Input: 75,000VA / 1.73 / 480V = 90A. Call it #3 or #4, depending on whether you play by the "next larger standard breaker size" rule.

Output: 75,000VA / 3 / 120V = 208A. If you call this 200A, it will take 3/0 cable. I don't know if the rules would allow fusing this at 225A; if so, you'd want to use 4/0.

The above assuming 75 degree wiring.

I would imagine that either the input or the output fusing is supposed to limit the TX to 75 KVA; I don't know what the rule is for this. In a vacuum, I'd say a 90A breaker on primary feeder would do the job.

Well, on a SQ D slide-rule my teacher gave us last semester, it states that a 75 KVA can hold a maximum of 208A at 208V, 180A at 240V, 90A at 480V, and 72A at 600V.

Then, going from 310.15, 4/0 for 208V, 3/0 for 240V, #3 for 480V, and #4 for 600V.

I barely finished my 1-year program, I'm barely going to start my Apprenticeship. Someone correct me if I'm wrong.

I suppose that answer was rather sparse. I also used the relationship that the VA of a three-phase circuit is sqrt(3) * the VA of a single phase circuit, at the same number of amperes per conductor.

My problem here is protection of the transformer. Using normal breakers, with a continuous, 100% load, you need to oversize by 25% because of the 80% rule. That would suggest 90*125% = 112.5A on the primary, and 208*125% = 260A on the secondary.

Also, there is some loss in the TX. Supposing 5% loss, you need 94.5A in to get 75kVA out.

So, my thought would be to use 100% rated breakers, with #3 fused at 100A on the input, and with 4/0 fused at 225A on the secondary.

This doesn't really limit the TX to 75 kVA. A 90A, 100% breaker would limit the input to 75kVA, but you won't get 75kVA out of it. Assuming this isn't a trick question where the premise is false, you said that the load is 75kVA; therefore, I have to assume that the TX is designed and listed to have an input large enough to supply that load. Given that, I'd feel comfortable with an adjustable, 100% breaker set to 95A supplying the primary. I have no idea if that would meet Code. And I'll stay with 4/0 at 225A, 100%, on the secondary.