Transformer rating in short circuit calculation

[vectormechanic]

When performing short circuit calculations that involve larger transformers with multiple VA ratings based on cooling (i.e. 19.8MVA ONAN, 26.1MVA ONAF, 32.7MVA OFAF) and/or multiple temperature rise ratings (i.e. 11MVA @ 45C or 12.3MVA @ 55C), which VA value is generally used when calculating available fault currents?

Thanks!

 

[kingpb]

Two choices:

In the US, the impedance Z is supposed to be based on the lowest rating. If it's not shown and you have no one to ask, then it's safe to assume low rating. It never hurts to ask the question though.

If the transformer is built to IEC standards it should be based on the maximum rating, but you need to verify for sure, I've seen it various ways. Typically, they will provide you the impedance "Z" and also tell you what base it's on.

 

[vectormechanic]

Much appreciated. I sort of answered my own question (doh!) after looking at some more of our transformers - the two that I used in the example are the two that didn't list impedance at a given voltage/tap and base MVA rating.