If the 1 00-A panel board of Figure 14-28 is within
25 ft (7 .62 m) of the transformer, the disconnect switch
can be eliminated, and a set of 1 00-A main fuses or a
1 00-A main circuit breaker can be installed in the panel board,
as shown in Figure 14-29. The minimum size
secondary wire would be No. 3 AWG copper THWN.
A 2-wire secondary from a single-phase transformer
is permitted to be protected by the transformer primary
over current device, NEC Section 240-3, Exception No.
5. However, there is a caution. The primary over current
device must be sized so that the secondary wire cannot
become overloaded. Equation 14.9 can be used to determine
the maximum size primary over current device rating.