Article 700 ATS Sharing Feeder on the Normal Line Side With Other Loads

[CrapperEpiphanies]

Looking at 2017 NEC: Reviewing a design where the normal, line side of a new Art 700 ATS is spliced into an existing Art 701/702 feeder that powers the normal, line side of Art 701/702 ATS. My initial reaction is that everything on the line and load side of the ATS should be strictly Art 700, and keep everything else separate to extent and exceptions allowed in Art 700.

But then reading 700.10(B), the section seems to address wiring separation "...from an emergency source or emergency source distribution overcurrent protection to emergency loads..." This feels like it's addressing the generator side and downstream of the ATS, and the handbook certainly has multiple graphical exhibits supporting this: generator output wireway feeding separate enclosures, or common bus but separate vertical sections, multiple generator feeders to separate enclosures, but I don't see any explicit verbiage or graphics addressing separation on the normal side of the ATS.

My question is what is meant by "emergency source" - do they mean the emergency line side of the ATS, OR any source to the ATS - normal and emergency?

Handbook commentary in 700.10(B) seems to support the general intent, but that's not enforceable since it's not code per se: "...wiring for emergency circuits must be completely independent of all other wiring and equipment. This practice ensure that a fault in any other system wiring [e.g. 701/702 ATS] will not affect the performance of the emergency wiring or equipment." Performance of Art 700 branch will definitely be affected if there's a fault on the 701/702 side that takes everything out, forcing the ATS to go on the alternate/emergency path.

Lastly - if you have a setup like this, can selective coordination even be accomplished per 700.32 through a splice? My understanding is that the entire purpose of selective coordination is to minimize unnecessary outage / interruption along the branch. In this case the entire non-emergency side can take out the emergency. So I feel like defining selective coordination as such gives me more of a leg to stand on than the verbiage in 700.10(B).

Thoughts?

 

[steve66]

IMO its OK as shown. The emergency source is the generator, not the normal supply.

Selective coordination is usually interpreted as being required between:
1. All breakers from the generator to the loads.
2. All breakers on the load side of the ATS's with all normal supply breakers.

However, coordination is not normally interpreted as being required between multiple breakers on the normal side. So as long as everything coordinates with the one Distribution breaker you show (and anything upstream of it), IMO its OK.

 

[CrapperEpiphanies]

IMO its OK as shown. The emergency source is the generator, not the normal supply.

Selective coordination is usually interpreted as being required between:
1. All breakers from the generator to the loads.
2. All breakers on the load side of the ATS's with all normal supply breakers.

However, coordination is not normally interpreted as being required between multiple breakers on the normal side. So as long as everything coordinates with the one Distribution breaker you show (and anything upstream of it), IMO its OK.

Thanks for the feedback @steve66
I've never seen this setup as it's probably less than ideal practice - agree? Appears to be a cost savings in this case, especially since the GC is trying to save max $.

 

[steve66]

Yes, a separate breaker for each ATS would obviously be better. Better for maintenance and testing for one thing.

There are a few other considerations too - the wiring to both ATS's would have to be rated for the breaker ampacity, and the breaker has to be large enough for all the loads. I also believe both ATS's would have to be rated at least as large as that breaker.

You don't show the distribution from the ATS, but I also think its its really bad design to put a main breaker right after an ATS.

 

[CrapperEpiphanies]

Yes, a separate breaker for each ATS would obviously be better. Better for maintenance and testing for one thing.

There are a few other considerations too - the wiring to both ATS's would have to be rated for the breaker ampacity, and the breaker has to be large enough for all the loads. I also believe both ATS's would have to be rated at least as large as that breaker.

Makes sense.

You don't show the distribution from the ATS, but I also think its its really bad design to put a main breaker right after an ATS.

@steve66 Do you mean for Art 700 selective coordination between branch and main? In my case there are more atypicals: the load side of the 700 ATS splits into an elevator tap (AHJ considers elevators Art 700) and an Art 700 panelboard, feeder and the tap are intercepted by enclosed circuit breakers.

Would it not be beneficial in this case to have MCBs / ECBs downstream of ATSs to trip first and not take out the feeder breaker upstream, which would take out both ATSs?