How the calculation works
Conductor areas come from NEC Chapter 9, Table 5 (THHN/THWN/XHHW); conduit areas from Table 4. Maximum fill per Table 1: 53% for one conductor, 31% for two, 40% for three or more. Fill % = total conductor area ÷ conduit internal area. Bare equipment grounding conductors count too – add a row for them. Conduits over 24 in. and nipple exceptions are not covered.
Worked example
Four 10 AWG THHN in 3/4" EMT: 4 × 0.0211 = 0.0844 in² ÷ 0.533 in² = 15.8% (limit 40%) → pass.
Common mistakes to avoid
- Leaving out the equipment grounding conductor, which counts toward fill.
- Using THHN areas for conductors with different insulation, such as XHHW.
- Applying 40% to a short nipple, which has its own rule.
Frequently asked questions
Why 40%?
Ensures heat dissipation and easier wire pulling. 3+ conductors are limited to 40% of the conduit cross-section.
What about CEC?
CEC Table 6 and Rule 12-1014 use 40% for 3+ conductors, but conductor dimensions differ slightly.
Do ground wires count in conduit fill?
Yes. Chapter 9 notes require equipment grounding and bonding conductors to be included in the fill calculation.
What is the nipple rule?
NEC Chapter 9, Table 1, Note 4 permits up to 60% fill for conduit nipples not longer than 24 in.
Read the guide
Conduit Fill Explained: NEC Rules and Examples
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