Overcurrent protective devices can be either Fully Rated or selected as a series combination rated system or Series Rated System.

In a series rated system, two overcurrent devices are used. The upstream overcurrent device can be either a fuse or circuit breaker and it will have an interrupting rating (AIC rating) equal to or greater than the Available Fault Current (AFC) at its line terminals. The downstream overcurrent device will always be a circuit breaker and will have an AIC rating less than the actual AFC at its line terminals but the two devices will have been tested by a nationally recognized testing laboratory, and found to work together in order to eliminate a fault condition that subjects the downstream circuit breaker to more AFC than it is rated for.

UL standard 67 for panelboards requires any series rated combinations to be marked on the label or book that comes with a new panelboard. The book or label is used to identify overcurrent devices that have been tested and found to be compatible as a series combination.

The compatibility of overcurrent devices used in a tested series rated system is required to be verified by the inspector who will use the label or series rating book that comes with the new panelboard and match up the combination.