Temperature Correction Calculator

What this temperature correction calculator does

This page starts from a table ampacity or a user-entered starting ampacity, then applies two separate derating steps:

Final usable ampacity = starting ampacity x ambient-temperature factor x conductor-count factor, capped by the terminal-temperature limit

Step 1: Ambient-temperature correction

Table ampacities are tied to a standard ambient condition. When the actual ambient temperature is higher, the conductor loses thermal headroom and the ampacity must be reduced. The factor depends on the conductor temperature rating you select.

Step 2: More than three current-carrying conductors

If more than three current-carrying conductors share the same raceway or cable, the conductors heat one another and the 310.15(C)(1) adjustment factor applies. The factor is separate from the ambient-temperature factor and both are multiplied together.

Step 3: Terminal limit

A 90C conductor column can be used as a derating starting point, but the final usable ampacity still cannot exceed the governing equipment termination column. That is why the calculator shows the adjusted ampacity and the terminal limit separately.

Worked examples

Example 1: A 12 AWG copper 90C conductor at 122F with 6 current-carrying conductors starts from 30A, uses a 0.82 ambient factor and a 0.80 conductor-count factor, and screens at about 19.7A before any project-specific decisions about upsizing.

Example 2: A 4 AWG copper 90C conductor at 45C with 4 current-carrying conductors starts from 95A, uses a 0.87 ambient factor and a 0.80 conductor-count factor, and screens at about 66.1A.

What this page does not decide

This page does not automatically add rooftop-sunlight temperature adders, does not decide whether a neutral counts as current-carrying, does not cover free-air or cable-tray ampacity, and does not replace a full conductor-sizing workflow. Use the Ampacity Calculator and the Wire Size Calculator when the project also needs table-column selection, material comparison, and final conductor choice.