There are two issues here. The protection of the transformer as found in 450.3 and the protection of the secondary conductors as found in 240.21(C). These are two independent (in most cases) issues. If the primary protection of the transformer is 125% or less of the rated primary current, there is no code requirement to provide protection for the secondary of the transformer. The conductors connected to the secondary of the transformer will require protection based on the rules found in 240.21(C).
