First calculate the current of primary side(HT) and derate the cable by atleast 0.7.
Current I after Derating =78.73/.7 =112A (0.7 if laid on the trays or value will vary if buried in the ground).
Cable equivalent to rating has to be selected.
Now Short circuit withstand capacity (sq. of I X t in sec) has to be checked from the cable catalogue.This value of cable should be higher then SC withstand capacity of the ground usually it is 50KA.it less go for next size and finalise.
HT Cable select, its depend of incoming capacity of feeder
line. say feeder capacity 100 MVA SO Calculate current I=
100*100/11*1.73=5.25kA. So we can select of HT Cable for
minimum short ckt current carrying capacity of minimum 5.2kA.
Assume PF of transformer is .8.
rating of transformer=1500kVA=1500X.8=1200kW
So that maximum current-
So cable size required if copper cable
1mm=4amp= 400sqmm(by consedering all factors)
For aluminium cable