NEC Table 450.3(B) - Transformer Primary OCPD Size

[Strathead]

There may be certain times when you cannot round up because NEC prohibits it, thus it is a violation of NEC and an incorrect answer on his exam.

As a second year instructor and an area I also teach, I don't have the problem you do and I disagree with this statement. In my experience, the questions on a code test aren't written to "trick" you in this way. If they are asking a question where "next larger size" or not, the question will also include the parameter that allows or disallows it.

 

[jim dungar]

I think that's backward. On a typical step-down transformer, the secondary is closer to the core, leading to lower impedance. That lower impedance leads to a higher inrush when back-fed.

For low voltage transformers, primaries are further away from the core primarily due to the need for the taps to be accessible. Inrush is a function of the complete core and winding design.
The problems with reverse feeding is the transformer was not designed that way, with the result being increased magnetising currents, backward operations of taps, and poor voltage regulation due to improper location of compensating windings.