700.10(B)(5)(b) Exception

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charlie b

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A standby generator provides power to a switchgear via a breaker on the switchgear. The switchgear has three other, separate sections, each of which has a breaker that provides power to its own ATS. One ATS serves article 700 loads, the second serves 701 loads, and the third serves 702 loads. The exception to 700.10(B)(5)(b) says this is OK, provided the breaker from the source satisfies article 700.28 coordination requirements. But 700.28 only says that the overcurrent devices in the emergency distribution system must coordinate with the upstream breakers. Looking only at the words as those words are written (IAW "Charlie's Rule"), the words could be interpreted as meaning that the breaker that brings in power from the generator does not need to coordinate with the breaker that gives power to the article 702 ATS.

Do you agree that this is a viable interpretation of the words, as written?

I don't think this was the intent. If they are allowing a breaker at the source, and if that breaker does not coordinate with the breaker serving 702 loads, then a fault on an optional standby load could take the generator off line. Looking at older editions of the NEC, this was clear in the 2008 version, and was made unclear in the 2011.
 

steve66

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Illinois
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Engineer
I haven't referred back to the 2008 text, but I tend to agree - they have created an unintentional loophole.
 

charlie b

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Here is the text of the exception in the 2008 edition:
Overcurrent protection shall be permitted at the source or for the equipment, provided the overcurrent protection is selectively coordinated with the downstream overcurrent devices.
Here is what it said in the 2011 edition:
Overcurrent protection shall be permitted at the source or for the equipment, provided the overcurrent protection complies with the requirements of 700.27.
The only change in the 2014 edition was related to paragraph numbers.
 
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