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Apply ambient-temperature correction (310.15(B)) and the more-than-three-conductor adjustment (310.15(C)(1)) to a base ampacity, then cap it at the termination temperature (110.14(C)). All three candidate values are shown so you can see which one binds.
Apply ambient-temperature correction (310.15(B)) and the more-than-three-conductor adjustment (310.15(C)(1)) to a base ampacity, then cap it at the termination temperature (110.14(C)). All three candidate values are shown so you can see which one binds.
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For a continuous 50-amp load on copper THHN at 75°C terminations, NEC Table 310.16 calls for #6 AWG (65A allowable ampacity after 125% sizing). Use #4 AWG aluminum for the same load. Always derate for ambient temperature above 30°C and for more than three current-carrying conductors in a raceway.
Conductors can be rated 90°C, but terminations on most devices are listed for 75°C — so you size from the 75°C column unless the equipment is specifically listed for 90°C terminations. Use 90°C ampacity only as the starting point for derating calculations.
NEC Table 310.15(B)(1) gives an ambient temperature correction factor — multiply the 90°C ampacity by the factor for your installation temperature, then verify the corrected value still exceeds 125% of the continuous load.