When you size transformer primary overcurrent protection up to 250% of primary current along with secondary protection at 125% of secondary current as per NEC table 450.3(B), are you required to size your primary conductors to the 250% rated breaker/fuse or can they be sized at 125% of primary current? I've always sized the primary conductors to the 250% rated OC device since I haven't found anything in the NEC that specifically says otherwise. However, in speaking with others on this subject I've found differing interpretations. Any clarification you could give me would be helpful.
Transformer primary conductors w/ OC protection @ 250%
- Thread starter kbreyla
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- Massachusetts
Unless you know of something in the NEC that allows protecting the conductors for this application beyond 100% of their rating you must size the conductors to the chosen breaker size.
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