If the primary full-load current is less than 9 A, the
primary over current device is not permitted to exceed
1.67 (167%) times the primary full-load current, NEC
Section 450-3( b )(I), Exception No. 1. If the primary current
is less than 2 A, the over current device is not permitted
to exceed 3.0 (300%) times the primary full-load
current, NEC Section 450-J(b)( 1).
Consider the case of a 3-k VA transformer stepping
down 480 V to 120 V to supply one 20-A single-phase
circuit. The primary full-load current is 6.25 A.