NEC 240.21 says all ungrounded circuit conductors require overcurrent protection, with a list of exceptions:
One of those exceptions is 240.21(C):
240.4(B) says that if you're under 800 amps, you can up-size your breaker to the next available size. But what I don't understand is the text of 240.21(C). It says the conductors connected to the secondary don't have to have protection, and also that the up-sizing of the (non-existent) protection isn't allowed. Why would you say "You're not allowed to up-size the breaker" right after saying "You're not required to have a breaker at all?" I must be misunderstanding something.