Generator quetsions...

jscozzNovember 11, 2012

Finally installing a Kohler 20RESA standby generator. I have a couple code related questions before I submit my permit paperwork...

I have all loads that I want backed up on a separate subpanel already. 150A panel (mainly for the extra slots), but fed by a 100A breaker off main panel.

1) I am hearing different interpretations about how the loads in the subpanel, the ATS size and the generator size need to correlate to meet code. The Kohler 20RESA will do 75A on nat gas. The backup power subpanel is a 150A panel (30 slots), 150 main breaker, fed from main panel by 100A breaker. Max normal load over a week (I have power monitors on this panel) is about 7kw. If I turn everything on it approaches 12 kw. I have read there is some code requirement that the generator or xfer switch has to be able to handle ALL loaded in a subpanel? Is it just xfer switch? 20kw generator is realistically more than enough for me and what I want to run, knowing that most loads to not run simultaneously... does code dictate whether I use a 100A xfer switch or 200A switch? And whether I need load shedding devices on my major loads to keep total under 18kw IF all loads were on at once?

2) Is a ground rod always required at the generator? I assumed the equipment ground was not switched in the ATS... and that the generator was automatically tied to building equipment ground and the existing ground roads at my meter socket. I thought only stand alone generator installations (not tied to grid power or xfer switch) needed a ground rod... am I wrong?

Thanks for your help!

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Ron Natalie

If you don't have an AUTOMATIC transfer switch, then you don't need to worry about the size of the generator other than it has to be big enough for what loads you need. They assume you can manually shed the loads while you are manually activating the switch.

If you have a automatic transfer switch, the generator needs to be large enough for the entire automatic switched load. Of course, my idiot engineers just use the full rated ampacity of the service which often ends up greatly oversizing the generator.

    Bookmark   November 11, 2012 at 9:21PM
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It will be an automatic transfer switch.

I guess I need to figure out how CODE says the TOTAL load is calculated. It does not make sense to use the subpanel main breaker, or the sum of all subpanel breakers... since there is no load that draws it's full breaker current. And many loads like receptacle circuits are ZERO unless something is plugged in.. so adding 20A of generator size for each receptacle circuit is ridiculous. How does code allow "TOTAL" to be calculated? How about real-world load measurements on the panel?

As I said, my subpanel with backup loads is 150A panel, 100A feed from main panel breaker, all breakers in the panel add up to 500A (I overdid the circuits)... but max draw over the course of a week is about 7kw (about 30A). It would be ABSOLUTELY RIDICULOUS for code to require me to put in a generator that could supply 500A.

    Bookmark   November 11, 2012 at 9:50PM
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Ron Natalie

It is neither the size of the main breaker/feeder breaker for the subpanel (though that is an upper bound) nor is it the sum of the breaker sizes (which is NEVER really a good load value for load computations. You need to do a proper load calculation.

    Bookmark   November 11, 2012 at 11:03PM
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" How does code allow "TOTAL" to be calculated? How about real-world load measurements on the panel? "

There are a couple different methods described in the code.

Actual loads that can run concurrently can be added up (either heating or cooling, but not both), then required loads, then an allowance for square feet of lighting load is one.

There is no provision in residential for measuring since there is no way t determine if that is a max, min, or typical loading.

    Bookmark   November 13, 2012 at 1:48PM
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If it were me, I would get a 100-amp manual transfer switch since that is the load the panel gets from the main panel. If you increase the size from the main panel (increase the breaker and the wire size), then you would need a new switch.

The generator side of this switch would be 80 amps.

If you're going to do an automatic switch, then you'll need a bigger generator, like 25kw or a bit more. If you have things on this panel with high starting loads (like AC or a HP), then larger still.

    Bookmark   November 13, 2012 at 6:07PM
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I have a 200amp Main panel and a 200amp auto transfer switch. My family knows not to run dryers and the electric stove at the same time simply because the generator only produces 12kw.
Its capable, I just want to avoid running it at maximum capacity.
This set up was approved by the local town inspectors.
12kw has proven itself capable of managing a water well pump, dryer, lighting, range-but as I say, never run them all at the same time.

    Bookmark   November 13, 2012 at 8:14PM
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