Derating NEC 310.15(B)(3)(a) applies when more than three current-carrying conductors are installed together in a raceway, cable, or enclosure.

• A. True
• B. False
• C. Depends on ambient temperature
• D. Only for 480V circuits

Answer: (A)

The main idea is that heat buildup from multiple current-carrying conductors sharing the same space requires reducing their allowable ampacity. NEC rules say that when more than three current-carrying conductors are installed together in a single raceway, cable, or enclosure, the ampacity of each conductor must be derated. This derating accounts for the extra heat produced by conductor proximity and ensures insulation isn’t overstressed. In practice, you count conductors that carry current (uncoated ungrounded and neutrals generally count, while equipment grounding conductors typically do not), and the more conductors there are, the greater the reduction applied to each conductor’s ampacity. Neutrals may count in some multi‑wire branch circuit arrangements, depending on how the circuit shares current. Ambient temperature and insulation type can further adjust the final allowable ampacity, but the trigger for derating—more than three current-carrying conductors in one enclosure—is the key concept. That makes the statement true.