Conduit fill and raceway sizing
CERT 3 · Commercial & Industrial Systems · ~11 min
Conductors in a raceway generate heat and need room to be pulled without damaging insulation, so the code limits how much of the conduit's cross-section they may occupy. The percentages come from NEC Chapter 9, Table 1 and are worth memorizing outright: one conductor may fill 53%, two conductors 31%, and three or more 40%.
The two-conductor case looks like a typo but isn't. Two round conductors side by side inside a round pipe leave awkward geometry and are the hardest combination to pull, so the allowance drops below the three-or-more figure. Exams ask this specifically because it looks wrong.
Working a fill problem: find each conductor's area from Chapter 9 Table 5 (it depends on insulation type — THHN is smaller than THW for the same AWG), total them, then compare against the allowed percentage of the conduit's internal area from Table 4. Note that equipment grounding conductors count toward fill even though they carry no normal current.
Exam focus: the 53/31/40 sequence, the fact that insulation type changes conductor area, and that grounds count. Fill questions are pure table navigation — speed comes from tabbing Chapter 9 before exam day.
Quiz — 3 questions
1. Maximum conduit fill for three or more conductors is:
2. The allowance for exactly two conductors is lower than for three or more because:
3. Equipment grounding conductors in a raceway: