Cable Ampacity & Conduit Fill Calculator

Ampacity is the current a conductor can carry continuously without exceeding its insulation temperature rating. The table value is where the calculation starts, not where it ends: the published figure assumes no more than three current-carrying conductors in a raceway at 30°C ambient, and both of those assumptions fail routinely in real installations. Conduit fill is the companion check — too many conductors in a raceway makes them impossible to pull without damage and traps the heat that ampacity derating exists to manage.

The formula

Worked example

Nine 12 AWG THHN conductors, each 0.0133 in², in 3/4 inch EMT with 0.533 in² internal area:

Fill = 100 × (9 × 0.0133) / 0.533 = 22.5%

Inside the 40% limit. But nine current-carrying conductors trigger a 70% ampacity adjustment factor, so each one now carries far less than its table value.

Which standard governs this

Ampacity from NEC Table 310.16. Adjustment for more than three current-carrying conductors from 310.15(C)(1), ambient correction from 310.15(B). Conduit fill limits from NEC Chapter 9, Table 1, with conductor and raceway dimensions in Tables 4 and 5.

What this calculation does not account for

The lookup gives the base table value before any correction. Real ampacity is the table value multiplied by both the ambient correction factor and the conductor-count adjustment factor, and either can be substantial. Terminal temperature ratings under 110.14(C) may limit the answer further regardless of the conductor's own rating.

Common mistakes

Using the 90°C column when the terminations are only rated 75°C — the termination governs. Counting the neutral as non-current-carrying on a circuit feeding non-linear load, where it carries harmonic current. Checking fill and forgetting the derating that the same conductor count triggers.

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