When to use: Use this calculator whenever conductors are installed in environments above 30°C (86°F) or when multiple current-carrying conductors are bundled together. NEC 310.15 requires derating for ambient temperature (Table 310.15(B)(2)(a)) and for more than 3 current-carrying conductors in a raceway (Table 310.15(B)(3)(a)). Failure to derate can cause insulation failure, tripped breakers, and fire hazards.
This free wire ampacity and derating calculator finds the adjusted current-carrying capacity of copper and aluminum conductors using the NEC 310.16 ampacity tables, ambient temperature correction factors, and conductor bundling adjustment factors. Enter the conductor material, wire size, insulation temperature rating, ambient temperature, and number of current-carrying conductors, and it returns the base ampacity along with the derated ampacity after correction. It's built for electricians, electrical engineers, designers, and inspectors who need to size conductors that stay within their temperature ratings in real installation conditions.
Adjusted ampacity starts from the base allowable ampacity in NEC Table 310.16, read at the conductor's insulation temperature rating column (60°C, 75°C, or 90°C). Two derating factors are then applied:
Ambient temperature correction — Table 310.15(B)(1) adjusts for ambient temperatures other than the 30°C (86°F) basis of the table. Hotter ambients use a factor below 1.0, reducing capacity.
Conductor bundling adjustment — when more than 3 current-carrying conductors share a raceway or cable, NEC 310.15(C)(1) applies an adjustment factor: 4-6 conductors → 80%, 7-9 → 70%, 10-20 → 50%.
Adjusted ampacity = base ampacity × temperature factor × bundling factor.
Finally, the terminal temperature limitation of NEC 110.14(C) caps the usable ampacity at the lowest-rated termination — typically the 60°C column for equipment up to 100 A and the 75°C column above that, even when 90°C-rated wire is used.
Worked example: #6 THHN copper (90°C) has a base ampacity of 75 A. In a 40°C ambient (factor 0.91) bundled with 6 conductors (factor 0.80): 75 × 0.91 × 0.80 ≈ 54.6 A. But because the terminals are rated 75°C, the wire's ampacity for sizing and protection is also limited to the 75°C column value of 65 A — so the governing adjusted ampacity here is about 54.6 A, the lower of the two.
1. Select the conductor material (copper or aluminum) and the wire gauge. 2. Choose the insulation temperature rating (60°C for TW/UF, 75°C for THW/THWN/XHHW, or 90°C for THWN-2/XHHW-2/USE-2). 3. Pick the ambient temperature range where the conductors are installed. 4. Enter the number of current-carrying conductors in the raceway or cable. 5. Read the base ampacity, the applied temperature and bundling factors, and the resulting corrected (derated) ampacity, then verify it against your terminal temperature rating.
A conductor carries current safely only as long as it stays at or below its insulation temperature rating. High ambient temperatures and bundled conductors both trap heat and raise conductor temperature for a given current, so the allowable current must be reduced. Skipping derating lets conductors run hotter than rated, which degrades and eventually fails the insulation, nuisance-trips or fails to trip overcurrent devices, and in the worst case creates a fire hazard. Correct derating keeps conductors within their thermal limits over the life of the installation.
Ampacity is the maximum current in amperes a conductor can carry continuously, under its conditions of use, without exceeding its insulation temperature rating. The base allowable values come from NEC Table 310.16.
The ampacity tables are based on a 30°C (86°F) ambient. When the actual ambient is higher, the conductor has less margin to shed heat, so NEC Table 310.15(B)(1) applies a correction factor below 1.0 that lowers the allowable ampacity. Cooler ambients can slightly increase it.
You apply the bundling adjustment factor of NEC 310.15(C)(1) when more than 3 current-carrying conductors are in the same raceway or cable: 80% for 4-6 conductors, 70% for 7-9, and 50% for 10-20. Three or fewer current-carrying conductors need no bundling adjustment.
NEC 110.14(C) limits the usable ampacity to the temperature rating of the connected terminals. You may use the 90°C column for derating calculations, but the final ampacity for conductor sizing and overcurrent protection cannot exceed the 75°C (or 60°C) column value when the terminals are rated for those lower temperatures.
Take the base ampacity from Table 310.16 at the conductor temperature column, multiply by the ambient temperature correction factor and by the bundling adjustment factor, then limit the result to the terminal temperature rating per 110.14(C). The governing value is the lower of the derated result and the terminal-limited value.
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