Are THHN 90°C conductors permitted to be used at their 90°C ampacities?

The correct answer indicates that THHN 90°C conductors can only be used at their 90°C ampacities if the circuit breaker is rated accordingly. This requirement is rooted in the National Electrical Code (NEC) standards, which dictate that conductors' ampacity ratings must align with the overcurrent protection devices used in the circuit.

The ampacity of THHN conductors is indeed based on their ability to handle higher temperatures, but in practice, this ability is qualified by the type of protection installed. When a circuit breaker is rated for the appropriate temperature, it allows the conductors to operate at the higher ampacities without risk of damaging insulation or creating hazardous conditions. If the breaker is rated for a lower temperature or the installation does not comply with the amperage rating, then the higher ampacities of THHN cannot be safely utilized.

Thus, ensuring that the circuit breaker is rated for 90°C conditions is essential for safely leveraging the full ampacity of THHN conductors, balancing performance with safety standards in electrical installations.