Overcurrent protective devices (OCPD’s) can be either Fully Rated or selected as a series combination rated system or Series Rated System.

In a Fully Rated system, all overcurrent devices are fully rated for the maximum amount of available fault current (AFC) at their line terminals which complies with the code language in NEC^® 110.9. An example would be a 200-amp circuit breaker or fuse with an ampere interrupting capacity (AIC rating) of 42k AIC or 42,000 amps, installed in a panelboard where there is 38,000 amps of available fault current.

In the image, the example shows one electric panel with feed through lugs feeding another panel to the right. This example intends to show that after considering the type of feeder conductor running between the two panels, the length of conductor, the raceway type and the AFC at the beginning of the run of wire, there would be less AFC at the load end of the feeder but still a significant amount. The amount of AFC at each panel location would need to be calculated in order to comply with NEC^® 110.9 and to be able to select the correct AIC rating of the circuit breakers since their purpose is to interrupt current at fault levels.

The image represents a Fully Rated System since all circuit breakers have an AIC rating equal to or greater than the AFC at their location.