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Free tool · NEC Chapter 9
Conduit Fill Calculator
Enter your THHN conductors and pick an EMT size. The calculator sums the conductor areas from Table 5 and checks them against the Table 4 fill area — the exact Chapter 9 method the exam tests.
1 · Conductors (THHN)
How many of each size are in the pull. Areas from Chapter 9, Table 5.
2 · The raceway
40% fill area shown after calc
How the Math Works
Three tables, in order — T1 = percent, T4 = conduit, T5 = conductor:
- Table 1 sets the fill percent: 53% for one conductor, 31% for two, 40% for three or more. A nipple 24 in. or shorter gets 60% (Note 4).
- Table 5 gives each THHN conductor's cross-sectional area in square inches. Add them all up.
- Table 4 gives the allowable fill area for each EMT trade size. If your conductor total fits under it, the pull is legal.
Equipment grounding conductors count toward fill like any other conductor. And remember: fill is about physical space — ampacity derating for 4+ current-carrying conductors (310.15) is a separate check.
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