Ampacity Chart — Cable Ampacity Table

Ampacity of a conductor is the current it can carry continuously without pushing its insulation past its temperature rating. The ampacity table above is organised the way the National Electric Code (NEC) organises its own: by conductor material, by insulation temperature column, and then adjusted by correction factors. Cable ampacity for a finished assembly follows the same conductor values, with the cable's own temperature rating as the ceiling.

Two adjustments apply before the number is usable. Correction factors for ambient temperature scale the value whenever the surroundings are hotter or cooler than 30 °C — roughly 0.91 at 40 °C and 0.71 at 50 °C on a 90 °C conductor. Adjustment for the number of current-carrying conductors applies from four upward: 0.80 for 4 to 6, 0.70 for 7 to 9, 0.50 for 10 to 20. Both multiply together, which is how a 30 A conductor legitimately lands near 17 A in a hot, crowded raceway.

Small conductors are limited by rule rather than by physics. 14 AWG copper is treated as 15 amps, 12 AWG as 20 amps and 10 AWG as 30 amps for overcurrent protection, even though the 90 °C column computes higher. Those three sizes cover most lighting circuits and receptacle branch circuits. Larger sizes read straight off the table: ampacity of #4 copper is about 80 A at 75 °C, and 750 MCM ampacity is around 524 A copper at the same rating.

Overcurrent protection is the other half of the pairing. The breaker protects the conductor, so its rating must not exceed the corrected ampacity, with the standard rounding-up allowance for smaller circuits. Undersized conductors behind an oversized breaker are the classic path to insulation breakdown and fire hazards, because the conductor cooks long before the device trips. Terminations cap it too: most equipment is rated 75 °C, so the 90 °C column is for derating math only.