The conductors of NM cable shall be rated at 90° C but the allowable ampacity of the conductors shall not exceed that of a _______ ° C rated conductor.

The correct answer indicates that although the conductors of NM (non-metallic) cable are rated for a maximum temperature of 90°C, the ampacity—essentially the maximum amount of electrical current a conductor or device can safely carry without overheating—must not exceed that of a conductor rated at 60°C.

This requirement is grounded in safety and performance. The reason for adhering to the ampacity limits of a 60°C rated conductor is due to the insulation type typically used in NM cables, which will generally have a lower thermal limit in practical applications, even though the conductors themselves can withstand higher temperatures.

If a conductor were to be rated merely by its maximum thermal tolerance, it could lead to situations where the conductor operates at an unsafe level, potentially resulting in overheating and increased risk of fire hazards. Therefore, the practical application in real installations often necessitates adhering to the more conservative 60°C ampacity limits to ensure reliable and safe operation.