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I just replaced an existing dimmer switch (with two dimmers). I noticed that the previous wiring used what I understood to be the wrong colour codes.

One dimmer had blue => C and brown => L1. The other had black => C and red => L1.

The blue and black wires are not banded or otherwise marked.

My understanding was that brown (and previously red) were permanent live and should go into C, so for my new dimmer I swapped them around and the lights work the same.

I know that doesn't mean they're safe, so should I keep the original wiring or use the new one based on colour? Is there any way to know for sure which is the switched live?

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  • are you asking which wire is hot when the switches are not connected?
    – jsotola
    Commented Jan 22, 2018 at 17:27
  • Yes. Establishing whether red and brown are really the permanent live wires or whether the wiring was correct and just coloured wrongly.
    – Tim
    Commented Jan 22, 2018 at 17:32

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You figure out the live by using a voltmeter or voltage detector. If a voltmeter you compare it to neutral (or earth will suffice for testing, if present.)

You figure out the switched-live by noting which of the live wires is off with the switch off and dead with the switch missing.

Generally you are stuck with the colors that twin-and-earth come in: blue and brown (or black and red, I assume you're going back to those colors after Brexit completes?) So using a different color wire is not a luxury you have. Thqt's why I own 10 colors of tape, but many installers do not bother especially if the usage is obvious.

If power (always-live and neutral) are coming through the switch, the wire past the switch to the light will need neutral, so its usual neutral wire color must be used; the live color gets used for switched-live. i.e. You will see two blues spliced together in the back of the box and two browns on the switch.

If it is what we Yanks call a "switch loop", power comes to the light and only a single twin-and-earth goes to the switch. Neutral is not in the cable even though a neutral-colored wire is present. One is always-live and one is switched-live, anyone's guess which. The switch either connects them to each other, or does not. Dimmers might care which is the always-live, but if it's wrong, they just wouldn't work.

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  • Thanks. There is only a single twin-and-earth going to the switch, but the always-live and switched-live do work both ways around.
    – Tim
    Commented Jan 22, 2018 at 18:04
  • I established that the original red and brown wires were indeed always-live. I used a non-contact tool instead though.
    – Tim
    Commented Jan 24, 2018 at 10:47
  • I am getting confused as to which wires are where. If you mean the red wire from the obsolete cable and the brown wire from the modern cable, that I'm guessing would be up in the lamp, then sure, that is right/normal.
    – Harper - Reinstate Monica
    Commented Jan 24, 2018 at 16:09
  • In one case I have brown (live) and blue (switched). In a second case I have red (live) and black (switched). So in both cases the wiring was initially the wrong way around
    – Tim
    Commented Jan 24, 2018 at 16:28

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