Construction
Conduit Fill Calculator (NEC)
NEC max fill is 40% for 3+ conductors. Overfill = heat buildup, insulation damage, code violation.
Fill percentage
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- Max allowed
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- Used (sq in)
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- Conduit ID (sq in)
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FAQ
Why different max for 1 or 2 conductors? +
NEC allows 53% for 1 conductor (less friction pulling), 31% for 2 (mechanical wedging risk), 40% for 3+.
What about derating? +
Past 3 current-carrying conductors, ampacity derates per NEC 310.15(B)(3)(a). Plan ahead.