I have read about the exemption for <240V and <125kva transformer. Also have read about the accuracy of each calculation method for different voltage levels. I have come across a situation where the transformer is 225kva, 12kv to 208v for a small pumping station. Which calculation method is most accurate for this? Turns out the 225 kva transformer is large because it feeds a variety of other loads in addition to the substation. The utility has given me a transformer fault current of 28,400A for the secondary side.

Typically the transformers are fairly small, dedicated to the load and the secondary voltage higher.

I appreciate any comments.

We use the 'standard' IEEE 1584 calculations for 208V systems.