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I would like to add an intermediate switch to an existing lighting circuit. I would prefer not to cut a new hole in my wall for a completely separate switch and so my ideal choice is to replace an existing switch for a different set of lights on a different lighting circuit (separate at consumer unit) with a double switch. So I would then have both circuits run into the same 1x1 switch box.

My main question is, is this allowed based on UK regulations and if yes, could someone direct me to the relevant bit of the rules.

I have a second question, which is basically just a soft version of the above: even if it is allowed, is it a bad idea, such that someone might get a shock from assuming that only one circuit could be going into one switch?

To be clear, the replacement switch I'm thinking of would be something this: tlc-direct double+intermediate switch in one unit

Many thanks

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  • Are there "double switches" that take line hot from two different breakers? The ones I am familiar with accept only one line hot but of course have two independent switched hots. Commented Dec 30, 2023 at 12:16
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    @JimStewart Most double switches in the UK are completely separate from each other.
    – Simon B
    Commented Dec 30, 2023 at 12:27
  • @Simon B, if the switches in a double switch are completely separate, this suggests to me that the UK code and practice allows a double switch to be handling power from two different circuits. Is this in fact the case? Commented Dec 30, 2023 at 13:01
  • @JimStewart I believe it is, and I know I have one switch in the house wired off two separate circuits. But I don't know the regulation in BS7671 that allows it.
    – Simon B
    Commented Dec 30, 2023 at 13:54
  • RE the switch i linked to above, here is a pic of the back: varilight.co.uk/images/back/XDI71S-BACK.jpg you can see that there are 4 terminals for the intermediate and 3 for the 2 way switch + a separate earth which is common.
    – aghsmith
    Commented Dec 30, 2023 at 14:23

1 Answer 1

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The switches exist, because it is allowed. The number of circuits is not the problem. It is common to have 2, 4 or even 6 switches. But multiple phases going into the same switch box can be an issue in some jurisdictions.

  • Again, danger wise, multiple circuits is fine, but not multiple phases, which could substantially increase the voltage.
  • It is possible that you mean that the two circuits are on different circuit breakers. In which case it would be dangerous and is probably illegal where you are. If this is the case, and if your breaker can handle the load (seeing its only lighting), put both circuits on the same breaker.
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  • The UK doesn't have multiple phases in one domestic building. Your second point is clear, though - 2 supplies to one switch is an accident waiting to happen.
    – Tetsujin
    Commented Dec 30, 2023 at 11:36
  • Thanks for the answer. For clarity, there is only one phase involved but there are separate circuit breakers (soon to be RCBOs). I have different lighting circuits for different floors, (5A). The switch I wanted to replace currently is part of a stairway multi-switch setup (so going between two floors).
    – aghsmith
    Commented Dec 30, 2023 at 11:54
  • Don't these double switches have only one terminal for line hot so only one breaker? Or are some of them two fully independent switches which could be supplied by two different breakers? Commented Dec 30, 2023 at 12:13

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