Conduit Fill & Wire Capacity Calculator (NEC)
Calculate electrical conduit fill percentage (40% maximum rule), wire capacity, and ampacity derating factors for EMT, PVC, and RMC with THHN wires.
Calculated Results
Frequently Asked Questions
What are the NEC conduit fill percentage limits?
Per NEC Chapter 9, Table 1: 1 conductor: maximum 53% fill; 2 conductors: maximum 31% fill (to prevent jamming during pulling); 3 or more conductors: maximum 40% fill.
Why does the NEC restrict conduit fill to 40%?
Restricting fill to 40% prevents severe mechanical damage to wire insulation during pulling through elbows and ensures adequate air circulation space to dissipate heat generated by electrical current.
What is wire ampacity derating for conduit?
When more than 3 current-carrying conductors share the same conduit or raceway, heat accumulates. The NEC requires derating allowable ampacity: 4 to 6 wires: 80% capacity; 7 to 9 wires: 70% capacity; 10 to 20 wires: 50% capacity.
Why does Schedule 80 PVC hold fewer wires than EMT?
Schedule 80 PVC has much thicker plastic walls to withstand physical impact damage. This reduces the interior cross-sectional area compared to thin-wall EMT or Schedule 40 PVC.