, and public buildings is invariably located in the basement, and housed in a separate well-ventilated electrical switchgear Other typesrooms. of metal-enclosed switchgear are: a) Metal-clad b) Compartmented (with one or more non-metallic partitions) c) Cubicle (with number of compartments less than that required for metal-clad or compartmented switchgear, usually having partition.

7 – SUGGESTED STEPS in BUILDING WIRING DESIGN Prepare an electrical load estimate based on areas of the building and other pertinent data; for office buildings, the P.E.C. has information on the estimated general illumination load, some other books can furnish data for other loads. An estimated load of 0.1 kilowatt per square meter of habitable area may be used to countercheck the estimated load. Consult the local company as regards the point of service entrance, service voltage, metering equipment and other requirements for power connections; the same should be done for the telephone system. Determine from other designers the exact electrical rating of all equipment, viz. HVAC, plumbing elevators and escalators, kitchen and others; the electrical designer may be asked for comparative characteristics of these equipment as regards the electrical supply.

Determine the location and estimated sizes of the different electric supply equipment such as load center, switchboards, electrical panel board, rooms or enclosures; this will enable the architect to allocate spaces for these equipment. This estimated space requirements could be checked and adjusted as may be necessary after the completion of the detailed plans. Design the lighting system, using either the lumens or point-by-point methods, after due consultation with the architect and lighting designer as to the type of luminaires, ceiling and wall finished. For clarity of the plans, the lighting design which is mostly a reflection of the ceiling is separate from those of power layout showing the floor plans. A separate sets of plans may be prepared for auxiliaries, viz. fire alarm, hold-up and burglar, paging and background music, noise masking and the like. Assign circuitry for all lighting and power system to appropriate panels including emergency lines, and compute panel loads.

Prepare riser or one line diagram to include main distribution panels, load centers, switchboards or switchgears and other service equipment. Compute feeder, sub-feeder sizes and all protective equipment ratings. Check and coordinate with other trades, architectural structural, VAC, mechanical, plumbing, others to minimize conflicts in the work execution. (the latter may belong to the project management team or the construction manager)

Table – 1: Typical Molded Case Circuit Breaker Frame Sizes, Trip Settings and Interrupting Rating

FRAME SIZE

TRIP SETTING

50 Amp

15 20 25 30 35 40 45 50

100 Amp

15 20 25 30 35 40 45 50 60 70 80 90 100

250 Amp

70 80 90 100 110 125 150 175 200 225

400 Amp and 600 Amp

125 150 175 200 225 250 300 350 400 450 500 600

800 Amp and 1200 Amp

250 300 350 400 450 500 600 700 800 1000 1200

1600 Amp

400 450 500 600 700 800 1000 1200 1600

3000 Amp

2000 2500 3000

4000 Amp

4000

5000 Amp

5000

6000 Amp

6000

Typical Interrupting Rating (r.m.s. symmetrical Amperes)

240 Volts

480 Volts

10 kA 18 kA 22 kA

14 kA

42 kA 65 kA 100 kA 200 kA

25 kA 30 kA 35 kA 50 kA 65 kA 150 kA

Standard Ampere Ratings for Low Voltage Fuses

0 – 600 Amp 15 20 25 30 35

40 45 50 60 70

80 90 100 110 125

150 175 200 225 250

300 350 400 500 600

0 – 600 Amp 601 650 700 800 1000

1200 1350 1500 1600 1800

2000 2500 3000 3500 4000

4500 5000 6000

References: • Distribution Switchgear (Construction, Performance, Selection & Installation) By : R. W. Blower • Electrical Wiring Commercial – based from NEC (6th ed) By : R.L. Smith & S.L. Herman • Design & Analysis of Building Electrical Systems By : J.H. Mathews • Philippine Electrical Code (part I & II) By : Institute of Integrated Electrical Engrs. Of the Phils. Inc.