Guide · NEC 2023
How to calculate conduit fill
Fill % = (total conductor area ÷ conduit internal area) × 100. Compare against Table 1.
Worked example 1 — same-size conductors
Three #12 THHN in 3/4″ EMT. Table 5: #12 THHN = 0.0133 in² each → 0.0399 in² total. Table 4: 3/4″ EMT = 0.533 in². Fill = 0.0399 ÷ 0.533 × 100 ≈ 7.5%. Three conductors → 40% limit. Pass.
Worked example 2 — mixed sizes + ground
Feeder: three #6 THHN + one #10 ground in 1″ EMT. #6 = 0.0507 × 3 = 0.1521. #10 ground (30A breaker per 250.122) = 0.0211. Total 0.1732 ÷ 0.864 ≈ 20.0%. Four conductors → 40% limit. Pass.
Jam ratio check
Ratio = conduit ID ÷ conductor OD. Between 2.8 and 3.2, three conductors can lock in a triangular jam. When flagged, go one trade size up — pulling labor drops far more than material cost rises.