Conduit Fill Calculator — NEC Chapter 9
Free · works offline · no account · NEC 2023 tables
Mix any THHN/THWN-2 sizes and get the minimum trade size for EMT, IMC, rigid, or PVC Schedule 40 — using the real Chapter 9 Table 4 and Table 5 areas, not rules of thumb.
Open the free calculator →How the math works
Add up each conductor’s Table 5 cross-sectional area, then compare against the conduit’s Table 4 total area times the Chapter 9 Table 1 fill limit: 53% for one conductor, 31% for two, 40% for three or more. The first trade size whose allowed fill covers the total wins.
Worked example
16 conductors of 12 AWG THHN in ¾" EMT
- Wire area: 16 × 0.0133 = 0.2128 in²
- ¾" EMT at 40%: 0.533 × 0.40 = 0.2132 in²
- 16 fits — the 17th doesn’t.
FAQ
- Why 40% fill?
- Chapter 9 Table 1: with three or more conductors, 40% of the conduit’s internal area is the limit — it manages heat and pulling damage. One conductor gets 53%, two get 31%.
- Are Cu and Al THHN the same size?
- For fill, yes — fill is about insulation outside diameter, so Table 5 areas apply to both at the same gauge.
- Do equipment grounds count toward fill?
- Yes — every conductor in the raceway counts toward fill area, grounded, grounding, or otherwise.