The I 20/208-V panel board in the example is rated at
100 A; therefore, it must be protected at not more than
100 A, NEC Section 384-16(a). An over current device
must be provided on the secondary side of the transformer
because the transformer is 3 phase and the feeder
consists of more than two wires, NEC Sections 384-16(d) and 240-3, Exception No.5. A fusible disconnect
switch is installed adjacent to the transformer. If 100-A
fuses are installed in the disconnect switch, a main
breaker will not be needed in the 100-A panel board. The
secondary feeder wire must have an ampere rating of a1
least 100 A. No. 3 AWG copper with THWN insulator
is adequate.
The primary over current device is not permitted to
exceed 250% of the primary full-load current.