Emergency, Legally Required, Optional Standby Loads

[msteiner]

I know emergency generators are permitted to serve all three classes of standby loads (Art. 700, 701, 702), but I've always had the understanding that separate transfer switches were REQUIRED for each class of load (in addition to the requirement that the generator be adequately sized, or incorporate selective load pickup/load shedding). However, I can't find this requirement for separate transfer switches in the NEC, going back to 2002. Was I dreaming?

 

[shamsdebout]

If this is for an hospital then 517.30(B)(4) would apply.

I don't recall any other section of the code requiring separate transfer switches.

 

[david luchini]

Art. 700 requires that emergency system wiring be kept entirely independant of all other wiring and equipment. This would require a separate transfer switch for the emergency system. I don't believe there is any requirement to separate legally required and optional standby systems.

 

[msteiner]

David - Great point, that's probably where I'm coming up with the separate transfer switch requirement.

 

[steve66]

Art. 700 requires that emergency system wiring be kept entirely independant of all other wiring and equipment. This would require a separate transfer switch for the emergency system. I don't believe there is any requirement to separate legally required and optional standby systems.

700.6 (D) is more specific: " Transfer equipment shall supply only emergency loads."

Article 701 and 702 loads can share a transfer switch if its not a hospital (701.6).

I have always thought that emergency egress and exit lights could not share an ATS with optional loads. However, now I'm not sure. Its not clear to me if if exit and emergency lights are considered article 700 or article 701 loads. If they are 701 loads, then they can share a transfer switch with optional loads.

 

[david luchini]

700.6 (D) is more specific: " Transfer equipment shall supply only emergency loads."

Thanks, Steve.

I have always thought that emergency egress and exit lights could not share an ATS with optional loads. However, now I'm not sure. Its not clear to me if if exit and emergency lights are considered article 700 or article 701 loads. If they are 701 loads, then they can share a transfer switch with optional loads.

See 700.16. Emergency lights and exit sign part of Art 700.

 

[steve66]

See 700.16. Emergency lights and exit sign part of Art 700.

I didn't realize that was right in the NEC. I thought one had to refer to the building codes to try and determine if emergency lighting was a 700 or a 701 load.
So emergency lighting does still need an ATS separate from other non-emergency loads.

I was starting to think I had been wrong for a long time.