A 75KVA 240 to 480 transformer has a rated primary current of 180A and a rated secondary current of 90A.
Generally the primary OCPD is _greater_ that the rated full load current of the transformer, in order to avoid 'nuisance tripping' when the transformer is energized. When a transformer is energized, there is usually a sudden inrush of current until proper magnetization is established.
However you are not _required_ to oversize the primary OCPD, and in fact it is certainly permissible to use OCPD that is _less_ than the full load rating of the transformer, again with the risk of nusiance tripping when the transformer is energized. For what its worth, in my lab I've used 30KVA transformers with primary OCPD about half the full load rating, with no nuisance tripping problem.
You must select OCPD that is properly sized for the load, including any 125% sizing if the load is continuous.
Note: you can also place a 175A breaker in a 200A main panel, as long as the calculated load is properly served by the 200A main panel. You could even put a 200A breaker in a 200A panel, as long as your load calculations tell you that the over-all load is properly served. The size of a feeder breaker doesn't tell you how much load that feeder _uses_; it simply says that the feeder conductors are protected at a certain current level.
-Jon
