Enter your Conductor Size, Material, Wire Type, Ambient Temperature, Conductor Count, Installation Type, and Terminal Rating into this Derating Calculator to find your Derated Ampacity, Base Ampacity, Temperature Correction Factor, and Fill Correction Factor.
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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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.
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).
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.
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.
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.
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.
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.
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.