Calculate the minimum wire size per NEC Table 310.16 with temperature correction, conduit fill derating, and termination rating verification. Shows which constraint controls your wire size.
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Wire sizing isn’t a single table lookup. The NEC requires you to check base ampacity (Table 310.16), apply temperature correction (Table 310.15(B)(1)), conduit fill derating (Table 310.15(B)(3)(a)), and verify the termination rating (110.14(C)). The required wire size is the largest demanded by any single constraint.
Conductor ampacity ratings in Table 310.16 assume a 30°C (86°F) ambient temperature. When conductors are installed in hotter environments — attics, rooftops, boiler rooms — the ampacity must be reduced. At 50°C ambient, a 90°C conductor loses 18% of its rated ampacity.
When more than 3 current-carrying conductors share a raceway, heat dissipation is reduced and ampacity must be derated. With 4-6 conductors the factor is 0.80; at 10-20 conductors it drops to 0.50. This is the most commonly overlooked derating in panel feedthrough and commercial installations.
Per NEC 110.14(C), most circuit breakers and equipment terminals are rated at 75°C. Even when using 90°C insulated wire (THHN), you can use the higher insulation rating for derating calculations, but the final ampacity cannot exceed the 75°C column value. This is why the “controlling factor” often shows as termination limit.