Final answer:
Derating factors for electrical conductors need to be applied when the number of current-carrying conductors in a wireway exceeds three, according to the National Electrical Code (NEC).
Step-by-step explanation:
The student's question revolves around the application of derating factors for electrical conductors. Derating factors are used to adjust the current-carrying capacity of a conductor based on various conditions, such as the ambient temperature or the number of conductors in a conduit or wireway. According to the National Electrical Code (NEC), derating is typically required when more than three current-carrying conductors are placed in a raceway, cable, or wireway. Therefore, the correct answer to the question is:
Derating factors shall only be applied when the number of current-carrying conductors in a wireway exceeds three (a).