Conduit Fill Calculator
Add up to three conductor groups, choose the conduit and see the fill percentage, PASS/FAIL result and the smallest compliant trade size.
Step-by-step
- Enter valid values to begin.
Conduit areas are from NEC Chapter 9 Table 4 and conductor areas from Table 5 (typed from the tables; verify against the NEC edition adopted in your area). Fill limits per Chapter 9 Table 1: 53% (1 conductor), 31% (2 conductors), 40% (3 or more).
Conduit Fill: How Many Wires Actually Fit
Core Engineering Principles
Conduit isn’t sized like a bucket. Round wires don’t pack tightly, so a conduit that’s 100% “full” by area would be unpullable. Heat is the second problem. Wires in a crowded raceway can’t shed heat, so the conductors run hotter than the table assumed. The NEC handles both with a simple area limit. Add up the cross-sectional area of every conductor, divide by the internal area of the conduit, and keep the answer under the fill limit.
The limit depends on the count. One wire can fill 53%. Two wires get only 31% because two round cables sit side by side and jam the pull. Three or more get 40%. Insulation matters too. A 12 AWG THHN is thinner than a 12 AWG XHHW, so it takes up less room. Count everything, including the equipment ground and the neutral.
Max count = floor(Aconduit × limit / Awire)
NEC & Standard References
The rules live in Chapter 9, Table 1 (the percentages), Table 4 (internal area by conduit type and trade size) and Table 5 (conductor areas). Note 1 points you to Annex C for same-size, same-type conductors, Note 3 says equipment grounding conductors count toward fill, Note 4 allows a nipple 24 in. or shorter to be filled to 60%, and Note 7 lets you round up when a count of identical conductors calculates to 0.8 or higher. EMT, PVC Schedule 40 and rigid have different internal diameters, so the same trade size holds different amounts of wire. PVC Schedule 80 is smaller still.
1. Table 5: 12 AWG THHN = 0.0133 in², so 3 × 0.0133 = 0.0399 in².
2. Table 4: 3/4 in EMT = 0.533 in²; 40% = 0.2132 in².
3. Fill = 0.0399 / 0.533 = 7.5%, well below the limit.
4. Maximum count = 0.2132 / 0.0133 = 16 conductors, but you wouldn’t run that many current-carrying conductors without derating.
- Fill is not derating. Sixteen wires might legally fit, but with more than three current-carrying conductors you also lose ampacity under 310.15(C)(1).
- Bends matter more than percent. No more than 360° of bends between pull points. A legal fill with four 90s will still be a bad day for whoever pulls it.
- Different insulation, different area. Use the actual insulation type on the job, not whatever was easiest to type.
- Plan for the next circuit. If you’re over 30% fill, go one trade size up. Conduit is cheap and rework isn’t.