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After a recent blizzard, I realized how SOL I'd be if I lost power, and purchased a 4500 watt, 120v inverter generator (3500 running watts). One of my requirements for a generator was "mobile" so I could lift it into the back of my SUV and take it to family/friends that need help. At under 80lbs, this one fit the bill. 240V generators were much heavier, and I have no critical 240V circuits.

I have 3 critical loads:

  • 120v furnace fan pulling ~550w (NG Furnace & NG HWT)
  • Fridge (200w)
  • Backup sump pump (Battery trickle charger, minimal)

I have a regular NG hwt. I would also like to be able to have some creature comforts; typical LED lights, internet, TV, functional garage opener, computers, all combined would barely add an additional 500w.

As extended outages are rare, my initial thought was to run extension cords, but this would be janky and annoying; would need to put in a plug/outlet for generator, pull fridge out to access plug, have wires all over the house, and not have any hardwired lights.

So, I thought an interlock would be my ticket, with a little game-day common sense: no toasters, or hair dryers, or dishwasher; I'd keep obvious no-touch circuit breakers flipped off. With 30A available I'd likely use 10A - 16A.

A licensed electrician took a look at my panel today and said, "no problem, just need to move a couple breakers to tandems to make room for the generator breaker" but then I showed him my 120v generator and he said "problem - you might just be electrifying half the box." He left and said he would have to do some homework.

I've done a little reading and on a basic level I understand the concept of MWBC/shared neutral but I don't know how I could determine:

  • do I have any MWBCs? How do I know for sure?
  • if I do have MWBCs, I could probably make do with half the breakers, I'd just have the sparky move a couple critical circuits around if necessary. But if this is the case, how can I determine whether a particular circuit is on which leg?

200amp service

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  • Most panels switch legs. The first(single) breaker would be on leg one, the next breaker right below it would be on leg two. All of your 240v/double breakers seem to be for 240v circuits, so do not think you have MWBC circuits.
    – crip659
    Jan 11, 2023 at 23:22
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    That's a tidy panel! Jan 12, 2023 at 1:45
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    Thanks - it's a big reason we bought the house! My first home almost burned down twice thanks to AL wiring and the previous homeowner that thought he understood residential electric. When I saw this panel I knew the house was wired with care.
    – user162124
    Jan 12, 2023 at 1:49
  • Say, would breaking the standby loads out into their own subpanel be an option for you? Jan 12, 2023 at 5:08
  • Certainly many options are available, but I'm trying to keep this under a relatively moderate cost, especially as it's a "may never use" emergency option. I would rather roll with an inexpensive interlock kit and some common sense restraint in the event of an extended outage which has only happened once in the past 10 years or so.
    – user162124
    Jan 12, 2023 at 13:06

2 Answers 2

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At first glance, it appears that you do not have any MWBCs. There are two tell-tale signs of an MWBC:

  • A 15A or 20A pair of breakers (double breaker or handle-tied) with black and red from one cable. I don't see anything like that - all the double breakers are larger, which is typical of regular 240V circuits, which you already know you can't put on the generator.
  • A set of wires from one cable (which would be black and red) or from one circuit in conduit (could be any two colors except white or green - e.g., black and red, black and black, red and red, red and blue, etc.) that go to separate breakers.

The rule about handle-tied or double breakers for MWBCs is from the 2008 NEC. Which was not adopted immediately, so a 2008 house (or even a few years later) might have MWBCs not handle-tied. But relatively unlikely.

The only way to tell for sure is to trace the wires going to each breaker. However, if your wires (except the really large ones) are in cables then it looks like you're OK.

And next to how to deal with the generator. Based on the labels on some of your breakers, it appears you have a Siemens panel. If that's the case (it isn't necessarily - sometimes people have a mix of breakers in a panel, and that can be a problems if they're not the right breakers for that particular panel), you can install a 30A 120V breaker, and install it in the appropriate place for an interlock with the main breaker. That will match your 3500W 120V generator.

However, there is a bit of a problem. If you rule out any MWBC issues, you should be able to connect the generator to both legs of your panel. However, you can't do that (as far as I know) using a double breaker because then you could have (for example) 25A on each leg, adding up to 50A total but not tripping the breaker because each leg would allow up to 30A based on the breaker. On the other hand, you don't want to use a 15A double breaker (which would protect the wire) because if your loads aren't balanced and you pull 25A all on one leg it will trip the breaker.

It may be OK, if there is a breaker in the generator (presumably 30A) protecting the wires, but I am not sure if that is code compliant or not.

In addition, there is the question of how to wire up the inlet. You would probably have to use a 30A 120V inlet, which it turns out is a fairly common thing for RVs. But that would mean that in addition to changing the wiring at the breaker if you decided to get a 240V generator, you'd also have to switch to a different inlet. That being said, I would at a minimum put in 3-wire (plus ground) cable from the inlet to the panel, so that someday when you decide to upgrade to 240V power you would be able to do so relatively easily.

I really do recommend a 240V generator. But I see that there is a cost to that - a 240V inverter generator costs quite a bit more than a regular 240V generator or a 120V inverter generator.

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    Really appreciate your thoughtful comments. I would have gone 240v if I had a need for that capacity and weight wasn't an issue. As it is, I can just manage to haul an 80lb generator into my trunk without throwing out my back. Based on your comments above, unless sparky figures out a solution to the problem you described, I might be best served getting my required loads onto 1 leg and using a single 30A breaker, then living with half the circuits.
    – user162124
    Jan 12, 2023 at 1:33
  • Wheels and a ramp?
    – Trout
    Jan 12, 2023 at 2:02
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    @Trout In theory yes. In practice, not so much. My generator (~7,500W 240V not inverter) is heavy - I just looked it up - 230 lbs! Which is typical for that capacity. Not easy to move even a few feet, even though it has big wheels on it. I consider it basically permanent - the most it moves is a few feet one way or another in my backyard. Jan 12, 2023 at 2:27
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Sure you can, even with MWBC, although your electrician is correct about you only energizing half your panel with the 120V generator. I would advise against trying to feed both legs with the same 120V generator. Instead, install everything from standard parts exactly the same way you would for a 240/120V generator so that future owners (and future you if you decide to get a new genset someday) will be able to simply plug in and go without having to change wire.

If they are installed and working correctly now, then your MWBCs will work fine with the exception that only one of the two grouped circuits will be energized while you are on generator power. The neutral will carry the full current of the energized half, just like it does under normal circumstances if the load was unbalanced.

I recently installed a subpanel for some new circuits and decided to set it up as a critical loads panel with a generator interlock. I already have a 120V 4kW generator, but I'm wiring the entire thing for 240/120V, knowing that if I get a 240/120V generator in the future I won't have to change anything. I'm just making sure that my really critical loads are all on one bus, and my nice to have loads are on the other. If the power goes out tomorrow, my really critical stuff is covered. If I upgrade to a bigger genset that provides 240/120V, then I get the rest as well.

Panel upgrades

  • Panel: SqD Homeline 20-space (with main breaker - breaker required for the interlock)
  • Generator breaker: 30A 2-pole in space 2,4
  • Interlock/holddown kit
  • Standard 30A 240/120V generator inlet (NEMA L14-30P)
  • 4x 10AWG conductors (L1,L2,N,G)

Generator

  • Champion 4kW 120V generator
  • NEMA TT-30P to NEMA L14-30R supply cord

The panel, breaker, interlock, and inlet are all standard parts for a 240/120V generator hookup. The wiring is standard as well. The only difference is my supply cord which only provides L1/N/G.

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