Overcurrent Protection for 120/208V Panelboards.

[Delcobob]

I need help understanding when 120/208V panelboards that are fed from 480V - 120/208V dry type transformers need a main breaker. According to 240.21 (C), and 408.36 (A) and (B), I see that a main breaker is necessary. However when I read Table 450.3 (B), I am not so sure. The panelboards in question are 225A filled with 1P20A breakers feeding receptacle circuits in classrooms and offices. Any help in clarifying this for me would be much appreciated.

 

[augie47]

Keep in mind, as noted by a FPN in 450.3, that Art 450 only addresses protection for the transformer with limits on the primary and/or secondary side. Art 240 address the OCP for conductors and Art 408 for panelboards.

 

[charlie b]

Let me clarify, though I fear this might not simplify. A panelboard never, Never, NEVER is required to have a main breaker. Period.

That said, a transformer?s secondary windings (let?s ignore the primary ones for this discussion), the conductors that run from the transformer secondary to the panel, and the panel itself all share the characteristic that they must be protected against overcurrent. A main breaker in the panel is often used to satisfy all three requirements. But an enclosed breaker or a fused disconnect located close to the transformer, or closer to the panel if it is not too far away, can do the same job. Under certain circumstances (that I don?t encounter very often), the breaker on the primary side of the transformer can be considered adequate to protect the secondary windings. But you still need to protect the secondary feeder and the panel, and the primary breaker cannot do these two jobs.