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NEC 240.21 says all ungrounded circuit conductors require overcurrent protection, with a list of exceptions:

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One of those exceptions is 240.21(C):

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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.

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  • $\begingroup$ Aren't they saying 240.4(B) does NOT apply, which means no breaker is required. $\endgroup$
    – StainlessSteelRat
    Nov 2, 2021 at 17:57

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See 450.3 and tables 450.3(A) and (B), 240.21(C) says the conductors connected to the secondary don't have to have protection at the point where the conductors receive their supply. It's often not the case that they need no protection at all, its just that the protection can be down stream from the transformer, like a main breaker in a panel. When applying these tap rules you can't use 240.4(B)

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