TradesHow this works
A conductor's table ampacity assumes a 30 C ambient and no more than three current-carrying conductors together. Heat and bundling both cut into that. This tool multiplies the base ampacity by the ambient correction factor and the conductor adjustment factor, then holds the result to the termination temperature limit, and tells you whether the wire still carries the load or needs to go up a size.
This is the step most single-number calculators skip. Derating from the 90 C column is the professional move that softens the penalty, but the terminations still cap what you can use.
Common questions
Why derate from the 90 C column but cap at 75 C?
You are allowed to apply the correction and adjustment factors to the higher 90 C ampacity, which softens the penalty. But the final ampacity you actually use can never exceed the temperature rating of the terminations, which is usually 75 C or 60 C under NEC 110.14(C).
Which conductors count toward the adjustment?
Only current-carrying conductors. A neutral that carries only the unbalanced current of the other conductors does not count, and equipment grounding conductors never count. Count the hots, and the neutral only where it carries current.
When do I have to derate for temperature?
Whenever the ambient is above 30 C, such as a hot attic, a rooftop in sun, or near heat-producing equipment. Rooftop runs in sunlight can add a large adder, so measure the real ambient the raceway will see.
Sources: NEC Table 310.15(B)(1)(1) ambient correction and Table 310.15(C)(1) adjustment (2023) · NEC 110.14(C) termination temperature limits, Table 310.16 ampacity (2023). Engine version 1.0.0. Risk tier 2.