NEC Conduit Fill
NEC Chapter 9 conduit fill calculation. Sum conductor areas (Table 5 for THHN/THWN/XHHW), compare to conduit interior area (Table 4 for EMT, IMC, RMC, PVC), apply allowable percentage fill from NEC Chapter 9 Table 1.
Defaults: 3-#10 THHN with 1-#10 ground in 1/2" EMT — typical 30A circuit. Allowable fill: 1 conductor 53%, 2 conductors 31%, 3+ conductors 40%.
- 1 conductor: 53%
- 2 conductors: 31%
- 3 or more conductors: 40%
Why fill limits exist
Conduit fill limits prevent:
- Pulling damage: tightly packed conductors can't be pulled around bends without insulation damage.
- Heat dissipation: tightly packed conductors can't shed heat — they de-rate.
- Future re-pulls: leaving room for adding circuits later.
The 40% fill rule for 3+ conductors balances heat and physics. For high-temperature applications or close-spaced conductors with simultaneous current carrying, additional ampacity de-rating per NEC 310.15(B)(3)(a) applies.
Areas — what's in NEC Tables 4 & 5
Table 4 lists the interior cross-section area of every standard conduit type and trade size. These vary by material:
- 1/2" EMT: 0.304 in² interior; 40% fill = 0.122 in²
- 1/2" RMC: 0.230 in² interior; 40% fill = 0.092 in²
- 1/2" PVC Sch 40: 0.286 in² interior; 40% fill = 0.114 in²
- 3/4" EMT: 0.533 in²; 40% = 0.213 in²
- 1" EMT: 0.864 in²; 40% = 0.346 in²
- 2" EMT: 3.408 in²; 40% = 1.363 in²
Table 5 lists conductor area (copper + insulation). THHN is most compact; XHHW is slightly larger; RHH is bulkier:
- 10 THHN: 0.0211 in²
- 10 XHHW: 0.0260 in²
- 10 RHH: 0.0353 in²
- 4/0 THHN: 0.3237 in²
- 4/0 XHHW: 0.3578 in²
Equipment grounding — count or don't?
NEC 250.122 requires an equipment grounding conductor (EGC) sized per Table 250.122. The EGC counts toward conduit fill — both for area and for ampacity de-rating. Common error: forgetting the green ground when sizing conduit, then having to upsize at install.
Compact conductors and ampacity de-rating
When 4+ current-carrying conductors share a conduit, ampacity is reduced:
- 4–6 conductors: 80% of Table 310.16 ampacity
- 7–9 conductors: 70%
- 10–20 conductors: 50%
- 21–30 conductors: 45%
- 31–40 conductors: 40%
- 41+ conductors: 35%
Don't crowd the conduit: a 1" conduit might be physically 100% fill-compliant, but if it requires de-rating to 70% ampacity for 7 conductors, the practical wire size needs to increase. Always check ampacity table compliance after fill compliance.
Reference: National Electrical Code (NFPA 70-2023), Chapter 9 Tables 1, 4, 5, 5A. Article 310.15(B). Stallcup's Electrical Design Book for tabulated conduit fill examples.