NEC 240.21 (C) states that conductors are permitted to be connected to a transformer secondary without overcurrent protection at the secondary. It further states that 240.4(B) is not permitted. 240.4(B) is for next size up of standard overcurrent devices as long as calculated load is less. At the same time, 450.3(B) allows 125% for secondary with next size up provision. If a 75kVA 480:120/208 at 125% is 260A, you can use a 300A breaker with 300A wiring (144% overload). The questions are why the restriction for next size up for the wiring for transformers and taps and not other wiring? At the same time allowing 144% over for a 75kVA transformer?