WIRE SIZE AND PROTECTION
Electrical wires must be adequate to supply the load
and protected from over current, according to the wire
ampere rating as given in NEC Table 310-16. Therefore,
wire ampere rating must also be considered when sizing
transformer over current devices. An example will help
to illustrate the procedure of sizing feeder wires and
over current protection.
A 1 00-A, 120/208-V, 3-phase panel board is supplied
from a 480-V, 3-phase system through a 37.5-kVA step down
transformer, Figure 14-28.
The first procedure is to determine the primary and
the secondary full-load currents.