Section 14 states equipment must be able to withstand the fault current of a circuit. This was not a big issue in the past when distribution systems were lower voltage and transformers had a higher impedance. Traditionally the breakers in a residence were 10.000-amp fault current carrying capacity. Today as we get closer to a substation the fault current can be up to 22.000 short circuit amps. Some of today’s breakers are rated to withstand a 22.000 amp fault current.
Short circuit capacity calculation is used for many applications: sizing of transformers, selecting the interrupting capacity ratings of circuit breakers and fuses
Transformer impedance is the percent of normal rated primary voltage which must be applied to a primary winding in order to make the rated load current flow in the secondary winding with the terminals short circuited.
The lower the impedance of the transformer the higher the short circuit current will flow under a fault condition.
The following is a direct method of determine the available short circuit fault current
100 KVA - 14.4 kv primary / 120/240 volt secondary impedance 4%