NEC 2023 · updates as you type · free, no signup
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Worked example — for nine #12 THHN conductors in 3/4″ EMT, the engine returns:
22.5% fill in 3/4" EMT — COMPLIANT (40% allowed)
How it works. Fill limits exist for two physical reasons: conductors in a crowded raceway run hotter, and pulling through an overfilled conduit damages insulation. The calculation compares the total cross-sectional area of your conductors against the usable area of the conduit at the allowed fill percentage, which depends on how many conductors are in the run. Conductor dimensions come from Chapter 9 Table 5, which covers round concentric-lay-stranded conductors; compact-stranded conductors are listed separately in Table 5A and are not modelled here.
What this covers: EMT/IMC/RMC/PVC-40/PVC-80, ½″–4″, THHN & XHHW only. Anything else fails loud rather than guessing.
Educational reference, not code. Based on NEC (NFPA 70) 2023 (Ch. 9 · T1/4/5). Your state or local AHJ may enforce a different edition or amend it — theirs is the one that counts. No NEC text is reproduced here.