10.3 Boxes, Conduit Bodies & Fill

10.3 Boxes, Conduit Bodies & Fill

Quick Answer: Box fill lives in 314.16. Add volume allowances for conductors, clamps, devices/yokes, and EGCs per 314.16(B); the total must not exceed the box’s marked or Table 314.16(A) volume. Conduit bodies follow 314.16(C)—splice only when the body is durably marked with volume. For conductors 4 AWG and larger, minimum pull/junction box size often shifts to 314.28 dimension rules instead of small-conductor volume counting.

Raceways and Enclosures on Arkansas Prov includes boxes because every raceway ends in an enclosure. Nonmetallic PVC/ENT runs still land in metal or nonmetallic boxes that must meet Article 314. This section is the companion to Chapter 9 conduit fill: conduit fill asks “will it pull?” while box fill asks “will the splices and devices fit without overcrowding?”

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What Article 314 covers

314 addresses installation and construction requirements for:

• Outlet boxes, device boxes, and junction boxes.
• Pull boxes and conduit bodies (LBs, LLs, LRs, Ts, Cs).
• Handhole enclosures (related requirements).
• Support, depth, covers, and accessibility.

Prov favorites: volume fill math, whether an LB can contain a splice, and whether a box is deep enough for a device plus clamps.

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Box volume — 314.16 overview

Boxes must be of sufficient size so that conductors and devices are not jammed. Volume is expressed in cubic inches (or cubic centimeters). Use:

• The volume marked on the box by the manufacturer, or
• Table 314.16(A) metal box volumes when unmarked standard boxes are used, plus
• Volume of plaster rings, domed covers, and extension rings that are marked or listed with additional volume.

314.16(B) assigns volume allowances based on the largest conductor present for each counting category (with stated exceptions). Open Table 314.16(B) for the cubic-inch allowance per conductor size (for example, 12 AWG and 14 AWG have different allowances—confirm exact values in your 2023 Code).

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Counting rules — 314.16(B) detail

Conductor fill — 314.16(B)(1)

Each conductor that originates outside the box and terminates or is spliced inside counts as one volume allowance of its size. A conductor that passes through unbroken (no splice/termination) also counts as one. Exceptions exist for equipment grounding conductors (handled in (B)(5)) and for small pigtails that do not leave the box under the article’s conditions—read the Exceptions carefully.

Practical counting: A cable with two hots and a neutral entering a box contributes three conductor allowances (plus EGC rules below) if all three are present and spliced/terminated.

Clamp fill — 314.16(B)(2)

Internal cable clamps present in the box count as one volume allowance based on the largest conductor present in the box (not one per clamp). External clamps or connectors ordinarily do not consume internal volume the same way—follow the exact text.

Support fittings — 314.16(B)(3)

Fixture studs and hickeys count as one allowance of the largest conductor size.

Device or equipment fill — 314.16(B)(4)

Each yoke or strap containing a device (switch, receptacle) counts as two volume allowances of the largest conductor connected to that device. A single receptacle on one yoke = two allowances; a double switch on one yoke still follows the yoke rule as stated—one yoke, two allowances based on the largest connected conductor.

Equipment grounding fill — 314.16(B)(5)

All equipment grounding conductors and bonding jumpers entering the box count together as one volume allowance based on the largest EGC in the box. Additional isolated/insulated EGCs may require extra allowances under the article—watch for isolated-ground receptacle stems.

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Worked box-fill examples

Example A — Device box with NM cable

Given: A nonmetallic device box with internal clamps will contain two 14/2 NM cables with grounds and one duplex receptacle. Largest conductor is 14 AWG.

Count (conceptual):

• Conductors: four insulated circuit conductors (two blacks, two whites) if both cables’ hots and neutrals are in play → 4 allowances of 14 AWG (adjust if a conductor is not present).
• Clamps: 1 allowance of 14 AWG.
• Device yoke: 2 allowances of 14 AWG.
• EGCs: 1 allowance of 14 AWG for all grounds together.

Total allowances × Table 314.16(B) volume for 14 AWG = required minimum volume. Compare to the box marking. This pattern—cables + clamp + device + single EGC allowance—is the bread-and-butter Prov device-box item.

Example B — Junction box splice only

Given: Metal 4 in square box, no devices, no internal clamps, three 12/2 NM cables with grounds, all conductors spliced.

Count: Six insulated conductors (three cables × two insulated each) + one EGC allowance. No device, no clamp allowances if using locknut connectors outside the volume rules. Multiply by 12 AWG volume from the table; verify against box volume (standard 4SQ depths differ—deeper boxes or extension rings add volume).

Example C — Mixed sizes

Given: A box contains 10 AWG and 12 AWG conductors plus a device fed by 12 AWG.

Approach: Conductor allowances use each conductor’s own size. Device allowances use the largest conductor connected to the device. Clamp/EGC allowances use the largest relevant conductor per 314.16(B). Mixed-size boxes require careful line-by-line tallies—do not convert everything to one size blindly unless the rule says so.

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Conduit bodies — 314.16(C)

Conduit bodies (LB, T, C, etc.) must allow conductors to be installed without damage. Splices and taps are permitted in a conduit body only when the body is durably and legibly marked by the manufacturer with its volume; then fill follows 314.16 volume rules using that marked volume.

Exam trap: An unmarked LB used as a splice can is a Code violation even if “it fits.” Many conduit bodies are intended as pull fittings, not splice boxes. If the stem says the LB has no volume marking and contains wire nuts, the installation fails 314.16(C).

Conduit bodies must also be supported so they are not used solely as the sole support for a raceway in ways the article forbids, and covers must be accessible.

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Pull and junction boxes for large conductors — 314.28

When raceways contain conductors 4 AWG or larger, 314.28 sets minimum box dimensions based on raceway trade size and whether the pull is straight, angle, or U-pull:

• Straight pulls: minimum length typically eight times the trade diameter of the largest raceway.
• Angle or U-pulls: minimum distance often six times the largest raceway trade size, with additional rules when more than one raceway enters.

These are dimension rules, not cubic-inch 314.16 counts. On Prov, if the stem involves 4/0 feeders entering a pull box, reach for 314.28, not Table 314.16(B).

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Depth, covers, and nonmetallic boxes

Boxes must be installed so the front edge is flush with combustible finished surfaces as required (314.20 family). Covers are required; metal covers on nonmetallic systems still need grounding/bonding continuity where applicable. Nonmetallic boxes used with metal raceways need bonding means—PVC raceway into a plastic box does not create a metal grounding path; the wire EGC must land on devices/boxes as required.

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Common Prov traps

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Timed open-book tips

Tab 314.16, Table 314.16(A), Table 314.16(B), 314.16(C), and 314.28. Sequence: (1) Small conductors/devices → volume count. (2) Conduit body splice? → marked volume required. (3) 4 AWG+ pull box → dimension rules. Combine with Chapter 9 when the stem asks both conduit fill and box fill—solve each with its own article.