Legally Required Generator

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shockin

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2011 NEC 700.5 (D) states that transfer equipment shall only serve emergency loads. This always seemed logical to me that a legally required generator could only serve EM loads because it is a life safety issue.

However in the handbook right under this article it states that a single generator can serve both EM and non EM loads if seperate transfer switches are used.

So I have two questions. Is the Handbook correct that you can serve both types of loads from a single generator? What is the purpose of seperate transfer switches.
 
There are three types of loads, not two. That is perhaps one source of your confusion. Emergency loads are addressed in article 700, and I think 700.4(B) has the answer to your question. Legally Required Standby loads are addressed in article 701, and they are not the same as emergency loads. Please note that article 701.5 is similar to 700.5. Finally, Optional Standby loads are addressed in article 702.

The purpose of separate transfer switches is to prevent a failure of a "lessor important" transfer switch (e.g., the Optional Standby one) from taking away power from the "more important" transfer switch (e.g., the emergency one).
 
shockin said:
2011 NEC 700.5 (D) states that transfer equipment shall only serve emergency loads. This always seemed logical to me that a legally required generator could only serve EM loads because it is a life safety issue.

However in the handbook right under this article it states that a single generator can serve both EM and non EM loads if seperate transfer switches are used.

So I have two questions. Is the Handbook correct that you can serve both types of loads from a single generator? What is the purpose of seperate transfer switches.
Click to expand...
1) Yes. See 700.4(B)

2) Non-emergency ATS are not required to be identified for emergency use :slaphead:. Easier to setup load shedding if required. Probably a few others...
 
shockin said:
2011 NEC 700.5 (D) states that transfer equipment shall only serve emergency loads. This always seemed logical to me that a legally required generator could only serve EM loads because it is a life safety issue.

However in the handbook right under this article it states that a single generator can serve both EM and non EM loads if seperate transfer switches are used.

So I have two questions. Is the Handbook correct that you can serve both types of loads from a single generator? What is the purpose of seperate transfer switches.
Click to expand...

Having two separate generators would be better. It would make it even less likely that a problem with the non-required load could affect the legally-required loads. But as you mentioned, that's not a code requirment.

I can only guess that the code making panels (CMP's) decided the additional safety that would be provided by requiring a second generator for non-required loads isn't worth the financial expense of installing and maintaining a second generator.

Having separate ATS's makes sense - they are farily cheap and easy to maintain. And having separate ATS's helps reduce the number of common circuit breakers between the required and non-required loads. (With separate ATS's, the generator main breaker would be the only common circuit breaker on the emergency side.)
 
A fault on a non essential load has less chance of disrupting the essential loads if on a separate branch of the standby system, this is true while operating from normal supply as well as standby supply. Just something to consider.
 
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