I have a nursing home design that requires a back-up generator for essential systems. Looking at FPN Figure 517.41, No.2 (2005 NEC), I need to keep the life safety and critical branches separate, but can use one ATS (less than 150KVA system.) The '05 Handbook Exhibit 700.1 indicates a switchboard used to separate the emergency loads from legally required loads, etc. They claim that a switchboard can separate the wiring physically (unlike a panel board, which has an open design.) This diagram shows the switchboard at the generator, but I am assuming a switchboard would be required in Figure 517.41, as well. I'd like to delete the switchboard all-together by using a double-lug rated ATS to feed two separate panel boards - Life Safety and Critical. Of course, we would have separate conduits for the two feeders. Do you see anything wrong with this?
Double lugging ATS load side for life safety/critical separation?
- Thread starter donw
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I don't see any problem with the single ATS and double lugs. Just be sure you provide the necessary OCP for the conductors. (Personally, I don't like to use the tap rules on emergency systems unless absolutely necessary, but that's just a personal design preference.)
I'm not sure you couldn't use a panelboard on the load size of the ATS. In your 517 application, I think both your emergency loads are Article 700 loads. The example in Article 700 shows 700 loads with 701 loads and 702 loads.
I'm not even sure I agree that the switchboard shown in Exhibit 700.1 couldn't be a panelboard. In both cases (switchboard and panelboard) the bus is the same for the different loads. Where in 700.9 does it say the different systems can share a common bus, but not common space for the breakers?
Exactly how far back toward the generator you have to keep the systems separated has always been a point for discussion. Some people interpert this article as requiring separate generators. (But that's clearly not required in your case, since you have a paragraph that says you can even share a transfer switch.)
Steve
I'm not sure you couldn't use a panelboard on the load size of the ATS. In your 517 application, I think both your emergency loads are Article 700 loads. The example in Article 700 shows 700 loads with 701 loads and 702 loads.
I'm not even sure I agree that the switchboard shown in Exhibit 700.1 couldn't be a panelboard. In both cases (switchboard and panelboard) the bus is the same for the different loads. Where in 700.9 does it say the different systems can share a common bus, but not common space for the breakers?
Exactly how far back toward the generator you have to keep the systems separated has always been a point for discussion. Some people interpert this article as requiring separate generators. (But that's clearly not required in your case, since you have a paragraph that says you can even share a transfer switch.)
Steve
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Steve66, I am looking at 517.41(D) "The life safety branch shall be kept entirely independent of all other wiring and equipment and shall not enter the same raceways, boxes, or cabinets with other wiring..."
I agree that it has to be connected somewhere - bus, lug - somewhere.
petersonra
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There is no entrance into the ATS in this case for the life safety branch, only an exit.donw said:
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petersonra
Senior Member
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- Northern illinois
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- engineer
no.donw said:
IMO, the wording says you cannot ENTER something with life safety circuits that already has other circuits in it. I would argue that double lugging at the ATS does not constitute entry into the ATS, since the wiring begins there.
But thats just my take on it. And it is not anything I deal with much.
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