It is all to do with ampacity of the load. Transformers are continuous loads and conductors are sized per all other continuous loads. Our electrical equipment is sized this way. It deals with the heating from the conductor on the breaker and the transformer terminals. Conductors are sized to the rating of the transformer and not the O/C device.
You size the conductors for a hot water heater or a lighting load the same way. It sometimes appears a little differently when we say you can only load a 15 amp breaker to 80% but we could think that is the inverse of 125%. In the case of a plug circuit we wire with #14 and the 15 amp wire is only loaded to 80% or 12 amps.
I hope I am making sense.
A 75 KVA transformer is 208 amps at 208 volts the wire must be 260 amps. (75000/208*1.73)1.25 The other way to look at it is what is the largest load than can be connected to 260 amp wire? 260*.8 or 208 amps @ 208 volts 3 phase(75 KVA)
