A 75kVA delta/wye transformer is capable of ( 90.2 x 4 = 360.8 line to neutral amperes at 120 volts per each phase.
The 90.2 amps is the line to line draw of a delta connected 480 volt source. The turns ratio is 4:1 going from delta to wye. ( 480/120 )
Typical practice is to only partially load these transformers, ie max the draw at 225A line to neutral -- each phase.
Even if the delta voltage were higher... the load side calculations would remain the same for a 75kVA transformer.
Your latest info indicates that you could double tap this transformer. Do some load calculations. If the transformer belongs to the utility present them and the AHJ these load calculations in your submission for increased service.
I sure hope that the three 100 amp breakers have a common handle tie; in which case it's a single three pole breaker ... not 3 breakers.
This must be the first time through for you. Hook up with someone more experienced as soon as possible. Farm it out.
