Nec Boxes Sizing Con Less Than or Equal 6AWG (10mm2)

Article 314 Introduction This Article contains installation requirements for outlet boxes, pull and junction boxes, conduit bodies, and handhole enclosures.

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314.16 Number of 6 AWG and Smaller Conductors in Boxes and Conduit Bodies

Boxes containing 6 AWG and smaller conductors must be sized to provide sufficient free space for all conductors, devices, and fittings. In no case can the volume of the box, as calculated in 314.16(A), be less than the volume requirement as calculated in 314.16(B). Conduit bodies must be sized in accordance with 314.16(C). Author’s Comment: The requirements for sizing boxes and conduit bodies containing conductors 4 AWG and larger are contained in 314.28.

Figure 314–5

(A) Box Volume Calculations. The volume of a box includes the total volume of its assembled parts, including plaster rings, extension rings, and domed covers that are either marked with their volume in cubic inches (cu in.) or are made from boxes listed in Table 314.16(A). Figure 314–5 (B) Box Fill Calculations. The calculated conductor volume determined by (1) through (5) and Table 314.16(B) are added together to determine the total volume of the conductors, devices, and fittings. Raceway and cable fittings, including locknuts and bushings, are not counted for box fill calculations. Figure 314–6 (1) Conductor Fill. Each conductor that runs through a box and does not have 6 in. of free conductor for splices or terminations in accordance 300.14, and each conductor that terminates in a box is counted as a single conductor volume in accordance with Table 310.16(B). Conductors that originate and terminate within the box, such as pigtails, aren’t counted at all. Figure 314–7

Figure 314–6

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Part 3: Rules 51–75 (2) Cable Clamp Fill. One or more internal cable clamps count as a single conductor volume in accordance with Table 310.16(B), based on the largest conductor that enters the box. Cable connectors that have their clamping mechanism outside the box aren’t counted. (3) Support Fitting Fill. Each luminaire stud or luminaire hickey counts as a single conductor volume in accordance with Table 310.16(B), based on the largest conductor that enters the box. Figure 314–9

Figure 314–7 Author’s Comments: • A  ccording to 300.14, at least 6 in. of free conductor, measured from the point in the box where the conductors enter the enclosure, must be left at each outlet, junction, and switch point for splices or terminations of luminaires or devices. • Conductor loops occupy space and a box can be excessively filled if we do not take into consideration the increased conductor volume. This can create a serious fire hazard, especially when an electronic device is installed in an outlet box without adequate room for heat dissipation. Exception: Equipment grounding (bonding) conductors, and not more than four 16 AWG and smaller fixture wires, can be omitted from box fill calculations if they enter the box from a domed luminaire or similar canopy, such as a ceiling paddle fan canopy. Figure 314–8

Figure 314–9

(4) Device Yoke Fill. Each device yoke (regardless of the ampere rating of the device) counts as two conductor volumes in accordance with Table 310.16(B), based on the largest conductor that terminates on the device. Figure 314–10

Figure 314–10

Figure 314–8

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Table 314.16(B)

Conductor AWG 18 16 14 12 10 8 6

Volume cu in. 1.50 1.75 2.00 2.25 2.50 3.00 5.00

(5) Equipment Grounding (Bonding) Conductor Fill. All equipment grounding (bonding) conductors in a box count as a single conductor volume in accordance with Table 310.16(B), based on the largest equipment grounding (bonding) conductor that enters the box. Equipment grounding (bonding) conductors for isolated ground circuits count as a single conductor volume in accordance with Table 310.16(B). Figure 314–11

Figure 314–12 Answer: (b) 5 Step 1: Volume of the box assembly [314.16(A)]. Box 30.3 cu in. + 3.6 cu in. plaster ring = 33.9 cu in. Author’s Comment: A 4 x 4 x 2 1⁄4 in. box would have a gross volume of 34 cu in., but the interior volume is 30.3 cu in., as listed in Table 314.16(A). Step 2: Determine the volume of the devices and conductors in the box. Two—receptacles 4—12 AWG Five—12 THHN 5—12 AWG Two—12 AWG Grounds 1—12 AWG Total 10—12 AWG x 2.25 cu in. = 22.5 cu in. Step 3: Determine the remaining volume permitted for the 14 AWG conductors. 33.9 cu in. – 22.5 cu in. = 11.4 cu in. Step 4: Determine the number of 14 AWG conductors permitted in the remaining volume.

