Wire & Cable calculator
Temperature Correction Calculator
Use this page as an ambient-temperature and conductor-count derating screen. It applies the NEC 310.15 ambient factor, the 310.15(C)(1) adjustment for more than three current-carrying conductors, and then shows whether the selected terminal-temperature column still caps the final usable ampacity.
Updated July 20, 2026
A 12 AWG copper 90C conductor at 122F with 6 current-carrying conductors screens at about 19.7A because 30A x 0.82 x 0.80 leaves less ampacity than many installers expect.
Derated ampacity = starting ampacity x ambient factor x conductor-count factor, then compare that result against the governing terminal-temperature limit.
Enter conductor material, size, temperature rating, ambient temperature, and conductor count below to screen ambient derating before final conductor selection
Example Calculations
How to Use
What this temperature correction calculator does
This page uses either 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 uses a 30A starting ampacity, 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 uses a 95A starting ampacity, 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.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
Why is the final usable ampacity lower than the ambient-adjusted result on some runs?
Because the ambient and conductor-count factors are only part of the check. The final usable ampacity still cannot exceed the governing terminal-temperature column at the equipment.
Does this page decide whether the neutral counts as a current-carrying conductor?
No. The conductor count must already reflect the rules in 310.15(E) and (F). Some neutrals count and some do not, depending on the actual circuit and load characteristics.
Can I use a 90C conductor column for derating and still land on 75C equipment?
Yes, as a derating workflow. The 90C column can be the starting point for the adjustment factors, but the final usable ampacity still cannot exceed the governing 60C or 75C equipment termination limit.
Does this page include rooftop temperature adders automatically?
No. Enter the effective ambient temperature that belongs to your installation. If a rooftop-sunlight rule adds temperature above the outdoor ambient, apply that before using this screen.
When should I use this page instead of the full ampacity calculator?
Use this page when the main question is the derating itself. Use the full ampacity calculator when you also need the broader conductor-table workflow, material comparison, and continuous-load context.
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