Fault Current Template

Fault Current Calculation Assumptions

Fault currents at an electrical service entrance are determined by the transformer size and impedance along with the length and size of conductor that supply the service.

Fault currents are calculated using ohms law and the impedances are estimated based on a worst case scenario. For example, St. Thomas Energy's transformer specification does not allow single phase pole mount or pad mount transformers to be built with impedances lower than 1.5%. For 3 phase pad mount transformers, the specification does not allow them to be built with impedances lower than 3%. When calculating fault currents, these are the impedances used for transformers, regardless what impedance is on the name plate of the transformer. Since the majority of the impedance of the fault current path at a customer service entrance is from the transformer and the secondary conductor supplying the service, the impedance from the transformer back to the generators is assumed to be zero.