What ambient temperature counts?

Use the highest ambient the conductor is exposed to along its run, not the average. For conduit on a rooftop, the NEC adds a temperature adder for sun exposure, so the effective ambient can be far higher than the air temperature.

Why does a 90 °C conductor derate less than a 60 °C one?

Higher-rated insulation tolerates more heat before it degrades, so the same ambient leaves more headroom. That is why electricians often spec 90 °C wire even though terminations limit the usable ampacity to the 75 °C column — the 90 °C column is the basis for derating.

Can a factor reach zero?

Yes. A 60 °C conductor has no usable ampacity at 60 °C ambient and above, and a 75 °C conductor reaches zero near 75 °C. When that happens the tool tells you to choose a higher-rated insulation.

Do I apply this before or after bundling adjustment?

They are independent multipliers and the order does not matter. Multiply the base ampacity by both the temperature factor and the Table 310.15(C)(1) bundling factor to get the final corrected ampacity.

Which code year are the factors from?

These match NEC Table 310.15(B)(1) in the 2017 and 2020 editions, with a 30 °C basis. Older editions used the 310.15(B)(2)(a) table number but the same 30 °C-based factors for most ranges.

Temperature Correction Derating Calculator (NEC 310.15)

Why ambient heat shrinks a conductor’s rating

A conductor’s ampacity is the current it can carry without its insulation exceeding its rated temperature. The published value in NEC Table 310.16 assumes the surrounding air is at 30°C. When the ambient is hotter — a rooftop conduit baking in the sun, a boiler room, a packed equipment closet — there is less of a temperature gap for the conductor to dissipate its own heat into, so the same current would drive the insulation past its limit. The code handles this with a multiplier from Table 310.15(B)(1) that you apply to the base ampacity.

How it works

The corrected ampacity is simply:

corrected = base_ampacity × temperature_factor

The factor depends on both the ambient temperature band and the insulation rating. At the 30°C basis the factor is 1.0. Below 30°C it rises above 1.0 (cold air actually lets a conductor carry slightly more). Above 30°C it falls: a 75°C conductor at 46°C uses a factor of 0.82, so a 30 A base ampacity becomes about 24.6 A. The higher the insulation rating, the slower the factor falls, because the insulation can absorb more heat before failing.

Example and notes

Take a 10 AWG copper THWN conductor with a 35 A base ampacity in the 75°C column. In a 50°C ambient the factor is 0.75, giving 26.25 A of usable ampacity — a 28% reduction that can easily push a circuit over the line if you sized it for the table value. Remember to combine this with the bundling adjustment when more than three current-carrying conductors share a raceway, and to use the highest ambient the run actually sees. For rooftop conduit, NEC 310.15(B)(3)(c) adds a temperature adder above the measured air temperature; add that adder before entering the ambient here.