5.2 Boxes, Enclosures & Conduit Bodies (0402)

Boxes, Enclosures & Conduit Bodies (NEC Article 314)

Quick Answer: NEC Article 314 governs boxes, conduit bodies, and enclosures. The core inspector skill is box-fill per 314.16(B) using the Table 314.16(B) volume allowances — each conductor entering the box counts by its own size, each device yoke counts as two conductors of the largest conductor connected to it, all internal clamps together count as one of the largest, and all equipment grounding conductors together count as one of the largest. 314.28 is a separate calculation that applies to pull and junction boxes and conduit bodies where conductors 4 AWG or larger are installed.

Box-Fill Volume Allowances — 314.16(B) and Table 314.16(B)

The volume allowance per conductor size from Table 314.16(B) is:

What counts as a "conductor" for fill

• Each conductor that enters or passes through the box counts as one of its size.
• Internal cable clamps (314.16(B)(2)) — all the clamps in the box together count as one conductor of the largest conductor in the box; five clamps do not mean five allowances. Cable connectors with their clamping mechanism outside the box count as zero.
• Fixture stud or hickey (314.16(B)(3)) counts as one conductor of the largest.
• Device or equipment yoke (314.16(B)(4)) counts as two conductors of the largest conductor connected to that device. A duplex receptacle on a single strap is therefore a 2× allowance, and a device wider than a single 2-in. gang counts two volume allowances for each gang it occupies. This doubling is the single most commonly missed line in the whole calculation.
• Equipment grounding conductors (314.16(B)(5)) all together count as one conductor of the largest EGC present.
• Pigtails do not count; only conductors entering the box.

Worked Box-Fill Example

A single-gang device box contains 4 #12 THHN conductors, 2 #14 THHN conductors, internal cable clamps, 1 receptacle device (single yoke) connected to the #12 conductors, and two #12 equipment grounding conductors. The tally:

The chosen box must be marked with a volume at least equal to 22.00 in³. A 4 in × 4 in × 2⅛ in square box (30.3 in³ from Table 314.16(A)) works; a 4 × 4 × 1½ box (21.0 in³) does not, and neither does a 3 × 2 × 2½ device box (12.5 in³). Note how much the doubled yoke allowance moves the answer: forget it and you land at 19.75 in³ and wrongly approve the 21 in³ box.

Common Metal Box Volumes — Table 314.16(A)

For metallic boxes that do not have a stamped volume, Table 314.16(A) provides default volumes. A few of the most common sizes:

When the manufacturer marks a higher volume on the box, the marked value governs. The maximum number of conductors permitted in each Table 314.16(A) box is also tabulated (without clamps, without devices, without EGC); those rows must be derated when any of those items are present.

Multiple Devices and Mixed-Wire Yokes

When a box contains more than one device, each yoke is counted separately — and each is worth two conductor allowances. A two-gang box with a duplex receptacle and a switch therefore carries four allowances of the largest conductor connected to each device, not two. A device or utilization equipment wider than a single 2-in. gang counts two allowances for each gang it occupies. A split-circuit receptacle on one strap is still one yoke (two allowances), but the conductors of both circuits are counted individually. When a device is connected to conductors of two sizes, the yoke allowance uses the largest conductor connected to it — a device with a #12 line and #14 load counts as 2 × 2.25 in³.

Conduit Bodies — 314.28

314.28 applies to boxes and conduit bodies used as pull or junction boxes where the installed CONDUCTORS are 4 AWG or larger — the trigger is conductor size, not conduit trade size. It also applies to conduit bodies with provisions for more than two conduit entries. Get this backwards and every dimension you compute is answering the wrong question.

• 314.28(A)(1) Straight pulls: the length of the box or conduit body shall not be less than 8 times the trade size of the largest raceway.
• 314.28(A)(2) Angle pulls, U pulls, splices, or tapped conductors: the distance between each raceway entry inside the box and the opposite wall shall not be less than 6 times the trade size of the largest raceway in that row, increased by the sum of the trade sizes of the other raceways in the same row on the same wall. The distance between raceway entries enclosing the same conductor shall not be less than 6 times the trade size of the larger raceway.

Remember it as straight = 8×, turn or splice = 6×. The straight pull is the bigger multiplier because the conductors must be pushed through in a line without a bending radius to help them. Where the conductors are smaller than 4 AWG, a conduit body is filled per 314.16(C) using the Table 314.16(B) volume allowances instead.

Boxes in Walls and Floors

• 314.20 — Flush-mounted installations. In walls or ceilings of concrete, tile, or other noncombustible material, the box front edge may be set back from the finished surface by not more than 1/4 in. (6 mm). In walls or ceilings of wood or other combustible surface material, the box must extend to the finished surface or project from it — no setback is allowed, because a setback box leaves combustible material exposed to arcing at the device.
• 314.21 — Repairing noncombustible surfaces. Gaps or open spaces around a flush-mounted box in a noncombustible surface must be repaired so there is no gap greater than 1/8 in. (3 mm) at the edge of the box.
• 314.24 — Depth of boxes. Outlet and device boxes must have an internal depth adequate for the equipment installed; the section sets minimum depths keyed to the size of the largest conductor entering the box.
• 314.27(B) — Floor boxes. Boxes listed specifically for floor installation must be used for receptacles located in the floor.

Supports, Enclosures, Boxes Supporting Luminaires

• 314.23 — Supports. Every box must be rigidly and securely fastened in place by one of the methods listed in 314.23(A) through (H): to the building surface, by nails or screws (314.23(B)(1) requires nails or screws to be attached using brackets on the outside of the enclosure or to pass through the interior within 1/4 in. of the back or ends), by mounting to structural members, by threaded rigid or intermediate metal conduit supports, and so on.
• 314.27(A)(2) — Luminaire outlet boxes. An outlet box used as the sole support of a luminaire weighing more than 50 lb (23 kg) must be listed and marked with the maximum weight it will support; otherwise the luminaire must be supported independently of the box.
• 314.27(C) — Boxes for fan support. An outlet box may not be used as the sole support of a ceiling-suspended (paddle) fan unless it is listed and marked for fan support, and marked for the fan weight where the fan exceeds 35 lb (16 kg). Fans over 70 lb require support in addition to a listed box.

Covers, Flush, Nonmetallic vs Metallic

• Boxes shall have covers, faceplates, or fixture canopies. Covers shall be flush with the wall when the box is set flush (314.25).
• Nonmetallic boxes (314.16(C)) list their volume on the box; metallic boxes use Table 314.16(A) for default volumes or marked volumes.
• 314.4 requires metal boxes to be connected to an equipment grounding conductor; metal faceplates and covers likewise depend on that connection.

Inspector Focus

Verify: (1) total box fill does not exceed the marked volume, remembering the doubled yoke allowance; (2) pull and junction boxes carrying 4 AWG or larger conductors meet 314.28 — 8× the largest raceway on a straight pull, 6× on an angle or U pull or splice; (3) boxes are properly supported under 314.23; (4) luminaire boxes over 50 lb and fan boxes carry the listing and marking required by 314.27; (5) metal boxes are connected to an equipment grounding conductor and covers are in place.