Selective Coordination and 700.27 Exception #2

[bcorbin]

In a hospital project I am reviewing, there is an emergency switchboard feeding a distribution panel with main CB through a transformer. The coordination study is saying the breaker feeding the transformer isn't sized correctly to handle the inrush, so either breaker could trip.

My question is this...does it really matter which breaker trips? Either way, the distribution panel is out. There is already an exception to 700.27 for same size OCPD's in series. How is this any different, outcome-wise? :-?

 

[charlie b]

No it does not matter. In fact, the 2008 NEC added an exception to explicitly address this situation. The same 2008 NEC added the exception you are talking about. So if you are under the 2008, then the primary and secondary OCPDs do not have to coordinate.

 

[bcorbin]

Great! Thanks, Charlie!

 

[mayanees]

No it does not matter. In fact, the 2008 NEC added an exception to explicitly address this situation. The same 2008 NEC added the exception you are talking about. So if you are under the 2008, then the primary and secondary OCPDs do not have to coordinate.

Agreed.... that they don't need to coordinate, but they both need to hold magnetizing inrush for the transformer. I would think the breaker gets rejected because of the likelihood of the xfmr inrush trip.

John M