When using the per-unit method there is an impedance associated with the fault duty. This obtained first by getting the per-unit apparent power which is found by using the 1000kVA rating of the transformer the system's base apparent power and converting it into MVA, which is 1MVA. Next the fault duty's per-unit value is S=40MVA / 1MVA = 40p.u. . Next the per-unit impedancnce associated with the fault duty is z = 12 / 40 = .025pu. Next we find the per-unit fault current is
if = 1 / (.04+.025) = 15.38pu.Next the base current on the secondary side of the transformer where the fault occurs is Ib = (1000*103) / (sqrt(3) * 480 ) = 1202.81Amps
Finally we determine the acctual fault current by multiplying the per-unit fault current by the base current on the secondary side of the transformer.
IF = if * Ib = (15.38)*(1202.81) = 18499 Amps.
