How Box Fill Is Counted
Each item gets a volume allowance from Table 314.16(B), based on wire size: 2.00 cu in for 14 AWG, 2.25 for 12, 2.50 for 10, 3.00 for 8 and 5.00 for 6 AWG. Then 314.16(B)(1) to (5) set how many allowances each item takes:
- Conductors: one allowance for each conductor that enters the box and ends or is spliced there, or passes through. Pigtails that never leave the box do not count.
- Internal clamps: one allowance for all clamps together, at the largest conductor size.
- Support fittings: one allowance each, at the largest conductor size.
- Devices: two allowances per yoke, at the largest conductor connected to that device.
- Grounding conductors: one allowance for all of them, at the largest size, plus a quarter allowance for each extra set of isolated grounds.
Up to four fixture wires smaller than 14 AWG, plus one grounding conductor, from a luminaire canopy are not counted. The total must not exceed the box volume, plus any marked volume of a plaster ring, extension ring or raised cover.
Worked Example
A receptacle box is fed by three 12/2 NM cables with grounds and has internal clamps. Count it:
- 6 insulated 12 AWG conductors: 6 × 2.25 = 13.50
- Clamps: 1 × 2.25 = 2.25
- One receptacle: 2 × 2.25 = 4.50
- Grounds: 1 × 2.25 = 2.25
Total = 22.50 cu in. A 4 x 1-1/2 square box holds 21.0 cu in, so it is too small. A 4 x 2-1/8 square box at 30.3 cu in works, or add a 4.5 cu in or larger marked plaster ring to the 1-1/2 inch box.
Limits
Box fill applies to boxes with conductors 6 AWG and smaller. Larger conductors fall under the pull box sizing rules in 314.28. Check the edition of the NEC your jurisdiction has adopted, and use the volume stamped on the box when it differs from the table.
Frequently Asked Questions
Do grounding conductors count in box fill?
Yes. Under 314.16(B)(5) all equipment grounding conductors together count as one conductor, based on the largest one. Each additional set of isolated equipment grounding conductors, used for isolated ground receptacles, adds a quarter allowance.
Why does a receptacle count as two conductors?
Section 314.16(B)(4) gives each yoke or strap holding a device a double allowance, based on the largest conductor connected to that device. A device wider than a single 2 inch gang counts double for each gang it needs.
Do wire nuts and pigtails count?
No. Wire connectors are not counted, and a conductor that starts and ends inside the box, such as a pigtail, is not counted either.
Sources
- NFPA 70, National Electrical Code, Section 314.16 and Tables 314.16(A) and (B).
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