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I am looking to hook up my power generator to my house in case city power goes out. I have 3 phase power supply from the city and 1 phase generator. I have connected my generator using rotary switch (like this https://www.reichelt.com/bg/en/rotary-selector-switch-2-poles-on-off-on-bac-2p-on-off-on-p236359.html) using this scheme:

Switch contact    Line
--------------    -----------
Common A          L1 (house)
Common B          N  (house)
1A                L1 (city)
1B                N  (city)
2A                L1 (generator)
2B                N  (generator)

The idea is to switch home power circuit to city supply by turning the switch to 1, to generator - by turning it to 2. When I switch to 1 - Common A connects to 1A while disconnecting from 2A, etc. However how do I connect L2 and L3? Can I leave them permanently connected to the home circuit or should I better use similar switch as for L1?

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  • Can you post a photo of your distribution board please? Jan 26, 2022 at 12:33
  • 1
    I haven't looked at the details of the switch, but will it prevent your generator power from feeding back out to the city's lines? You don't want to be responsible for pushing the power into a line that the linesman thought was not energized, zapping him and killing him.
    – FreeMan
    Jan 26, 2022 at 12:58
  • State where you are. Most users here assume US electrical code that is absolutely irrelevant if you are in a 230V country with 400V 3-phase grid.
    – fraxinus
    Jan 26, 2022 at 13:21

2 Answers 2

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Deducing from your URL, you are in Bulgaria, so 230V grid and 400V between phases (or 220 and 380V for us older people).

  1. Be absolutely aware that your 3-phase equipment (if you have some) will not only fail to work, but also can be damaged if powered from 1-phase supply.

Most houses don't have any 3-phase outlets even if subscribed to 3-phase service so this is usually not a problem.

  1. The switch in your link is not of any use. You will need a 3-phase switch (it will have 12 terminals). You should absolutely disconnect everything from the utility grid when you switch to a generator.

  2. At the generator side of the switch, just tie L1 + L2 + L3 together. You will get 230V wherever it is expected and zero between phases (instead of 400V).

Of course, if unsure, hire an electrician.

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  • Thank you for your ideas. I do not have 3-phase outlets nor 3-phase equipment in the house. 3 phases are used to connect 3 different circuits in the house. I.e. L1 - lighting and power outlets on the 1st floor, L2 - on the 2nd floor, L3 used for power equipment (electric stove, fridge, etc.). Are you still saying that I can not connect L1 via the provided switch while leaving L2, L3, N and GND connected to the house?
    – ggv
    Jan 26, 2022 at 15:26
  • 2
    You should be either connected to the utility grid, or be connected to another power source (e.g. generator) but NEVER, EVER connected to both! It is up to you if you want to power all circuits in your house off the generator, but the rule is as simple as that - first disconnect everything (all 3 phases together with the neutral) from the utility and then connect whatever you want to the generator. Otherwise, you risk leaking power from your generator to the (obviously defunct) utility grid and (in the worst case) electrocuting the utility workers who are fixing the grid for you.
    – fraxinus
    Jan 26, 2022 at 15:44
  • Should also compare the size of generator output to the amount of power you use. Unless you got a whole house sized generator, then you probably can only power certain devices at a time, and some you cannot power with the generator(required too much power).
    – crip659
    Jan 26, 2022 at 17:40
  • OK, so the solution is to disconnect all the 5 wires from the grid and connect L1 and N of the house circuit to the corresponding wires of the generator. How will I then know when the power from the grid is restored? There must be some neat solution as this situation is very common in many houses.
    – ggv
    Jan 26, 2022 at 20:00
  • You can connect an indicator lamp (one or even 3) connected to the utility-side Or small voltmeters. Or even have a separate breaker set that is not switched between utility/gen and are always connected to utility. Just do your best to never interconnect anything between the switched and non-switched part.
    – fraxinus
    Jan 27, 2022 at 8:19
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DO NOT DO THIS! You'll blow up your house.

Here's the problem. It's subtle.

By disconnecting neutral from utility, but not disconnecting the other two phases, you create a "Lost Neutral" situation between those phases.

There will be 400 volts between L2 and L3. However, there will be nothing keeping either one of them at 230V. All the loads on L2 (together) will be in series with all the loads on L3 (together). In other words, L2 will act like a split-phase North American system, but with 400V at the ends, and absolutely nothing holding neutral in the middle. And then, it will behave like a "Lost Neutral" scenario.

So the two phases will add up to 400V, but might be 100/300V or 50/350V depending on the loads on the two phases. Obviously that is more than they're rated for, so it will burn stuff out or set it on fire.

Mind you, all this is happening on L2/L3 after utility power returns. Up until that point, everything will seem like it is working.

The simplest answer is "Don't switch neutral".

That keeps neutral attached to the utility, so phase L2 and L3 will stay at their proper 230V-to-neutral voltage.

You would connect it to the generator also, but you'll need to comply with local Codes as far as whether the generator has its neutral and earth jumpered together.

If those codes require. you to switch neutral, then by golly, you'll need a 3-phase switch (4 pole). Leave earth connected. Connect the generator on 1 pole and neutral. The switch interrupts the other 2 poles just to keep the above problem from happening.

"I can just use 2 switches right?" No. They must throw together as one action.

That switch isn't nearly big enough

The problem isn't how much power will flow through it on generator.

The problem is how much will flow while on utility. It needs to be rated for full utility current - whatever your circuit breaker is.

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  • OK. But how will I know when the power is restored? Connect relay to grid L2 and N and have a buzzer sound ?
    – ggv
    Jan 27, 2022 at 6:29
  • @ggv at night that's really easy - look out the window to see when the neighbors' lights turn on. During the day, listen for their TVs to suddenly start making noise again.
    – FreeMan
    Jan 27, 2022 at 14:44
  • @GGV I would expect more of a "bang" and then the smell of burnt electronics. Wait, that was why you wanted to leave L2-L3 on the utility? That can work, but you have to find a connection method that doesn't disconnect neutral from utility then. At worst, an isolation transformer if that is the Code issue. Jan 27, 2022 at 19:13
  • @Harper-ReinstateMonica Yes, exactly ! I was thinking I can have all the neutrals connected?
    – ggv
    Jan 28, 2022 at 6:40
  • @FreeMan Well, the problem is that there no houses close by. So I can't really see if they have their lights on during the day time. Not to mention TVs making noise :)
    – ggv
    Jan 28, 2022 at 7:43

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