Electrical Box Fill — Limited US NEC Worksheet
US NEC 2023 / 314.16 educational volume arithmetic; not BS 7671 and not a compliance verdict. Single-size insulated and grounding conductors only. Confirm scope before calculating.
US NEC 2023 / 314.16 educational volume arithmetic; not BS 7671 and not a compliance verdict. Single-size insulated and grounding conductors only. Confirm scope before calculating.
No current calculation
Enter and review the parameters, then calculate. The fixed example below is not your current result.
Method and boundaries
Volume allowances (cu in): 14 AWG=2, 12=2.25, 10=2.5. Larger sizes are unsupported here. Insulated conductor count must already reflect the applicable counting rules. Up to four entering grounding conductors use one allowance; each additional grounding conductor adds one-quarter allowance (2020/2023 rule). Each device yoke uses two allowances and the internal clamp group one. Support fittings require manual review and block calculation. Ordinary wire connectors do not each add a conductor allowance. Mixed sizes, isolated grounding sets, special devices, long unbroken loops, integral-clamp exceptions and other counting exceptions require manual review; do not use this worksheet for them. Enter the box marked volume, not external dimensions.
Fixed worked example — not a live result
Fixed example: 4 counted 12 AWG insulated conductors, 4 grounds, 1 device yoke and one internal-clamp group = (4+1+2+1)×2.25=18 cu in. Five grounds change the total to 18.5625 cu in. A positive volume balance is not a full code check.
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Minnesota DLI checklist, 2023 NEC, item 046: 14/12/10 AWG volumes and extra-ground rule
NFPA 70: verify adopted edition and omitted Article 314.16 requirements before use
Worksheet version 2026-10-15. Verify source applicability and all omitted requirements before any real-world decision.
Next calculation, separate decision
Review the bounded conductor-temperature example. Each worksheet has different assumptions and does not approve a design.