Transfer Switches per Article 701 & 702

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bdingman

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There have been questions recently concerning the number of transfer switches required on an emergency generator. Article 700.6(D) requires a seperate transfer switch for emergency loads only. This paragraph does not appear in Articles 701 and 702. Some clients have asked if this means that one transfer switch can be used for 701 and 702 loads. Can anyone clairify this in the NEC?
 
I reviewed Article 701.6 and it appears to refer to the power source, i.e. the generator, being of the capacity to supply 701 and 702 loads. Article 700.5(A) & (B) is similar in that it allows 700, 701 and 702 loads on the same power source.
 
I believe what George was getting at is that in 701.6 you are given two alternatives. 1) Sized the generator so that it can adequately carry the legally required AND optional loads or 2) provide a means to shed the optional load when the combination of the two loads exceeds the generators capacity. So, the way I read it, in case #1, only one transfer switch is required. In case #2, two transfer switches are required with automatic load sensing control that will shed the optional load if necessary.

Thats how I read it. Please correct me if I am wrong George!
 
If you read 700.6(D), if the load is truely emergency and not just legally required or optional, it must be on a separate transfer switch. There are some exceptions such as very small hospitals, etc. see 517.30, but in general "emergency systems" must have a separate transfer switch. If I correctly read Mike Holt's recent e-mail about this, 701 and 702 systems are generally not considered emergency systems.

Jim T
 
ramdiesel3500 said:
So, the way I read it, in case #1, only one transfer switch is required. In case #2, two transfer switches are required with automatic load sensing control that will shed the optional load if necessary.
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In the 700/701/702 generator scenario, then there doesn't seem to be a way around 700.6(D). This is probably redundant anyway, because the wiring is required to be separated from non-emergency circuits (700.9(B)) So, one dedicated transfer switch would be required for the emergency loads, which is controlled by a device that would shut off the 701/702 loads when they became too big a burden for the generator. The 701/702 loads would then be governed by 701.6.

In the 701/702 scenario (no emergency system present), I would say that if a transfer switch has the capabilities of "automatic selective load pickup and load shedding", then one transfer switch can do the job. (I don't have any idea if such a thing exists, I'm just giving my opinion on the explicit meaning of the text.) If that's impractical (or non-existent, or nonsensical) then another option would be two switches, controlled by a load-shedding device.

Assuming there is no emergency system: The easiest route to think about would be sizing the generator to handle all the 701/702 loads as outlined in 701.6(2). Then one transfer switch could serve both. It may not be the most cost effective solution, however.

Disclaimer: Bear in mind, I've not dealt with any of these systems in practice.
 
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IMHO, sizing the generator to carry both 701 and 702 loads is usually your best value. Much more simple and reliable. I would only resort to load shedding options in the case where additional loads are being added to an existing generator installation.
 
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