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#191273 - 12/23/09 10:52 AM Sizing Generators
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
When I look at Article 702 Section 702.6(2) in the '08 NEC I think it is saying that when an automatic transfer switch is installed on an optional standby system the generator must be sized to carry the full load. No more relying on the overcurrent protector on the generator to protect the generator from overload. These words were not in the '05 code and they are not shown for Legally Required Standby System. Am I reading it correctly? I see a situation where these small Generac generators (7-15kw) installed at residential occupancies and they are probably not complying with 702.6(2)
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#191277 - 12/23/09 11:23 AM Re: Sizing Generators [Re: George Little]
renosteinke Offline
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Registered: 01/22/05
Posts: 5300
Loc: Blue Collar Country
We just had a thread about this, and I have serious qualms about the 702 requirement.

It comes down to defining the load. I can't put my finger on the source, but I believe that the code somewhere mentions actual measurements as one way to determing loads, as opposed to calculations. I'm just having a senile moment ... but I'd like to be able to use that approach in 702 sizing. After all, you CAN do it that way in 700 and 701!

It seems silly to construe 702 as requiring the generator to be able to supply all the power the OCPD will allow. Good grief, even the PoCo feeders are often not sized that way (they're sized to the calculated load, not the 'next breaker up.')

Sizing the generator to the service size also puts you in the untenable situation of destroying the generator. Simply put, running the generator at light load is destructive, and the regular maintenance/ exercise program will hasten this damage.

In that regard, 702 is in conflict with 110, in that every generator maker wants you to use a different set of design principles. Conspiracy nuts, don't blame the manufacturers for this one; the generator makers want you to use a smaller generator.

I sure hope this is addressed in the next cycle!

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#191280 - 12/23/09 11:44 AM Re: Sizing Generators [Re: renosteinke]
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Reno, can you point me to the "thread" you referenced?
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#191285 - 12/23/09 01:23 PM Re: Sizing Generators [Re: George Little]
renosteinke Offline
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#191312 - 12/23/09 06:27 PM Re: Sizing Generators [Re: renosteinke]
HotLine1 Offline

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Registered: 04/03/02
Posts: 6786
Loc: Brick, NJ USA
George:

"(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
(b) Load Management. Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system. "

Could you not define (a) as the actual load, documented by POCO info (billing), or data from a recorder device?

Most resi gens & xfr switches that I have seen go to a 'sub or 'gen' panel, thru the transfer switch, so the load in the gen/sub panel with a xx amp 'main' to 'match' the OCP on the gen.
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#191313 - 12/23/09 06:32 PM Re: Sizing Generators [Re: renosteinke]
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Thanks Reno, I even contributed to that thread and I must of forgot to keep up with it. I'm embarrassed.

Having now reviewed that thread I guess the code does require a generator to match the load, meaning the connected load when an automatic transfer switch is used. Guess the old adage holds true that the code may not make sense but it's still the code.
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#191318 - 12/23/09 09:35 PM Re: Sizing Generators [Re: George Little]
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
John-I just now saw your post and I think your suggestion to have a sub-panel is a work around that would make the job code compliant. Sorry I missed your post.
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#191327 - 12/24/09 08:47 AM Re: Sizing Generators [Re: George Little]
renosteinke Offline
Cat Servant
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Registered: 01/22/05
Posts: 5300
Loc: Blue Collar Country
A sub-panel, splitting off the 'backed up' circuits from the rest of the house is not a 'work-around,' it's a perfectly legitimate form of 'load shedding.' Then the question becomes one of sizing the genny to the sub-panel.

And, again, the specs of the generator companies come into direct conflict with the 'method' espoused by some in the other thread, a method that seems to be what the code language wants.

Let's say I just split off my well, sump pump, refrigerator, gas furnace, and a couple light/receptacle circuits off to this little panel. Just try finding a 30- amp panel with enough spaces; you're almost certain to have a 100-amp panel. Chances are, the lugs won't even secure #10 wires. The end result is very likely to be at least a 50 amp breaker feeding that panel.

If you size the generator to that breaker, you'll have twice as much generator as you'll ever use. I didn't say 'need,' because this is a completely voluntary system; you don't "need" any generator at all.

So, what's the problem with having a big generator? Simply put, the generator starts breaking things if it's not operated fairly regularly under nearly a full load. Often generators will be installed to automatically operate for a period every month for this purpose.

When the generator 'exercises,' one of two things happens. Most often, the transfer switch automatically switched the house over to generator power for a while. If you have a 30 amp household load being powered by a 100-amp (25kw) genny, that genny is not being properly 'exercised.'

The other way to 'exercise' a genny is to use a 'load bank.' A load bank is simply a series of giant toasters, which progressively switch on until the genny is fully loaded. In our example, the 30 amp house would be supplimented by 60 amps of the 100-amp load bank operating. That's a lot of heat - and in a residential setting, I would have concerns about grass fires, etc.

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#191328 - 12/24/09 09:49 AM Re: Sizing Generators [Re: renosteinke]
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Reno- I happen to have a 13kw generator feeding an automatic transfer switch rated at 100a. Integral with this automatic transfer switch is a MLO panel with 12 spaces. There is no main in the transfer switch nor is it SUSE rated. The other side of the transfer switch is fed from a 60a. breaker in my 150a main panel. When running on generator, my range and dryer and several other circuits are not connected to the generator. I see this as code compliant. As for the exercising the generator, the unit starts every week and runs for 10 minutes to circulate the oil and charge the battery. The load does not transfer during this exercise cycle. On this generator panel is my central air, furnace, microwave, refrigerator, two freezers and three lighting circuits.
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#191331 - 12/24/09 10:31 AM Re: Sizing Generators [Re: George Little]
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5300
Loc: Blue Collar Country
13Kw genny supplying a 60 amp breaker? Personally, I think that's about as close as it gets - though the reasoning applied in that other thread would ask you to reduce that panel feed to 50 amps. Naturally, that also could lead to a discussion of whether to consider the 'surge' rating of the generator as well.

I think it's clear we're well beyond the scope of proper code concerns, and into design issues. Time for the 702 code panel to back off, IMO.

As for your maintenance program ... generator makers have been adamant about the need for the generator to operate 'under load,' which they define as about 80% capacity. This is due to concerns over 'wet stacking.' I'm not entirely clear on that issue; I take their word for it. The reccommended exercising is for a somewhat longer period as well; I think they want everything to become fully warmed up.

To be fair, I haven't installed anything that was smaller than 50kw, and it's possible that these issues are not as critical in smaller units. The units I've installed have typically been set up to run for an hour, under load, once a month.

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