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Our house is basically two houses stacked: each floor with its own load center located in the garage. The load centers for each floor are fed from two 200 amp main cutoffs located outside the garage under the meter. Neither inside panel has a main cutoff negating the ability to fit an interlock, but my plan is to run backfeed breakers in each one with #8 appliance cables with plugs attached to junction boxes below and fed up to the breakers with #8 romex. Since each load center has its own main cutoff, there shouldn't be any issues with neutral as they are completely isolated from each other. My generator (13,000W) has a 50 amp and a 30 amp plug on it, so in the case of a blackout, I would kill the outside main breakers, set up the generator outside the garage, plug in the two connecting cables, get the generator up to speeed, then flip the backfeed breakers to "on". I've read other posts where the panels share the utility feed and forced the owner to try to isolate the crucial circuits to one panel because of the neutral complications, but since my panels are independent of each other, I'm assuming my plan is okay.

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    There may be neutral/ground issues - I'm not 100% sure. But lack of interlocks is a major problem. Aside from "doing things out of sequence and killing a lineman", I think there are additional possible problems here because of the double panels. Can you upload a picture of your meter main or whatever it is that has those 2 x 200A main breakers? Dec 28, 2022 at 20:11
  • It is important to LOCK the main breaker in off position so it is not activated by mistake, since the power will flow to outside and kill the neighbor
    – Traveler
    Dec 28, 2022 at 21:44
  • Can you take an inventory of what loads you need/want to run off the generator and which panel they're on? Is moving the generator loads to their own subpanel an option? Dec 29, 2022 at 4:44
  • Your panels are not independent of each other. It would be an interesting but off topic question whether neighbors a hundred feet apart with no shared metal plumbing could share a generator. Off topic because it gets into third party power distribution territory that is governed by different rules.
    – jay613
    Dec 29, 2022 at 21:24

3 Answers 3

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There are no two ways about it: either you have an automatic transfer switch or you have a manual interlock. A checklist item that says "turn off the disconnect outside" will not suffice.

You could replace the two indoor load centers with load centers having a main circuit breaker. It may even be possible that your load centers can be converted from main-lug to main-breaker type without having to be replaced. Show us a photo of the labels and/or interior of the load center and we'll be able to advise on that. A load center with a main breaker provides solid foundation for an interlock.

In any case, once you have the interlock situation resolved, then you can have a backfeed breaker and an appropriate inlet connector to which the generator cable can be plugged. An inlet is the inverse of an outlet: it is male prongs recessed into the wall so that the female end of an extension cord can be pushed onto it.

I didn't quite understand what you were proposing with the #8 appliance cables with plugs, but.. don't do that. The inlet, wired only to the interlocked backfeed breaker, is the proper and safe way to feed local power into the system. A male-prong appliance pigtail/cord screams "dangerous rookie workmanship; your life may be in peril!" even if it actually is wired the same way a proper inlet would be.

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On this forum, we've been around the block on this issue several times. People often want to feed two panels typically because the two panels are the two 200A panels fed by a 400A service.

It cannot be done by any method you'd like to use.

Since each load center has its own main cutoff, there shouldn't be any issues with neutral as they are completely isolated from each other

Sorry. Your main cutoffs do not interrupt neutral. And that's the problem! There is no way to feed two panels from one generator when their neutrals are tied together by the system wiring.

I would kill the outside main breakers, set up the generator outside the garage, plug in the two connecting cables, get the generator up to speeed, then flip the backfeed breakers to "on".

This forum has a Be Nice Policy, and that absolutely prohibits me from providing feedback on this. That is literally all I can say.

I've read other posts where the panels share the utility feed and forced the owner to try to isolate the crucial circuits to one panel because of the neutral complications, but since my panels are independent of each other, I'm assuming my plan is okay.

Well, it's your house, your relationship with your family members, insurance company, local inspector, mortgage lender, and district attorney. You do you. Best of luck. And that is all I can say.

What would Harper do?

First, I'd take a close look at those two master panels, the ones without main breakers. Are they "Convertible"? (to a main-breaker panel). If they are, then absolutely I would convert them and apply appropriate generator interlocks.

