Free conduit fill calculator · NEC Chapter 9 · No signup
Pick a conduit type and size, add your THHN/THWN conductors, and get the fill percentage versus the NEC 53% / 31% / 40% limit — with a clear pass or fail.
NEC Chapter 9, Table 1 sets the maximum percentage of a raceway's area that conductors may occupy: 53% for one conductor, 31% for two, and 40% for three or more. The limits exist so conductors can be pulled without damaging insulation and so bundled heat can dissipate.
The math: sum each conductor's cross-sectional area (Table 5 — this tool uses THHN/THWN values) and divide by the conduit's internal area (Table 4, which varies by type — EMT, IMC, RMC, PVC Sch 40/80). Compare to the limit for your conductor count. Every conductor counts, including neutrals and equipment grounds.
Getting this right at design avoids a failed inspection and a re-pull. Field PM keeps your panel schedules, submittals, and QA/QC records together in the job book so the rough-in matches the plan. For non-THHN insulations, substitute the conductor areas from NEC Chapter 9, Table 5.
Per NEC Chapter 9, Table 1: one conductor may fill up to 53% of the conduit's internal area, two conductors up to 31%, and three or more up to 40%. This calculator applies the right limit automatically based on how many conductors you enter.
Add the cross-sectional area of every conductor (NEC Chapter 9, Table 5) and divide by the conduit's internal area (Table 4). Compare the result to the 53% / 31% / 40% limit for your conductor count.
It uses THHN/THWN-2 conductor areas — by far the most common building wire. Other insulations (XHHW, RHW, USE) have different diameters; for those, substitute the area from NEC Chapter 9, Table 5.
Yes. Every conductor in the raceway counts toward fill, including equipment grounding conductors and neutrals. Add them as rows here.
The percentages come from heat dissipation and the practical ability to pull conductors without damaging insulation. More conductors means more bundled heat and friction, so the allowable fill drops from 53% (one) to 40% (three or more).
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