Figure 314–11

11.4 cu in./2.0 cu in. = 5 conductors (C) Conduit Bodies.

Author’s Comment: The conductor insulation is not a factor for box volume calculations. Question: How many 14 THHN conductors can be pulled through a 4 in. square x 2 1⁄4 in. deep box with a plaster ring with a marking of 3.6 cu in.? The box contains two receptacles, five 12 AWG conductors, and two 12 THHN equipment grounding (bonding) conductors. Figure 314–12 (a) 3

(b) 5

(c) 7

(d) 9

(2) Splices. Splices are only permitted in conduit bodies that are legibly marked, by the manufacturer, with their volume. The maximum number of conductors permitted in a conduit body is limited in accordance with 314.16(B). Question: How many 12 AWG conductors can be spliced in a 15 cu in. conduit body? Figure 314–13 (a) 4

(b) 6

(c) 8

(d) 10

Answer: (b) 6 conductors (15 cu in./2.25 cu in.)

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Figure 314–13

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314.23 Support of Boxes and Conduit Bodies

Figure 314–21

Boxes must be securely supported by one of the following methods: (A) Surface. Boxes can be fastened to any surface that provides adequate support. (B) Structural Mounting. Boxes can be supported from a structural member of a building or from grade by a metal, plastic, or wood brace. (1) Nails and Screws. Nails or screws can be used to fasten boxes, provided the exposed threads of screws are protected to prevent abrasion of conductor insulation. (2) Braces. Metal braces no less than 0.020 in. thick and wood braces not less than a nominal 1 x 2 in. can support a box. (C) Finished Surface Support. Boxes can be secured to a finished surface (drywall or plaster walls or ceilings) by clamps, anchors, or fittings identified for the purpose. Figure 314–21 (D) Suspended-Ceiling Support. Outlet boxes can be supported to the structural or supporting elements of a suspended ceiling, if securely fastened by one of the following methods: (1) Ceiling-Framing Members. An outlet box can be secured to suspended-ceiling framing members by bolts, screws, rivets, clips, or other means identified for the suspended-ceiling framing member(s). Figure 314–22 Author’s Comment: Luminaires can be supported to ceiling framing members as well [410.16(C)].

Figure 314–22 (2) Independent Support Wires. Outlet boxes can be secured, with fittings identified for the purpose, to independent support wires that are taut and secured at both ends [300.11(A)]. Figure 314–23 Author’s Comment: See 300.11(A) on the use of independent support wires to support raceways and cables. (E) Raceway Support—Boxes and Conduit Bodies Without Devices or Luminaires. Two intermediate metal or rigid metal conduits threaded wrenchtight can be used to support an outlet box that does not contain a device or luminaire, if each raceway is supported within 36 in. of the box, or within 18 in. if all conduit entries are on the same side. Figure 314–24

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Figure 314–25 Figure 314–23 (H) Pendant Boxes. (1) Flexible Cord. Boxes can be supported from a cord that is connected to fittings that prevent tension from being transmitted to joints or terminals [400.10]. Figure 314–26

Figure 314–24 (F) Raceway Support—Boxes and Conduit Bodies with Devices or Luminaires. Two intermediate metal or rigid metal conduits threaded wrenchtight can be used to support an outlet box containing devices or luminaires, if each raceway is supported within 18 in. of the box. Figure 314–25

Figure 314–26

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