Wire Ampacity Calculator

How it’s calculated. Base ampacity is NEC Table 310.16 (not more than three current-carrying conductors, 30°C/86°F ambient). The ambient correction uses √((Tc − Ta) ÷ (Tc − 30)) (matching Table 310.15(B)(1)); bundling uses Table 310.15(C)(1) (4–6 = 80%, 7–9 = 70%, 10–20 = 50%, …).

Terminations are commonly listed for 75°C — don’t exceed the 75°C column at the lugs. Small conductors are also limited by 240.4(D) (14 AWG Cu = 15 A, 12 AWG Cu = 20 A, 10 AWG Cu = 30 A). This is a sizing aid, not a substitute for an engineered design.

Worked example. A 2 AWG THHN copper conductor starts at 130 A in the 90°C column of Table 310.16. Run it across a Tucson rooftop where the raceway sits at 50°C (122°F) and the ambient correction becomes √((90 − 50) ÷ (90 − 30)) = 0.82, so three conductors in the raceway are good for about 106 A, governed by the ambient correction — already below the 115 A that the 75°C terminations on the breaker and the equipment would otherwise cap it at. Put two three-phase circuits in the same conduit and there are six current-carrying conductors: 130 × 0.82 × 0.80 = 85 A. At a 30°C ambient with three conductors the same wire corrects to its full 130 A, but the 75°C termination limit of 115 A governs at the lugs. That is the whole reason to specify 90°C insulation: the extra headroom is what you correct and adjust from, not what you get to load. A neutral that carries only the unbalanced current of a three-phase, four-wire circuit is not counted (310.15(E)), and equipment grounding conductors never are (310.15(F)).

Picking the ambient. The 30°C base of Table 310.16 is a mild room, not a Tucson summer. Use the temperature the raceway actually sees: an unconditioned electrical room or mezzanine runs warmer, an attic or a sun-exposed wall warmer still, and a raceway lying on a roof deck in direct sun must add the 33°C (60°F) rooftop adder of 310.15(B)(2) to the outdoor ambient unless it is supported at least ⅞ in. above the roof. With Tucson’s summer design ambient already above 40°C, that adder pushes the conduit past 70°C, where a 75°C conductor has almost no ampacity left and even a 90°C conductor keeps barely half of its table value — which is why rooftop raceways here are lifted off the deck as a matter of course. The derating guide walks through the Arizona cases.

Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.