Answer:
The fault current of a power transformer will depend on the following;
Transformer Rating (in KVA/MVA)
per unit impedence of the transformer (%p.u.)
line/phase Voltage (VL/VP)
the following formula can be used to find the fault current on the secondary side of a transformer
Fault Current = Transformer Rating /(per unit impedance x phase voltage)
The Values of Transformer Rating, per unit impedance & phase/line voltage will usually be mentioned on the transformer rating plate / data sheet
As an example a 500kVA, 11kV/400V/3-Phase/50Hz transformer with 5% p.u impendence will have the following fault levels on the secondary side
Fault level = 500/(5%)=10000kVA
S=1.732 * VPP * IP
Fault current = 500/(5% x 400 x 1.732) = 14.4 kA
Remember to use 3phase voltage!