Conduit fill rules keep conductors from overheating and make pulling wire practical. The limits come from NEC Chapter 9, Table 1, and the areas from Tables 4 and 5.
The fill limits
- One conductor: 53% of the conduit's internal area.
- Two conductors: 31%.
- Three or more conductors: 40%.
Short conduit nipples of 24 in. or less may be filled to 60% under Table 1, Note 4.
How to calculate
- Find the area of each conductor in Chapter 9, Table 5 (THHN/THWN is 0.0133 in² for 12 AWG, 0.0211 in² for 10 AWG).
- Add them all up, including the equipment grounding conductor.
- Divide by the conduit's total internal area in Table 4 and compare with the limit.
Example 1: passes
Nine 12 AWG THHN conductors in 1/2 in. EMT: 9 × 0.0133 = 0.1197 in². The 1/2 in. EMT has 0.304 in² total, so fill is 39.4%, just under the 40% limit.
Example 2: fails
Ten 12 AWG THHN in the same conduit: 0.133 in² ÷ 0.304 in² = 43.8%, which exceeds 40%. Move up to 3/4 in. EMT (0.533 in²), where the same wires fill 25%.
Practical tips
- Mixed insulation types need their own areas from Table 5.
- Fill passing is not the same as an easy pull; long runs with several bends need more room.
- Ampacity derating is separate and still applies when there are more than three current-carrying conductors.
Try the calculators
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