The 208 input current would be 30,000 /208/1.732=83 amps. Breaker would be 83 X 1.25=103 amp (next size up=110 amp.)
The 480 load side would be 30,000/480/1.723=36 amp. Secondary breaker = 36X 1.25=45 amp.
I'm not agreeing with a 110A breaker. It seems overkill...to me.
That 103A 208 current is based on the capacity of the transformer and not the actual load.
Does the transformer need to have conductors and an OCP rated for the transformer...or just the load?
The load already had 25% added to the 21 kW load get to 26kW and then that was bumped 15% to 30kVA and then the load of that 30kVA transformer had another 25% added to get to 103A.
In this example, the actual load is 480V * 25A * 1.732 = 20,784. The primary side would see 20,784/208/1.732 = 58A plus 25% (for continuous)...58 * 1.25 = 72A. Add 15% for transformer losses for a total of 80A. 90A breaker or you can apply 450.3B and go with a 100A breaker.
Primary conductors: #4 Cu or #2 AL. 1" EMT for AL.
With a 103A current, conductor size: #2 Cu or 1/0 AL 1.25" EMT for AL.
Not much of a difference is the run is short and the bends are few but if we're talking a 100'+ run, that could add up. Or make a difference in other situations when maybe you are dealing with an existing transformer or getting a good deal on a used transformer.
Basically, I just seeing if everything can be based on that actual load of the machine.