Short Circuit is a fault which allows the current to flow through its unintended path with very low impedance, which results into excessive current flow through the path.
a) Supply Source capacity, eg, Transformer, Generator
b) Supply Source Impedance
c) Fault Impedance
d) Distance of Fault from Source to determine loop impedance.
e) Motor Contribution
Common Assumption is there is Infinite source to Transformer and the only Impedance limiting the fault current is Transformer Impedance.
For Tranfo 1000KVA to 2500KVA with 6.25% Impedance, Short Circuit Fault is 16 times of full load current.
Motor contribution is the Current feed back by Motor into System during short Circuit condition. Due to Rotor innirtia, it keeps on running for sum time, even if there is a deep in supply voltage, and now motor becomes soruce and feeds back in to network. This Amount of Current is dependent on the Motor Impedance. Duration of this condition is normally up to 4 to 5 Cycles.
To calculate highest value of Short Circuit Fault at the main PCC Level, If we consider Infinite utility source to Transformer primary Plus 4 times Full load current of Motor as Motor Contribution to short Circuit fault, this will be highest possible Short Circuit Current that system will be able to feed for a Short Circuit Fault at any point in the electrical network.