Problem $3$:

A unit substation has a $13\mathrm{.}8kV-208\frac{Y}{120}V,$ three$-$phase, $750$ kVA transformer. Transformer has an impedance of $3\mathrm{.}5\%.$ The utility short$-$circuit capability is $500,000$kVA fault current at the main secondary$.$ The unit substation services $10$ sub$-$feeders.

$($e$)$ Size the main secondary feeder.

$($f$)$ Determine the short$-$circuit current at the end of the main feeder if its length is $50$ ft $.$

$($g$)$ Determine the short$-$circuit current at the end of one of the sub$-$feeders that has a continuous load of $300-$A and a length of $200-ft.$