Derating Calculator

Electrical wires have a maximum safe current capacity — but heat and crowded conduits can force that limit down, a process called derating. The Derating Calculator applies NEC correction factors to find your wire's actual safe current limit. Select your conductor size, conductor material, wire/cable type, ambient temperature, number of current-carrying conductors, installation type, and terminal temperature rating to get the derated ampacity, along with the base ampacity, temperature correction factor, conductor adjustment factor, and terminal limited ampacity.

°F

Temperature of surrounding environment

Total number of current-carrying conductors in conduit or cable

Temperature rating of connected equipment terminals

Results

Derated Ampacity

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Base Ampacity (90°C)

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Temperature Correction Factor

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Conductor Adjustment Factor

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Terminal Limited Ampacity

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Frequently Asked Questions

What is conductor derating and why is it necessary?

Conductor derating reduces the allowable ampacity of electrical conductors based on environmental conditions. It's necessary because high ambient temperatures and multiple conductors in close proximity generate additional heat, which can exceed the conductor's temperature rating and cause insulation failure.

How does ambient temperature affect conductor ampacity?

Higher ambient temperatures reduce conductor ampacity because the conductor has less thermal capacity to carry current safely. NEC provides temperature correction factors that decrease allowable ampacity as ambient temperature increases above the standard 30°C (86°F).

Why do multiple conductors in conduit require derating?

When multiple current-carrying conductors are bundled together in conduit or cable, they generate mutual heating effects. The NEC requires adjustment factors to reduce ampacity when more than three current-carrying conductors are installed together to prevent overheating.

What's the difference between 75°C and 90°C conductor ratings?

The temperature rating indicates the maximum operating temperature of the conductor insulation. 90°C conductors can carry more current than 75°C conductors, but the final ampacity is often limited by the terminal temperature rating of connected equipment.

How do I determine the number of current-carrying conductors?

Count all ungrounded conductors that normally carry current. This includes phase conductors and neutral conductors that carry unbalanced current. Equipment grounding conductors and neutral conductors that don't carry current under normal conditions are not counted.

What happens if my terminal equipment is rated for 75°C but I use 90°C wire?

You must use the ampacity based on the lower temperature rating for conductor sizing. However, you can apply derating factors to the 90°C ampacity and then compare to the 75°C terminal limitation - use whichever is lower.

Do I need to derate for both temperature and conductor bundling?

Yes, when both conditions exist, you must apply both derating factors. First apply the temperature correction factor, then apply the conductor adjustment factor. Both factors are multiplied together with the base ampacity.

What wire sizes are most commonly affected by derating?

All conductor sizes require derating when conditions warrant it, but smaller conductors (14-10 AWG) are more commonly affected in residential and light commercial applications where multiple circuits are run in common conduits or cable trays.