Calculate available short-circuit current (AFSC) at the transformer secondary and downstream panels using the point-to-point method. Verify equipment interrupting ratings.
The point-to-point method calculates available fault current at each distribution point by accumulating impedances from the utility source through each transformer and feeder. At each point, the available symmetrical RMS short-circuit current equals the source voltage divided by the total impedance to that point.
Equipment must be rated to interrupt the available fault current at its installation point. NEC 110.9 requires that circuit breakers and fuses have an interrupting rating not less than the available fault current at the point where they are installed.
Standard molded-case circuit breakers (MCCBs) are typically rated 10 kAIC or 22 kAIC. High-interrupting-capacity breakers are rated 65 kAIC, 100 kAIC, or higher. For systems with high available fault current (e.g., close to a large utility transformer), standard-rated breakers may be inadequate and series-rated or high-IC devices are required per NEC 110.9.
Transformer impedance (Z%) is the percentage of rated voltage required to circulate rated current through a short-circuited secondary. A higher Z% limits fault current — a 10% impedance transformer will produce roughly half the fault current of a 5% unit of the same kVA. Utility engineers sometimes specify higher impedance transformers to keep fault current within equipment ratings.
NEC 110.9 requires interrupting ratings to match the available symmetrical RMS fault current at the point of application. Breaker interrupting ratings (kAIC) are expressed in symmetrical RMS amps. Asymmetrical (crest/peak) values are used for momentary-rated devices like fuses and switchgear closing ratings.
Yes. Motors act as generators during a fault and contribute short-circuit current back into the system. Motor contribution is typically estimated at 4 times the motor full-load amps and is added to the transformer contribution. This is especially important in industrial facilities with large motor loads connected close to the fault point.