I have removed an old dimmer switch to find 3 black wires and 3 red wires. The black wires were connected together as per the photo. The single red wire was in L1 and 2 red wires were in L2. I tried this on the new dimmer but it didn’t work. I then swapped them round but again it didn’t work. I’m assuming I must have a live and a neutral so one red and one black? If that’s correct then can I just join the other reds together and the other blacks together?enter image description here

  • Pictures of the old switch wiring? Is the new switch "smart?" Dumb switches don't need neutral, "smart" ones do, usually. Where on the planet - looks like old UK/Commonwealth 230V wiring...
    – Ecnerwal
    Commented Dec 29, 2023 at 16:00
  • I will assume that black is neutral and red is live/hot. In this case one of the reds connected together is live and the second one goes to another light/receptacle. The single red should be your switch live going to the light.
    – crip659
    Commented Dec 29, 2023 at 16:00
  • 3
    Wire colors vary around the world. Neutral wires are not necessarily present (because they are not always needed) in switch boxes. Many different possibilities here, some according to code, some not. What country are you in? Commented Dec 29, 2023 at 16:04
  • I’m uk based. All the downstairs lights are on the same ring circuit.
    – Duncan
    Commented Dec 29, 2023 at 20:35
  • The UK doesn't use rings for lighting, only for sockets. tbh, I wouldn't attempt to try diagnose that without looking at every switch & ceiling rose on that floor, & testing with a multi-meter… to try figure out what on earth they's done. There should only be two/three wires in there [1 or 2-way]. The only time you should have cables connected to each other but not the switch is if it is part of a 3-way switch intermediate, when it would be a carrier. I definitely wouldn't expect to find a neutral in an old UK light switch. It should be in the ceiling.
    – Tetsujin
    Commented Dec 30, 2023 at 13:35

1 Answer 1


The light needs one (switched) live and one neutral.

A dumb switch (i.e. any switch that is "old") does not connect to neutral at all. A dumb switch has live, and switched live. Live comes from the "consumer unit" via fuses or breakers, and switched live goes to the light, which has an unswitched connection to neutral to complete the circuit and light the light when switched live is on.

Either the live (more commonly) or the switched live (or both) may be more than one wire - either to supply two lights with switched live from one switch, or to supply some other device with unswitched live.

So, the most common setup for what you show, assuming old-UK-style wiring is that you have unswitched live (two reds) switched live to light (one red) and neutrals (3 blacks.)

If the new switch does not work the same as the old switch, something is different about it. The most likely thing these days is that the new one is "smart" and requires a neutral, as well as live and switched live; but you have not specified. If the old switch worked, a completely equivalent new switch should work exactly the same.

  • The old switch worked, but the bulbs which were not led blew and when replaced with led ones which I know worked, they didn’t. The new switch is led dimmable.
    – Duncan
    Commented Dec 29, 2023 at 20:38
  • …bur are the LEDs dimmable? Most aren't.
    – Tetsujin
    Commented Dec 30, 2023 at 9:33
  • They were sold as dimmable
    – Duncan
    Commented Dec 30, 2023 at 10:01

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