When a bolted short circuit occurs, the current that flows is limited only by the impedance between the source and the fault — and that available fault current is what every protective device in its path must be able to interrupt. It sets the AIC/kAIC interrupting rating required of breakers and fuses and the withstand rating of busways and switchgear, making it a fundamental input to any protection and equipment-rating study under NEC 110.9 and 110.24.
At a transformer secondary the standard first estimate assumes an infinite primary bus — a zero-impedance utility source — so only the transformer itself limits the fault: Isc = FLA × 100/%Z. A 1500 kVA, 480 V transformer with 5.75% impedance has a full-load current of 1804 A and therefore delivers 1804/0.0575 = 31.4 kA into a bolted secondary fault; the downstream switchgear needs at least that interrupting rating before any feeder reduction is credited.
Away from the transformer the picture improves: conductor impedance between the source and the fault point limits the current, captured by the IAEI/Bussmann point-to-point method through the f factor and multiplier M = 1/(1 + f) — longer runs and smaller conductors mean lower downstream fault current. Running motors push the other way, briefly feeding roughly 4–6 times their full-load current back into a fault during the first cycles, which is why studies add a motor contribution so equipment is not under-rated.