When do I use 40% on conduit fill, and when not?
The 40 percent limit is for three or more conductors. One conductor gets 53 percent. Two conductors get 31 percent. Three or more get 40 percent. The trap is reaching for 40 percent on every problem. Count your conductors first, then pick the limit. The count sets the percent, not habit.
Last reviewed June 2026 · Answered by JourneymanIQ staff
Count the conductors first
Before you touch a table, count how many current-carrying and grounding conductors are in the raceway. That count is what picks your fill percentage from Chapter 9 Table 1.
Match the count to the limit
One conductor, 53 percent. Two conductors, 31 percent. Three or more, 40 percent. Most exam problems have three or more, which is why people autopilot to 40 percent and miss the one and two conductor cases.
Then run the five-step fill
Wire areas from Table 5, total them, apply the percent, and pick the raceway from Table 4 whose area at that percent equals or beats your total. Same steps every time.
Count first. One is 53, two is 31, three or more is 40. The count picks the limit.
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