How box fill is calculated
NEC 314.16(B) turns everything in the box into a number of "allowances", and each allowance is the Table 314.16(B) volume for one conductor (2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG, on up to 6 AWG).
- Conductors (B)(1): one each for every conductor that comes into the box, at its own size. Pigtails count zero.
- Internal cable clamps (B)(2): one allowance for all of them, at the largest conductor.
- Support fittings (B)(3): one allowance for each type of fitting (fixture studs, hickeys), at the largest conductor. Two studs still count once.
- Devices (B)(4): two allowances per yoke, at the largest conductor connected to that device.
- Equipment grounds (B)(5): one allowance for up to four, plus a quarter for each one after that, at the largest EGC. Enter how many enter the box and the calculator adds the quarters.
required in³ = Σ (allowances × volume per allowance) ≤ box in³
Worked examples
- How many 14 AWG in a 4" square, 1½" deep box? Its Table 314.16(A) volume is 21.0 in³, enough for 10 14 AWG or 9 12 AWG with nothing else in the box. Clamps, a device and the grounds all come out of that count.
- A receptacle on 12/2 NM, in and out, in a 3 × 2 × 3½" device box (18.0 in³) with internal clamps. Four insulated 12 AWG are 9.00 in³, the clamps 2.25 in³, the receptacle 4.50 in³ and both grounds together 2.25 in³. Total 18.00 in³, so the box is large enough, and that's the deepest standard 3 × 2 there is.
Step by step, with the smallest box for a switch loop: How to calculate box fill.
Common mistakes
- Counting each ground separately, or leaving them out. Up to four count once; it's the fifth and later that add a quarter each.
- Counting the device as one allowance.
- Looking up a plastic box in the metal-box table.
Worked examples on this page are computed by the same engine as the Voltkit app. The page cites NEC 2023 sections but doesn't reproduce NFPA text or tables; use your copy of the code for the exact wording.