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.