Three methods of providing over current protection
for transformers is covered by the National Electrical
Code. Both the primary and the secondary winding
must be protected. The procedure begins by calculating
the primary and the secondary full-load current, using
Equation 14.4 for single-phase transformers, and Equation
14.5 for 3-phase transformers.
A transformer can be protected by one over current
device on the primary side rated at not more than 1.25
(125%) times the primary full-load current, Figure 14-
24. This over current device can be a set of fuses in a
panel board, a fusible switch, or a circuit breaker.
Consider that a 25-kVA, single-phase transformer
has a 480-V primary and a 120/240-V, 3-wire secondary.
The primary full-load current is 52 A.