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I have a unique power scenario at my house that I’m considering trying to take advantage of. My house is fed by an underground service that comes from the neighborhood above my property. My detached garage has an overhead service that comes from the neighborhood below my property. It is not uncommon for one grid to be down while the other is still up. I started thinking about the possibility of adding a second ATS and making the detached garage service the 2nd option in the system. Option one would be normal power in ATS 1. Option 2 would be output of ATS 2 running to the generator input of ATS 1 which would leave garage power running to normal power input of ATS 2 and actual generator power connected to generator input of ATS 2. That way, the progression would be:

  1. Does house have utility power? If no: Does garage have power? If yes, ATS 1 switches to supply from ATS 2 which stays in normal power position.

If no, ATS 2 tells generator to fire and moves to generator input position, causing ATS 1 to move to input from ATS 2.

When power to house or garage is restored, either option shuts off generator and utilizes utility power. Once house power is restored, all return to default position.

How would neutrals work? Would communication wires be connected in both ATSs? Other issues I am overlooking?

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    Utility to utility transfer is definitely doable (it's reasonably common in large institutional facilities) but M-T-M or other such bussed setups are only found in switchgear, and most residential ATS controllers are going to struggle with the different programming required to deal with a utility/utility transfer situation (vs utility/generator) Nov 24, 2023 at 5:07
  • Where are you located ? Do you only have one phase? No mixtures of 110 and 220? Nov 24, 2023 at 5:07
  • Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
    – Community Bot
    Nov 24, 2023 at 5:07
  • Both sources are 200 amp, single phase 220.
    – Paul King
    Nov 24, 2023 at 5:56
  • What do you intend to happen when grid power to the garage is lost, but not to the house? How should the garage then be powered, from the house grid or from the generator? Nov 24, 2023 at 10:03

1 Answer 1

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You can do this… but you cannot use a Reliance 6-8-10 style transfer switch. In fact, you'll need to be very choosy about transfer switches, since the transfer switch MUST switch neutral.

Most interlocks and transfer equipment does not switch neutral, because it's cheaper, and the generator is an isolated source and can simply be forced to same neutral potential.

Why do you need to switch neutral? Because your neutrals will be sourced from 2 different electric distributions. If for any reason there is a potential difference between the two sets of neutral, all current needed to equalize them will flow via your transfer equipment if you don't switch neutral. And that flow will be continuous Do your neutral wires have circuit breakers? No, no they do not. So they could flow 2000 amps and nothing would impede that, outside of the wires catching on fire.

So yeah… neutral switching transfer equipment.

And let me add to this that you'll need double cables. That is, you won't be able to multiplex on the same feeder cable, you will need to run two complete sets of feeders. While appalling, this will be cheaper than the complex coordinating control equipment you would need to get it done in one.

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  • Yeah -- they'll also need transfer equipment that's fully programmable so they can set the timings up correctly for utility-to-utility transfer. Nov 26, 2023 at 4:24
  • @ThreePhaseEel good point - I was envisioning dual feeders, but forgot to make that clear. You know better than I but I think the average consumer will find it easier to work with plain old garden variety ATS's and that going to the kit you're talking about could cost orders of magnitude more than a second feeder. Nov 26, 2023 at 20:47
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    It kinda depends on what precisely you're looking at, but there are a few ATSes that are sort-of-available to to the home-gamer that do support that level of configurability. That said, having to switch the neutral automatically puts them out of home-gamer territory anyway...\ Nov 27, 2023 at 1:59

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