By Zencalcy Editorial TeamLast reviewed About us
A box must have enough cubic inches for every conductor, device and clamp inside it. The NEC assigns volume by conductor size — 2.00 in³ for 14 AWG, 2.25 for 12, 2.50 for 10 — and counts each device yoke, each clamp and all grounds as additional allowances.
Electrical box fill against the NEC conductor count.
Required volume = Σ(conductor count × allowance) + devices + clamps + grounds
Allowances per conductor: 18 AWG 1.50 in³, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50. Each device yoke counts as two conductors of the largest size, each clamp as one, and all equipment grounds together as one.
| Symbol | Meaning |
|---|---|
| V | Box volume in cubic inches. |
| n | Number of conductors. |
Six conductors × 2.25 = 13.5 in³, the device = 4.5, the clamp = 2.25 and the ground = 2.25, totalling 22.5 in³ — exactly at the box's capacity.
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Yes, including pigtails, but a conductor that passes through without a splice counts once. Grounds count as one allowance total.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
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Content updated: September 2026