We are building a new fire house. It is not a COPS building. It will include sleeping quarters, fire apparatus garage, offices and other space. The entire building will have generator backup. Even if the transfer switch and generator meet the requirements of NEC 700, it would appear to me the code requires mutable transfer switches. (One for emergency power-egress lighting-and one for other optional power.) It seems to me we add expense and complication to the system without adding and even diminishing reliability. If I am reading this code section right, does anyone understand the reasoning behind this requirement? Plainly if the power goes out all lights will become emergency egress lights so why two transfer switches?
NEC 700
- Thread starter wyboy
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packersparky
Senior Member
- Location
- Wisconsin
- Occupation
- Inspector
It's all about where a fault most likely to might occur. A fault is more likely to occur in an optional system where there are no restrictions on what is connected to the system.
Emergency lights vs. frig, printer, coffee maker, computers, cubicle lights, any receptacle that i need right now= Emergency lights win.
It is all about getting people out of a building in an emergency. The lights are the highest priority.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
I worked next door to a high energy physics lab at Stanford.
There was a subset of circuits that were backed up by generator for equipment for which unexpected loss of power could damage it or other equipment.
The only problem was so much equipment ended up plugged into them that one night the feeder breaker popped.
It took weeks to get some experiments running again.
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There was a subset of circuits that were backed up by generator for equipment for which unexpected loss of power could damage it or other equipment.
The only problem was so much equipment ended up plugged into them that one night the feeder breaker popped.
It took weeks to get some experiments running again.
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How is this related? Or are you going to let readers make the connection?
Hendrix
Senior Member
- Location
- New England
Not too difficult to make a connection.
I suppose... but going back to packersparky's post, faults are handled by OCPD's (such as the lab mentioned by GD) not generators and transfer switches. How would have separate emergency and optional transfer switches made any difference in GD's scenario?
No, not at all. Only the lights required for egress are allowed to be connected to the Article 700 transfer switch. An engineer or architect needs to design the lighting and determine which lights are necessary for egress.
Because a fault can take out an upstream circuit breaker, disabling the entire emergency system. I've seen it happen.
If the contractor doesn't want to foot the bill for a second ATS, the other option would be to install battery powered emergency and exit lights.
I understand that (and that should be dealt with through selective coordination). How does that justify separate transfer switches?
The separate ATS requirement does pre-date the selective coordination requirements, although breakers can still fail, and other things can go wrong, so the selective coordination requirement doesn't really guarantee anything.
Minimizing the loads, and the wiring, and the number of breakers on the Article 700 ATS is the best way to ensure that the emergency branch will function when necessary. Separation of wiring in separate Jboxes and conduits is also a requirement, other wise an arc or flash from an optional branch circuit could damage an article 700 circuit.
There are also some load shedding requirements that require a second ATS.
That's the only substantial reason mentioned thus far.
I strongly disagree.
But if you think the requirement is overkill, why not submit a proposal to eliminate it for the next NEC cycle?
On a different, but related subject (one which probably deserves a new thread), I really don't understand the exception that allows a single ATS for Hospitals with a total emergency demand less than 150KVA. A hospital is one place where you really want the life safety and critical branches to be rock solid. They are more important in a hospital where people may be on life support systems. Why do they get an exception that nobody else can use? I think it's an exception that should be eliminated.
Maybe the code making panels think design, construction, and maintenance is better in a hospital? That's just my WAG.
- Location
- Illinois
- Occupation
- retired electrician
I would expect that there are very few items that are actually covered by Article 700, and that would be mostly the exit and egress lighting. I would just use "unit equipment" for those loads and then you are not using the generator to supply Article 700 loads.
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