I have an old house circa 1730 - much of the wiring has been in for at least 20 years. The pull-switch in the bathroom has broken. I have removed the casing to find two red wires and one yellow/green earth wire.
I am confused by the markings on the casing which don't tally with the modern COM/L1/L2 markings etc. One is marked 'C' and the other is marked 'IN'.
Can anyone tell me if 'C' is likely to be the common/live supply please? Why would 'IN' be used? Thank you Julie

  • Seventeen-thirty?? – isherwood Nov 25 '17 at 21:43
  • The house may be old but if the wiring is only 20 years old, it should conform to modern electrical codes. What do you mean by casing? How are the existing wires connected to the fixture? – Barry Nov 25 '17 at 21:49
  • 1
    Hi Barry. I didn't say the wiring is only 20 years old - I said it has been in for at least 20 years, which is how long I have lived here for. By casing I mean the plastic case/housing which contains the wiring. One red wire goes into a hole marked 'C' and the other into a hole marked 'IN' - both fixed by screws. – Julie Schofield Nov 25 '17 at 22:04
  • 1
    Isherwood - 1730 yes, it's a grade 2 listed building. Don't think the wiring's quite that old though ;-) – Julie Schofield Nov 25 '17 at 22:06
  • 1
    Buckingham in the UK. – Julie Schofield Nov 25 '17 at 22:34

If by "common" you mean what North Americans (and I thought UK also) call neutral (blue, black deprecated, white North America)... A switch doesn't necessarily have neutral/common.

A switch connects two wires: a supply wire that is "always hot", and a "switched hot" wire to the lamp which is hot when the switch is on (lamp is intended to be on).

All the switch is doing is connecting those two wires, or not, depending on how you've thrown the switch. Since it is only connecting, it doesn't care which is which.

Since both wires are hot at least some of the time, it is correct to use a "hot" wire color like red for both.

Where youm may have seen a neutral/common colored wire on a switch is this oddity. Much house wiring is done with twin-and-earth cable, and usually twin-and-earth comes in only one color pair (brown/blue, deprecated red/black, North America black/white). Neutral isn't used in a switch loop, but you're stuck with the wire colors available, so one ends up being black/blue/white even though it's not neutral. However I have heard of twin-and-earth being made red/red for exactly this application. Maybe this was required previously.

In a similar vein they might have required (or chosen) metal conduit. Then they would use individual wires of appropriate colors, and the wires could be newer than the conduit.

Up at the lamp, the lamp connects to switched-hot and neutral. That is the only place neutral enters the picture.

  • Hi Harper, 'common' in the UK equates to the live supply which should be a brown wire (formerly red with older wiring such as mine). Your comment "All the switch is doing is connecting those two wires, or not, depending on how you've thrown the switch. Since it is only connecting, it doesn't care which is which." helps enormously, thank you. I'll bear that factor in mind. Really grateful for your input :-) Julie – Julie Schofield Nov 26 '17 at 10:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.