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BuyWire fill calculator
NEC Chapter 9 — check fill percentage for mixed conductors in any conduit
Enter conduit type and trade size, then add your wire mix — insulation type, gauge, and quantity. Get total fill percentage, pass/fail against NEC Table 1 limits, and ampacity derating reminders when you exceed three current-carrying conductors.
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What this calculator does
This calculator checks NEC Chapter 9, Table 1 fill limits for conduit runs. You enter the conduit type and trade size, then add your wire mix — type, gauge, and quantity. It sums conductor areas from Table 5 and compares the total to the allowed fill percentage for your conductor count.
NEC fill limits — Chapter 9, Table 11 conductor: 53% fill allowed. 2 conductors: 31% fill. 3 or more conductors: 40% fill. These limits apply to the internal cross-sectional area of the conduit.
What each input means
- Conduit type — EMT, PVC Schedule 40/80, RMC, and IMC each have different internal areas in Table 4.
- Trade size — Nominal conduit size; actual inside diameter comes from Table 4.
- Wire type — THHN/THWN-2, XHHW, and THW use different areas in Table 5.
- AWG / kcmil — Conductor size; area is looked up per insulation column.
- Quantity — Every conductor in the raceway counts, including equipment grounding conductors.
Frequently asked questions
Does the ground wire count toward conduit fill?
Yes. Equipment grounding conductors count toward conduit fill. Box fill has different rules — conduit fill does not.
Can I mix wire sizes in the same conduit?
Yes. Sum each conductor's Table 5 area and compare to the fill limit for the total conductor count.
What's the difference between wire fill and conduit fill?
Wire fill (this tool) checks a known conduit with a mixed wire list. Our conduit fill calculator finds minimum conduit size or max conductors for one wire size.
Do I need to derate if I'm at 40% fill?
Fill and ampacity derating are separate. More than three current-carrying conductors can trigger NEC 310.15(C)(1) derating even when fill is compliant.