If they're not convertible, I'd take note of their "Box Number" or enclosure size. Often that's on a sticker right on the box or "interior" (trade name for the huge bus assembly with everything on it except accessory ground bars). Otherwise I'd look up the model in legacy catalogs and the catalog will state the box number. From there, I'd check the catalog for convertible or main breaker panels that use the same exact box number. Acquire those panels, swap "interiors" and covers. You do have to mess with 3 big wires and all the neturals; you don't have to remove the branch circuit breakers from their wires. Let them dangle.

If that doesn't work, then I'd examine the Load Calculation for each panel and ask "Does this really, really need to be 200A?" Because very often 200A is chosen simply by default with no real reasoning to it. If I can get the Load Calculation down to 125A, those are available as branch circuit breakers. So I can convert it to a "backfeed main breaker" setup, and install an appropriate interlock with the generator breaker. For that matter, a couple of manufacturers offer branch circuit breakers as large as 200A, so interlock allowing, there may not be need for an ampacity downgrade. (though they do take 4 spaces).

Next, I would look at the cost of a 200A transfer switch, the big Frankenstein knife switch variety. I expect them to be pricy, that's why I'm doing them fourth. If possible, get one 3-pole transfer switch and also switch neutral - that will solve the original problem nicely. You only have to switch neutral on one of the panels. (both wouldn't hurt, but it's not necessary, one will suffice).

But how does Harper solve the neutral problem?

Well, the 3-pole transfer switch on one of the panels would suffice. However that's a longshot so let's pretend it didn't happen.

This is where you scour Craigslist for a very particular industrial transformer. This cannot be bought on eBay/Amazon/AliExpress. This thing is a supply transformer, isolating, with a 240/480V primary (jumperable) and a 120/240V secondary (jumperable). The KVA needs to be no larger than the KW the panel is expected to need from the generator. (KVA is similar to KW). 5 KVA is a common size and runs $100-150 used typically. That'll give 20A @ 240V.

7.5 KVA takes a 30A breaker and 10 KVA takes a 40A breaker. "too many KVA" is perfectly fine. So if a 15 KVA unit lands in your lap and you want to put it on a 40A breaker that's fine.

So one of the generator inlets will feed the primary of this transformer, which will be jumpered for 240V (not 480V). The secondary of the transformer is jumprered for 120/240V and feeds the hot-neutral-hot of one of the panels (presumably the smaller).

The other panel just goes straight to an inlet.

And that's it.

Now if you want to, you can actually power the transformer from a breaker in the other panel. That would allow you to have one inlet powering both panels (the second via the transformer).

Also I'd use proper inlets and not do a hick job of it. I prefer it when my home insurer pays.

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You can't feed two panels from one generator because if one neutral fails, and assuming both panels are ground bonded to the same plumbing or grounding rods, current will be routed through the grounds (which has one set of dangers) and then all of it back to the generator through the other panel's neutral (which has another set of dangers). There may be other reasons, I'm always learning more ... but that's one compelling reason I learned when contemplating a similar case.

You need real interlocks. It has to be impossible to backfeed the grid, you cannot rely on your best efforts. If your main panels can accommodate main breakers, even if they don't need them you could install a main breaker on one panel so that you can install an interlock. Otherwise you could install a manual transfer switch between the existing main breaker and its panel, but those are expensive.

A plug in a box, if you make it that simple, will not be to code. There isn't a plug that is designed to be "permanent wiring". You would need two boxes, one of them permanent in nature, and the other designed to be "outside" the permanent wiring, maybe with a lid. What you'd be doing is building an inlet from scratch. It would be awkward and ugly, and catch the eye of building inspectors and buyers' inspectors. An actual inlet is cheap and will look more professional so just buy one.

You didn't ask, but since you can't feed your house from both of the generator's outlets and you can't utilize all its power from the 50A outlet, the only way to utilize all its power would be to 1) Use the 50A outlet, VERY carefully balance the loads so you can use all its available power, and then plug in a single 1kW load to one of the 120V outlets that the generator undoubtedly also has.

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  • “and then plug in a single 1kW load to one of the 120V outlets” … why?
    – nobody
    Dec 29, 2022 at 3:49
  • @nobody -- 50A @ 240V = 12kW, and the OP has a 13kW generator Dec 29, 2022 at 4:43

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