How it works
Crowded boxes overheat and are hard to work in, so NEC 314.16 sets a minimum volume. Each item that takes up room is given a volume allowance based on the size of a conductor, from Table 314.16(B): 2.00 in³ for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG, and so on.
Each conductor counts once. Each device (a switch or receptacle yoke) counts twice, based on the largest conductor connected to it. All internal cable clamps together count once, all support fittings count once each, and all equipment grounds together count once. Multiply the total by the allowance for the conductor size and compare it to the volume on the box.
Worked example
A 3 × 2 × 3-1/2 inch device box with four 12 AWG conductors, one receptacle, two ground wires, and no clamps (4 + 2 + 1 = 7 allowances):
- Allowances = 4 conductors + 1 devices × 2 + 0 clamp set + 1 ground set + 0 fittings = 7
- Required volume = 7 × 2.25 in³ = 15.75 in³
- Box volume 18 in³: 2.25 in³ to spare
| Input | Value |
|---|---|
| Conductor size | 12 AWG |
| Conductors that start outside the box and end in it (count each) | 4 |
| Devices (each switch or receptacle yoke) | 1 |
| Internal cable clamps | None |
| Equipment grounding conductors | 2 |
| Support fittings (fixture studs, hickeys) | 0 |
| Box volume | 3 × 2 × 3-1/2 in device box, 18.0 in³ |
| Marked box volume (if "Other") | 20 in³ |
| Result | Value |
|---|---|
| Required volume | 15.75 in³ |
| Box volume | 18 in³ |
| Volume left over (negative means too small) | 2.25 in³ |
Assumptions and limits
- Uses one conductor size for everything. If the box has mixed sizes, add each size's allowances separately, or run the tool with the largest size for a conservative answer.
- Conductors that pass through the box unbroken count once; conductors that originate outside the box and end in it count once each. Short jumpers wholly inside the box (pigtails) do not count.
- More than four equipment grounds add a fraction of an allowance each under the 2020 and later NEC. This tool uses one-quarter allowance per extra ground. Confirm the exact fraction in the code book edition your jurisdiction adopts.
- Presets list a few common box volumes. For any other box, use the volume marked on the box.
Common questions
How many 12 AWG wires fit in a standard single-gang box?
In a 3 × 2 × 3-1/2 inch box (18.0 in³), a receptacle takes two allowances and a ground takes one, leaving room for about five more 12 AWG conductors, or four if there are cable clamps.
Does the ground wire count?
Yes. All equipment grounding conductors together count as one allowance at the size of the largest one.
How is a device counted?
A switch or receptacle counts as two conductor allowances at the size of the largest conductor connected to it.
What if I mix 12 and 14 AWG wire?
Count each conductor at its own size, and use the largest size connected for the device and grounds. The result is the sum of all allowances.
Sources
- NEC (NFPA 70) 314.16(B) and Table 314.16(B), volume allowance per conductor: 18 AWG 1.5, 16 AWG 1.75, 14 AWG 2.0, 12 AWG 2.25, 10 AWG 2.5, 8 AWG 3.0, 6 AWG 5.0 in³.
- Preset box volumes (3 × 2 × 2-3/4 = 14.0, 3 × 2 × 3-1/2 = 18.0, 4 × 1-1/4 round = 12.5, 4 × 1-1/2 square = 21.0 in³) are from NEC Table 314.16(A) as quoted in trade-magazine explanations.
Updated 2026-09-30