Whole house standby generators and 310.12

G

Goggles

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North GA
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Electrician
Does 310.12(B) govern the conductor size of a whole house standby generator that carries the entire load of a single family dwelling? 24kw Generacs are shipping with 100amp breakers now I believe. If that is indeed the case, do I still need to comply with 110.14(C) as the breaker in question is labeled 75c? Or does 310.12 supersede that rule by allowing a smaller conductor for the rated ampacity?
 
In my opinion, as the conductors from the generator to the panel are feeders, then that would apply.

I have seen existing installations where the conductors were sized like that, and presumably passed inspection. I just pulled an old Cummins out and replaced it with a Generac liquid cooled, and the existing wiring was sized like that and I happen to know the original installation was permitted.

However, I do not size them like that. I usually size my generator to be a little bit bigger than what is expected, but I want all of the capacity available.

It is my opinion that 310.12 addresses the fact that there is so much fluff in the load calcs, that it is so unlikely that everything in the house will be running at full capacity all at the same time, that this reduction is allowed.

In my opinion, a standby generator may be called upon to supply near its full capacity, so I do not size that way. And the minimal savings is not worth it to me.
 
I use #2 al SER for generators with 100a breakers with no hesitation.

In my opinion, a service is more likely to exceed rating than a genny.
 
Most generators in this size class are technically undersized by the Article 220 load calcs. Therefore, some degree of load shedding is usually used. When this is the case, I think that the "entire load" thing no longer applies, so it should probably be wired full size.

If the generator were simply sized to the load, without any load shedding, it should apply. But if a plan checker or inspector wanted to make it a hill to die on, was it worth it? The runs are usually not that long.
 
I would say to you first question yes. You did say "entire load", but to be more complete:

1. Generator supplying entire building with no load shedding - yes
2. Generator supply a critical loads panel - no
3. Electrical system has any sort of load shedding - no

Yes you still need to comply with 110-14c, but I don't see what that has to do with anything.
 
Birken Vogt said:
Most generators in this size class are technically undersized by the Article 220 load calcs. Therefore, some degree of load shedding is usually used. When this is the case, I think that the "entire load" thing no longer applies, so it should probably be wired full size.

If the generator were simply sized to the load, without any load shedding, it should apply. But if a plan checker or inspector wanted to make it a hill to die on, was it worth it? The runs are usually not that long.
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I'm wondering why load shedding has anything to do with it. If you are shedding or "locking out" a particular piece of equipment under generator load, it's no longer in the equation. Say I have a house that has an electric dryer and I lock it out under generator load. Whats the difference of not having a dryer to begin with?
 
I tend to agree that the only time you can reduce the generator conductor size is when the generator carries the entire load. Anything else changes the formula
 
frofro19 said:
I'm wondering why load shedding has anything to do with it. If you are shedding or "locking out" a particular piece of equipment under generator load, it's no longer in the equation. Say I have a house that has an electric dryer and I lock it out under generator load. Whats the difference of not having a dryer to begin with?
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Your entire dwelling load calculation includes the dryer.

disable the dryer, you now have a new load to calculate, which load shedding does lessen load but load diversity is no longer the same. The allowance for smaller conductors to supply a dwelling is based on the entire dwelling load and the load diversity that you normally run into with dwellings.

In the end it is pretty likely you don't overload the conductor especially if this is a 100 amp service to begin with but you may not technically be in compliance with the overcurrent protection rules either.

If it is a 200 amp service and you have load shedding (and probably on more than just a dryer) it may be possible you could be supplying up to the 100 amps rating of the generator breaker. You probably selected the unit you did because you wanted that capacity if situation demands it.
 
kwired said:
Your entire dwelling load calculation includes the dryer.

disable the dryer, you now have a new load to calculate, which load shedding does lessen load but load diversity is no longer the same. The allowance for smaller conductors to supply a dwelling is based on the entire dwelling load and the load diversity that you normally run into with dwellings.

In the end it is pretty likely you don't overload the conductor especially if this is a 100 amp service to begin with but you may not technically be in compliance with the overcurrent protection rules either.

If it is a 200 amp service and you have load shedding (and probably on more than just a dryer) it may be possible you could be supplying up to the 100 amps rating of the generator breaker. You probably selected the unit you did because you wanted that capacity if situation demands it.
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I agree you could be supplying up to 100 amps on the generator breaker but you could be doing the same thing with an apartment with a 100 amp service with no generator back-up if the load calculation allows this and just need a conductor rated for 80 amps. If you had a house with a gas dryer or one that's locked out under generator load, what's the difference? The diversity would still be there if you had a gas water heater as it would if you locked out the dryer. Dryer just used as an example and BTW, I always run #3 cu on the 100 amp genny wire. It costs a little more than the AL but pulls WAY easier.
 
LarryFine said:
Actually, dryers don't automatically restart, so they need not be locked out.
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Was just using a dryer as an example. Or let's replace dryer with water heater. A house could have a gas water heater or lock it out under generator load. IMO, that would be the same thing and you would have the same diversity and could size the conductors according.
 
frofro19 said:
Was just using a dryer as an example. Or let's replace dryer with water heater. A house could have a gas water heater or lock it out under generator load. IMO, that would be the same thing and you would have the same diversity and could size the conductors according.
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Agreed. I often find the best method, especially with a service that already has more than one panel, is a separate generator-load panel.
 
It is nonsensical to say that 310.12 should apply if there is no load shedding but should not apply if there is load shedding, i.e. it is nonsensical to require a larger conductor only because there is less load.

If the service is rated 100A then 310.12 can be used because in no case is the feeder required to be larger than the service.

The 110.14(c) part is interesting, and illustrates how the language in 310.12 has gone down the wrong road for the last several code cycles. When it was a table in 310.15(B)(7) there was no confusion about using the table to determine the ampacity but now we have to infer that in order for 310.12 to have any meaningful application.
 
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You guys misunderstood what I wrote. Most load shedding is done automatically. Not "locked out". So the water heater, heat strips, dryer, etc. CAN all run, just that the less important of them will be dropped as the generator reaches 100%.

So the load factor on the 100 amp generator feeders will be much, much higher this way than the typical underloaded 200 amp service.
 
electrofelon said:
Nonsensical, yes. But it's what the code says.
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I disagree. 310.12 says nothing about load shedding. I would argue that the feeder is still serving the entire load, whichever subset of load is connected. It's not the case that a different feeder or source is supplying the loads that are shed. The intent of that language in 310.12 is merely to stop 310.12 from applying to feeders that are smaller than the service or main feeder.
 
Put another way, 310.12 shouldn't stop applying because loads are automatically shed any more than it should stop applying because light switches or breakers are turned off manually.
 
If you have a 100A dwelling unit service and a 100A generator breaker, then 310.12(B) always applies to the feeder supplied by the generator.

So the interesting case is where you have say a 125A dwelling unit service and a 100A generator breaker. Suppose the load calc comes out to under 100A and there is an ATS that transfers the entire load with no load shedding. Does 310.12(B) apply to the feeder supplied by the generator? Seems like it does, as when that feeder is being used, it is carrying the entire load of the dwelling unit.

Now suppose the load calc comes out to over 100A, but the generator has some automatic load shedding so that the load on the generator will be just under 100A. Does 310.12(B) apply to that feeder? Does it matter whether the loads shed are loads that are optional for a dwelling unit, vs load mandatory for a dwelling unit?

Cheers, Wayne
 
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