Questions arise regarding the derating required when circuit breakers Listed to UL 489 are used in rack power distribution units.
While the design and construction of this type of circuit breaker is not affected by temperature and loading effects, UL’s guidelines for use require that the circuit breaker continuous loading not exceed 80% of the circuit breaker rated current.
While there are some circuit breakers that are Listed and marked for 100% use, circuit breaker size and the required enclosure space preclude use of these circuit breakers in Rack PDUs.
Unless otherwise marked, circuit breakers should not be loaded to exceed 80% of their current rating, where in normal operation the load will continue for three hours or more.