Wire Ampacity Calculator
Size conductors from NEC Table 310.16 with ambient-temperature correction and conductor-bundling adjustment — the two derates a Phoenix summer makes unavoidable.
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 Phoenix 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 Phoenix 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 Phoenix’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.
Need it engineered and stamped?
Sizing conductors for a commercial build? Talk to a commercial electrician in Phoenix — we design it, stamp it, and install it across Arizona.
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Read the field guide
Ampacity is where Arizona heat bites first. Start with the derating guide, then the motor branch-circuit and aluminum-feeder guides, which apply the same corrections to real circuits.
Why ambient correction is not optional in Phoenix
In a mild climate, ambient temperature correction and conduit-fill adjustment are arithmetic you can often skip. In metro Phoenix they are the difference between a conductor that is sized and one that is undersized. A conductor picked straight off Table 310.16 and run in conduit across a rooftop in July is undersized — that is not a judgement call, it is what the correction factors in 310.15 produce once you put a real Valley ambient into them.
This calculator applies both corrections and holds you to the 75°C termination column, which is where most commercial gear actually lands regardless of the conductor insulation rating. We built it for our own crews and estimators pricing commercial electrical and service and distribution work across Maricopa County, and left it open.
Rooftops, slabs and sun-exposed raceway
The cases that catch people are the ones where the raceway itself heats: conduit on a roof, raceway in a sun-exposed exterior wall, and runs through unconditioned attic or ceiling space in a building with a hot roof deck. Rooftop solar feeders, RTU circuits and site lighting home runs all live there. Size them for the ambient they will actually see in August, not the 30°C the table assumes.
Which code edition applies
Phoenix, Mesa, Chandler, Glendale and Buckeye are on the 2023 NEC; Scottsdale is on the 2020; unincorporated Maricopa County is still on the 2017. For Gilbert, Peoria and Tempe we confirm the adopted edition with the jurisdiction before design starts.
Sizing conductors for a permit set? The engineering page covers what a Valley reviewer expects in the load study and one-line, and the form at the bottom of that page gets a real set priced.
Frequently asked questions
Do I really need to derate for ambient temperature in Phoenix?
Yes. The NEC ambient correction factors are based on a 30 degree Celsius reference. A conduit run across a Phoenix rooftop or through a sun-exposed wall in summer sits far above that, and the correction reduces the allowable ampacity enough that a conductor sized from the raw table is undersized. This calculator applies the correction for you.
Why does the calculator use the 75 degree column?
Because that is where the terminations on most commercial breakers, lugs and gear are rated. A 90 degree conductor can be used for the derating math, but the final ampacity is limited by the 75 degree termination rating, so the calculator holds you to it.
Which conduit runs are most affected in the Valley?
Rooftop raceway, raceway in sun-exposed exterior walls, and runs through unconditioned attic or plenum space under a hot roof deck. Rooftop equipment circuits, solar feeders and site lighting home runs are the usual cases.
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Send us the plans, the scope, or just the problem. We will tell you what it takes. Call (602) 396-5216 or request a bid online.