Derating Factor

Cable ampacity tables are published for specific reference conditions — typically 30°C ambient air temperature, a single circuit, and no thermal insulation. Real installations rarely match these ideal conditions. Derating factors quantify the reduction in current-carrying capacity caused by each deviation. The ambient temperature factor (Ca) accounts for hotter or cooler environments. The grouping factor (Cg) addresses mutual heating when multiple loaded cables are installed together. The thermal insulation factor (Ci) applies when cables pass through insulated walls or ceilings. For buried cables, soil thermal resistivity and ground temperature factors apply. These factors multiply together — for example, if Ca = 0.87 and Cg = 0.70, the combined derating is 0.609, meaning the cable can carry only about 61% of its tabulated capacity. Overlooking any derating factor risks overheating, insulation degradation, and potentially fire. The required tabulated current becomes It = In / (Ca × Cg × Ci × Cc), where In is the protective device rating.