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NEC Article 314
Box Fill Calculator
Count everything in the box, not just the conductors, and see whether it fits.
Pick a box and add what is in it.
The grounding conductor count changed in the 2020 edition. Before that, every ground in the box counted as a single allowance no matter how many there were. Pick the edition your jurisdiction has adopted.
Each device yoke counts as two allowances, sized on the largest conductor landed on it. This is the item most often left out of a hand calculation.
How it adds up
| What | Allowances | Cubic inches |
|---|
A necessary note. Box volumes and per conductor volume allowances come from NEC
Article 314, cross checked against two independent public reproductions of the tables. This
covers the standard counting rules for conductors, clamps, support fittings, devices, and
grounding conductors. It does not cover conduit bodies, boxes over 100 cubic inches, or the
separate pull and junction box sizing rules that apply to 4 AWG and larger conductors. It is
a planning aid, not a substitute for the adopted code or your inspector.