How to calculate the short circuit current flow in a three phase circuit?

To calculate short circuit current flow in a 3 ph. Circuit, we should know % impedance of source like transformer or generator. Fault mva = base mva / % impedance. On obtaining fault mva, we could find out a.

In short:

If the impedance of the transformer is 5%
If the full load current of the transformer is 1000A
The short circuit current is 20 times the full load current

i.e. short circuit current = 20,000A

STC is maximum fault current that a system is designed to withstand.For obtaining STC it will be as following example.

Conider a 10 MVA transformer with 6.5% Impedance. System voltage is 13.8/4.16KV. Now Fault MVA = base MVA/PU Impedance

So Fault MVA=10/0.065=153MVA

Now Fault Current = 153*1000/(1.73*4.16)=21259A = 21KA

I hope this can help