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I plan to install a sub panel next to my main panel. Mainly because as I do electrical upgrades, I'm accumulating more circuit breakers. Since I need one anyway, I'd like to be able to power it with a backup generator during power outages. I'd also like to isolate the generator from the grid with and automatic transfer switch. Something like this: sub panel, generator, and ATS

I would prefer this method over a lock-out plate or manual transfer switch. I can't seem to find examples of people doing this. Does the NEC allow it?

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  • I'm guessing that "NEC" here is the US National Electrical Code, and adding the appropriate tag. Please correct and expand if that's wrong Commented Mar 20, 2023 at 16:34

4 Answers 4

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Sure, this isn't an issue

What you're proposing isn't a problem at all from a Code standpoint. In fact, it's the normal way automatic transfer is done in facilities that have more complex power needs, as it allows for better management of loads (i.e. what's on generator power vs. what's not on generator power) than putting the transfer switch at the service entrance does.

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Don't add an empty subpanel just for new future loads. You should thoughtfully move some existing circuits to the new panel, leaving room in both panels for new future circuits. In the end the sub-panel will contain all those exact circuits that you would want powered during a service outage, and both panels will have room for new breakers.

If you do it that way you can call it a "critical loads panel". It's the best way to do it.

An added benefit that any lights still on the main panel serve to notify you when power is back.

6kW is small for an ATS. The load on the subpanel can never exceed the generator capacity and that has to be managed proactively, not when an outage occurs. That is problematic if you have large critical loads that are not manually controlled. I for example have two sump pumps and two fridges, which to me are the most critical loads. It would be nuts to have a critical loads subpanel and not include those. But if they all decide to come on at once, that's 5kW even without considering all their startup draws. So if I also have some lights, the gas furnace, my router and other things on there, I'm over the limit. I can manage that with a manual switch but with an ATS I can't set it up that way on a 6kW generator.

If you're going to all the expense of an ATS and a suitable generator for it, I suggest you look at a 20kW+ generator. Then you can put almost everything in the new subpanel leaving behind only a few very large and not critical things ... central air, clothes dryer, car charger, etc.

If you prefer a smaller generator you should get a manual transfer switch (which is just an interlock for the subpanel's feed, very cheap). Then you can put more than the generator's capacity on the subpanel, and manage the load manually.

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  • Thank you. A critical loads panel is what I have in mind. The fridge, freezer, and a couple outlets. Nothing that would require a large generator.
    – Mike Gray
    Commented Mar 21, 2023 at 22:04
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First, if you're placing a subpanel near the main panel, I want you to run at least 5 conduits between the panels. If they're next to each other this amounts to short "nipples" they call them. You'll thank me later. The reason is, as long as you're not in Canada, a circuit that is physically wired into panel 1 can leave its ground in panel 1, and then extend all its hot(s) and neutral to panel 2 and land on breakers and neutral bar there. That makes it much easier to "move" circuits to the other panel. Five conduits because if the panels are right next to each other, having some high and low gives you a better chance of reaching with the original wire. If they're more than 2 feet apart, you're limited by NEC 310.15(B)(3)(a) to 4 circuits per conduit. (fewer on 30A+ circuits).

Yes, a "critical loads subpanel" is legal and in fact is mandatory given your requirement for an automatic transfer switch. NEC says the generator must be sufficiently large for the load to be served. With a manual interlock, you can manually cut out large loads and NEC leaves it to you since you are there. However, with an ATS, that may throw over while no competent persons or no-one at all is present. You can't have a situation where the generator bogs and no one can correct that. Since I gather you want to use modestly sized generators, you will need a modestly sized critical loads subpanel.

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A 6KW generator is smallish and I DK if you could find one that is compatible with an ATS. Normally what happens when the power goes out, the ATS sends a signal to the generator to start, then it gives it about 30 seconds to "warm up" and stabilize, then the ATS transfers power to the generator.

I have to disagree with jay613 regarding a couple of things.

First, you do not have to have the generator capacity meet EVERYTHING in the sub-panel since even during normal usage, we seldom use all circuits at full capacity. Personally I have a 90 amp generator connected to a 200 amp transfer switch to a 200 amp panel and have never had a problem. This all passed inspection.

Second: Using a manual transfer switch means you have to go and find a flashlight, wander to the panel and operate the plate and breakers. Not only that, you have to be home to do it. If not there, maybe on vacation, having a fully automated system can save a fridge and freezer full of food from spoiling, and maybe keep the heat on in freezing weather to keep pipes from freezing and bursting.

I do agree with Jay in that a 6KW is smallish and the long pole in the tent would be to find one of that size which would work with an ATS.

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    I think we agree on "everything", I just didn't go into much detail. "Everything" doesn't mean literally everything but the max anticipated load. You must however allow for all the things that might come on by themselves. In my case, two sump pumps and two fridges plus everything else. 90A is plenty for most reasonably planned installations. 6kW is not. I agree also on the benefits of an ATS but it's an x15 cost that is somewhere on my long to do list.
    – jay613
    Commented Mar 20, 2023 at 19:04
  • @jay613 ""Everything" doesn't mean literally everything but the max anticipated load." No... everything does mean everything. That's what makes it different from "max anticipated load". (George and I can't be the only ones who read "Everything" and thought you actually meant everything.)
    – RonJohn
    Commented Mar 21, 2023 at 2:02
  • Thanks both for the comments, I have (hopefully) improved my post accordingly.
    – jay613
    Commented Mar 21, 2023 at 12:50
  • Thank you. The sub panel would be a critical loads panel for just a few things. Maybe 15-20 Amps maximum at any one time. I figured a bit more for startup draw.
    – Mike Gray
    Commented Mar 21, 2023 at 22:10

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