When is conductors derated under NEC 310.15(B)(3)(a)?

• A. When ambient temperature is above 40°C
• B. When more than three current-carrying conductors are installed together in a raceway, cable, or enclosure
• C. Only for circuits operating at voltages above 600V
• D. Derating is not required under NEC 310.15(B)(3)(a)

Answer: (B)

Derating is required to prevent overheating when many current-carrying conductors share the same path. NEC 310.15(B)(3)(a) specifically says that when more than three current-carrying conductors are installed together in a raceway, cable, or enclosure, the ampacity of the conductors must be reduced according to the derating factors. This reflects the fact that mutual heating increases with the number of conductors, so the allowed current must be lowered to keep temperatures within insulation limits. The exact derating steps are, for example, 4–6 conductors at 80% of listed ampacity, 7–9 at 70%, and 10 or more at 50%. Ambient-temperature corrections are a separate consideration under another part of 310.15, not the derating scenario described here. Therefore, the condition described by having more than three current-carrying conductors together is when derating under 310.15(B)(3)(a) applies.