Wire Ampacity Temperature Correction for High Ambient Heat

Why hot ambient lowers ampacity

A conductor's ampacity is the current at which it reaches its insulation temperature limit while shedding heat to the surrounding air. The table values bake in a 30°C ambient. Raise the ambient and the conductor starts closer to its ceiling, so less current takes it the rest of the way. Attics, rooftops in sun, boiler rooms, and runs near process heat all push ambient well above 30°C, and the correction factor is how the NEC keeps the conductor inside its insulation rating.

Use the range that contains the ambient

Table 310.15(B)(1) lists correction factors in 5-degree Celsius bands, keyed to the conductor's insulation column. You apply the factor for the band that contains the ambient temperature, not a value interpolated at the exact degree. The bands that matter most for 90°C conductors:

Like the conductor-count adjustment, correction starts from the conductor's own column. A 90°C conductor (THHN, THWN-2, XHHW-2) is corrected with the 90°C factor even though it will still terminate on 75°C lugs. That higher starting factor is the payoff for running 90°C-rated insulation in hot spaces.

Worked example: 3/0 copper on a 50°C rooftop

A 3/0 THHN copper feeder is rated 225 A in the 90°C column of Table 310.16. The rooftop run sees 50°C ambient, which lands in the 46-to-50°C band. The 90°C factor there is 0.82.

Use the actual design ambient for the location. Raceways exposed to sunlight on a roof, and runs through unconditioned attics, are commonly the hottest ambient on a job, and the design ambient there can sit well above the outdoor air temperature.

When correction pushes past the termination

Take #6 THHN copper, rated 75 A at 90°C, in a 43°C boiler room (the 41-to-45°C band, 0.87 at 90°C):

The heat math left room, but the 75°C lug did not. Whenever the corrected value climbs past the termination column, the termination is the limit. Small conductors add a third ceiling: a #10 copper corrected for a 40°C attic gives 40 × 0.91 = 36.4 A, but its 75°C column is 35 A and NEC 240.4(D) still caps its overcurrent device at 30 A.

Cold ambient and the base-table cap

Below 30°C the factors rise above 1.0, which would raise the corrected ampacity. On a run with no other derating that gain is usually erased by the termination limit, because 110.14(C) still caps the conductor at the 30°C base value of the termination column. Cold factors earn their keep mainly when combined with conductor-count adjustment, where the extra headroom can offset the count penalty.

Combine with conductor-count derating

Ambient correction and the more-than-three-conductors adjustment are independent multipliers applied to the same 90°C base. Eight #6 THHN copper conductors in that 45°C attic take both: 75 × 0.87 (ambient) × 0.70 (eight conductors) = 45.7 A, well under the 65 A termination limit. The conductor-count side is worked in full in NEC 310.16 derating for 8 current-carrying conductors.

Reference table (90°C copper, corrected)

All base ampacities are the 90°C copper column of Table 310.16; governing ampacity is the lower of the corrected value and the 75°C termination column.

NEC reference

NEC 2020. Correction factors from Table 310.15(B)(1) (formerly Table 310.15(B)(2)(a)). Base ampacities from Table 310.16, 90°C and 75°C columns. Temperature-column and termination rules from 110.14(C) and 310.14(A). Small-conductor overcurrent limits from 240.4(D).

Related

• Wire Ampacity Calculator: apply ambient, conductor count, and termination limits in one step.
• NEC 310.16 Derating for 8 Current-Carrying Conductors: the conductor-count side of derating, worked step by step.
• Wire Size Calculator: size a conductor for ampacity and voltage drop together.