Three $25-$kVA $24000/480-$V single phase transformers are connected in A$-$A$.$The open$-$circuit test was performed on the low$-$voltage side of three phase transformer and the following data were recorded: Voc$=480$V$,$loc $=3\mathrm{.}5$A$,$Poc$=840$W$.$The short$-$circuit test was performed on the high$-$voltage side of three phase transformer, and the following data were recorded: Vsc$=1200$V$,$Isc$=2\mathrm{.}00$A$,$Psc$=1050$W

a$)$Find the equivalent circuit of this transformer referred to the high voltage side.

b$)$Find the voltage regulation of this transformer at the rated load and $0\mathrm{.}85$PF lagging.

$$c$)$What is the transformer's efficiency under these conditions