Hi.

I am trying to figure out why the transformer nameplate impedance, for example 4.75% is specified as 'per phase' on the nameplate but we still use that same value in 3phase fault level calculations?

Thanks

 

4.75% is not an impedance.

 

It's a bit general statement that impedance is specified for single phase. For three phase power transformers I have seen impedance percent is result of short circuit test. On the short circuit test three secondary phases are shorted and (all three phases) primary voltage is increased until current equals nominal transformer current. This voltage is compared to nominal primary voltage and the ratio is impedance percent.

Example. Nominal trafo primary voltage is 1000 V. Nominal primary current is 100 A. On short circuit test it was found out 100 V results 100 A primary current. Impedance percent is 100 V / 1000 V which is 10 %. To estimate short circuit current (bolted on three phase short on the secondary terminals of the trafo) when used in 1000 V supply network is 100 A/10% =100 A/0.1=1000A.