I have a 75kva transformer, 600v to 208v. The primary rating is 72.1 amps.
26-256(1): The primary overcurrent is to be 125%, or 90.125 amps; and, that will protect the secondary conductors rated at 125% of the rated secondary.
The rated secondary current is calculated to be 208.18 amps, so the secondary conductors must be good for 260.2 amps. I select 250 MCM copper with an ampacity of 265 amps.
26-256(3) provides that where 125% of the primary current does not correspond to the standard rating of an overcurrent device, the next highter rating shall be permitted. 90.125 amps is greater than the standard size of 90 amps, so the next standard size is 100 amps.
There is no stipulation that when the larger overcurrent is used, the size of the secondary conductor must be increased.
The inspector suggests that rule 14-606 must be applied to the conductors. I disagree that the rule about panels applies to conductors.
It is my position that the code allows for overcurrent to be greater than the ampacity of the conductors in certain circumstances. Motor feeders are an example, as is 14-104(a) and the associated Table 13. 26-256 is just another example.
Are we missing something? What do you think?
