Ampacity Calculator

Ampacity is a thermal limit, not an electrical one. Current flowing through a conductor dissipates power as heat, and the published ampacity is the current at which the conductor settles at its insulation temperature rating and stops getting hotter. Exceed it and the insulation degrades long before the copper itself is in any danger.

Every figure in NEC Table 310.16 is quoted at two specific reference conditions: an ambient of 30 °C, and no more than three current-carrying conductors sharing a raceway or cable. Both conditions concern how easily heat escapes. Raise the ambient and the conductor has less temperature difference to shed heat across; add conductors to the same pipe and each one is heating its neighbours. So the calculation is a lookup followed by two independent reductions:

• Correction for ambient, from Table 310.15(B)(1). Below 30 °C the factor is greater than one and the conductor may carry more than its table value.
• Adjustment for bundling, from Table 310.15(C)(1). Four to six conductors gives 0.80, seven to nine 0.70, and it falls to 0.35 above forty.

Where both conditions exist the two factors multiply rather than applying in sequence, and the result is rounded once at the end. Two further limits then sit on top of the answer without changing it: 110.14(C) caps the circuit at the ampacity column matching the lowest-rated termination, and 240.4(D) fixes the maximum overcurrent device for 14, 12 and 10 AWG whatever the arithmetic produced. Those two constrain the breaker, not the conductor's thermal capacity, which is why this calculator reports them separately rather than folding them into the headline figure.

Take a 10 AWG copper THHN conductor pulled through EMT across an unconditioned attic. Four current-carrying conductors share the conduit — two circuits, no shared neutral — and the attic reaches 40 °C on a summer afternoon. The breakers and lugs at both ends are listed for 75 °C, which is typical.

Step one, the base value. THHN is a 90 °C insulation, so the starting point is the 90 °C copper column of Table 310.16. For 10 AWG that is40 A.

Step two, ambient correction. 40 °C falls in the 36–40 °C band of Table 310.15(B)(1). Reading down to the 90 °C column gives a factor of0.91. So 40 × 0.91 = 36.4 A.

Step three, bundling adjustment. Four current-carrying conductors falls in the 4–6 band of Table 310.15(C)(1), a factor of 0.80. So 36.4 × 0.80 = 29.12, which rounds to 29 A.

Now the two ceilings. The 75 °C value for 10 AWG copper is 35 A, so 110.14(C) is not binding here — 29 A is already below it. The 240.4(D) cap for 10 AWG copper is 30 A, also above our figure. In this case the derating is what governs, and the conductor is good for 29 A.

The largest standard device at or below 29 A is 25 A. Note how easily this could have been missed: the same conductor in a 30 °C room with three conductors would have been a straightforward 30 A circuit. The attic and the fourth conductor between them cost roughly a quarter of the capacity.

Below is the whole of the base table this calculator draws on: every size from 14 AWG to 750 kcmil, in copper and aluminium, across all three insulation columns. Read across from the conductor size, pick the column matching the insulation printed on the jacket, and that is the ampacity before any correction — at 30 °C with three or fewer current-carrying conductors.

Use the table directly when your installation actually is at those reference conditions, which covers a great deal of ordinary interior work: a two-wire branch circuit in a conditioned space needs no factors at all, and the table value is the final answer. Reach for the calculator above as soon as ambient, bundling, or both come into play, because the factors compound and the intermediate figure is easy to round in the wrong place. The dashes in the aluminium row for 14 AWG are not an omission — Table 310.16 does not list aluminium below 12 AWG.