Re: XFMR PRIMARY & SECONDARY WIRE SIZING
I am one that likes to know what trouble I can get into be for I do an not be surprised by it.
I am to assume that it is a 75kva 480-208y/120. As a former transformer application engineer I'm not familiar with a 120/208-208.
Also, as a former applications engineer I have become accustomed to looking at the entire application such as the FLA would be 90a and the inrush current to energize a 150degC unit would be about 900a. If your 100a breaker were to have a magnetic pickup of about 10x (hopefully yours happens to be as some may be lower), that would place it at about 1000a and, as such, the transformer should be able to be energized with out nuisance tripping. But knowing that breakers are calibrated commonly +-20% that would place it between 800-1200a, which could create a conflict.
If you say that you have a transformer rated 115 0r 80gedC, or has a K factor that inrush will be much higher, anywhere from about 1100-1400a, which can pose to be a significant problem.
Remember Table 450.3(B) which allows you 125% (90x125%=112.5a) 112.5a is not a standard rating which you are allowed to go to the next higher standard rating but not over, which is 125a.
After having to field numerous complains about a 75kva transformer causing nuisance tripping and requests for warranty replacements because of a defect I find that the contractor has backed himself in a corner with a panel that can only accommodate a maximum of a 100at breaker.