Generator Sizing

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mkgrady

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Massachusetts
When calculating a standby generator size for a residence where load shedding is part of the ats. do I use the same calculation as sizing a service which considers the demand factor? I would leave out large loads like AC. Range. Dryer. Etc because the ats drops them in and out as the generator can handle the load?

In other words how do standby generators get sized when using the socalled whole house smart ATS?
 
You would use Art 220 to calculate any load that will be connected to the standby generator. But your question seems to imply not just load shedding, but a demand controller. The typical ATS with load shedding, such as a Generac, only does simple load shedding, not demand control. While this can be done it gets a lot more involved.
 
texie said:
You would use Art 220 to calculate any load that will be connected to the standby generator. But your question seems to imply not just load shedding, but a demand controller. The typical ATS with load shedding, such as a Generac, only does simple load shedding, not demand control. While this can be done it gets a lot more involved.
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I meant that it seems like a demand should be considered like it would when calculating a service. It seems the gen manufacturers would have you add up all of the loads even though you would not expect to run everything at once.
 
mkgrady said:
I meant that it seems like a demand should be considered like it would when calculating a service. It seems the gen manufacturers would have you add up all of the loads even though you would not expect to run everything at once.
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I think we are using the term demand differently. If you do load calcs per Art 220 that takes into account demand. But you mentioned "the ats drops them in and out as the generator can handle the load" which implied to me that, say, if the oven came on the ATS would turn off the A/C as an example to limit the load on the generator. Simple load shedding does not have that kind of smarts-it just says if the system is on generator drop X loads. True demand control (that I thought you meant by "drops in and out as the generator can handle the load") is a lot more complex.
 
702.4(B) tells us to use article 220 or "by other approved method".

I use 220 and follow a formula as I would for a service in single family dwellings with automatic transfer, and load shed big loads above the capacity and rating of the gen-set such as air conditioning and electric hot water.

Load shedding would be an example of "other approved method". Other types that may be approved could include manual control of loads exceeding the capacity and rating of the gen-set. The AHJ will be the one to consult on the other approved method.

shortcircuit
 
texie said:
You would use Art 220 to calculate any load that will be connected to the standby generator. But your question seems to imply not just load shedding, but a demand controller. The typical ATS with load shedding, such as a Generac, only does simple load shedding, not demand control. While this can be done it gets a lot more involved.
Click to expand...

Texie, you are over thinking this. :)
 
shortcircuit2 said:
702.4(B) tells us to use article 220 or "by other approved method".

I use 220 and follow a formula as I would for a service in single family dwellings with automatic transfer, and load shed big loads above the capacity and rating of the gen-set such as air conditioning and electric hot water.
shortcircuit
Click to expand...

So in your example above, when sizing the generator you are not including the A/C or the water heater in the calculation?
 
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