How to Calculate Box Fill
Box fill makes sure a junction or device box is big enough for everything inside it, so wires aren't crammed and overheating. NEC 314.16 turns each item into a volume allowance you total against the box's cubic inches.
Volume allowance per conductor
| Wire size | Allowance |
|---|---|
| 14 AWG | 2.00 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.50 in³ |
| 8 AWG | 3.00 in³ |
| 6 AWG | 5.00 in³ |
How to count everything (314.16(B))
- Each conductor entering and terminating (or passing through) = 1 allowance for its size.
- Each device (yoke/strap) = 2 allowances of the largest conductor connected to it.
- All internal cable clamps together = 1 allowance of the largest conductor.
- All equipment grounds together = 1 allowance of the largest ground.
Worked example
A device box with three 12 AWG conductors, one receptacle, internal clamps, and one 12 AWG ground: (3 × 2.25) + (2 × 2.25) + 2.25 + 2.25 = 15.75 in³. That fits an 18 in³ box.
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Frequently asked questions
How do you calculate box fill?
Add the Table 314.16(B) allowance for each conductor, plus two allowances per device yoke (largest conductor), one for all clamps, and one for all equipment grounds. Compare the total to the box's cubic-inch volume.
How many cubic inches does a 12 AWG wire need?
2.25 cubic inches per 12 AWG conductor. 14 AWG needs 2.00, 10 AWG needs 2.50.
Do ground wires count in box fill?
Yes — all equipment grounding conductors together count as one allowance of the largest ground present.
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