I finally figure out 480 to 120 1 phase transformer the calculation offered by tesla might hold up. The transformer ratio is not 4 it is 2.3. Think of a 480/277 to 208/120 transformer the ratio is 2.3. I know this is a delta/wye configuration but the ratio is the same you take the primary voltage phase to phase and divide by the secondary line to line voltage. The only way to connect this transformer and get 120 volts is wye on the secondary. If you could connect the secondary at 120 volts Delta IE a 480 to 120 volt Delta / Delta then the secondary current is 75000/(120*root3)=360 amps but the current is 208 amps if it is a 120/208 transformer. The secondary OC is 208 amps or if protected on the primary at 100amps the seconday is 230 amps and a 225 amp panel is too small without secondary OC protection. If a 90 amp fuse holds then a secondary panel rated at 225 amps could be installed without secondary OC protection.
