3-way light switch with additional 1-way required

[Mendip]

I have a single light switch which is part of a 3-way switch for the lounge lights. One cable has earth + red connected to L1 and black connected to L2. Another cable has earth and just red connected to c. All works OK. There is another earth + red cable in the switch which is not connected to anything and I am hoping that it is connected to a cable which hangs out of the outside wall as I want to install a light there. If I replace the single switch with a double and wire up 1 switch as it is at present, how do I wire up the the other switch as a 1-way switch to the outside light using the unused red cable?

 

[Steve]

Get an electrician in straight away - the earths should never be used as conductors like this. The switch system needs rewiring before you do anything. cowboys.

 

[Mendip]

Thanks Crafty1289 but I suspect I may have mislead you when I said earth + red are connected to L1. All the earths are connected together and fastened to the metal switch box - what I meant was that the first cable had an earth connected as above, and a red connected to L1, and a black connected to L2. I assume this makes a difference?

 

[Steve]

ah, my apologies - though you did phrase it badly.

Firstly, there are no neutrals in this switch. You need to get a neutral from somewhere. I would ignore that other "spare" cable - if it isnt connected to anything, and if you dont have any way to find out its continuity and insulation properties.

The best way to wire this new light will be to run a new live and neutral down from the ceiling.

There may not even be any permenant lives in that box either, so this is the way to go.

 

[Adam_151]

I'm guessing the cable that hangs out of the wall outside already has a neutral and its picked up from somewhere else?

Test using a multimeter if the red wire connected to com is live all the time (as opposed to just when the room lights are on), if it is, use another piece of wire to bridge it to com of the second switch, and put the wire that goes through the wall into L1 of this switch

 

[Steve]

that com may be perm live or switched live (its 50/50)

And the wire hanging outside - is it actually exposed to the elements? How long has it been like this? What sort of wire is it?

 

[Mendip]

The wire outside is 'open to the elements' and according to the person we bought the house off has been like that since the house was built in 1980 - there was meant to be an outside light there but they put a window in instead of the patio doors on the plans! The wire is red/black/earth (1sqmm?) in grey sheath.

Generally, all the wiring seems of a high standard in the house, and if as I suspect the neutral is connected somewhere to the other neutrals (everthing is buried behind a high vaulted T&G ceiling!), then if the com isn't permanently live (I'm getting a multimeter tomorrow to test it) do I have any way to connect this unattached red wire or do I somehow have to trace it back through the ceiling or start again!?

If the com is a switched live and I cross connect the coms and connect the unattached red wire to L1 in the second switch should it still work as long as I have the lounge lights on? If it should, then at least this would prove that it is the wire I'm looking for to activate the outside light.

 

[Steve]

It sounds like it should work if you connect it as you describe. It sounds like they were going to put a light there, but forgot for whatever reason. This being the case, the 2-way switching should have been wired so that the com in this switch is permenant live - making reconnection of this outside light easier.

However, the wire thats exposed outside may not now be in a useable condition, having been exposed to the elements for 26 years You may have to trace this back to its junction box and replace it.

 

[Adam_151]

Generally, all the wiring seems of a high standard in the house, and if as I suspect the neutral is connected somewhere to the other neutrals (everthing is buried behind a high vaulted T&G ceiling!),

Might be wise to check this with your multimeter, also verify continutity of the live wire to the wire in the switch, and earth to anything 'earthy' like the backbox of the switch you have open

then if the com isn't permanently live (I'm getting a multimeter tomorrow to test it) do I have any way to connect this unattached red wire or do I somehow have to trace it back through the ceiling or start again!?

Unfortunatly, if its not permantly live, there is not a lot you can do (in some arrangements you can pick up live from one of the 'strappers' but not the way yours is wired) however the fact that someone has set it up ready for an outside light, makes it seem likely to me that there will be permant live in that switch box

If the com is a switched live and I cross connect the coms and connect the unattached red wire to L1 in the second switch should it still work as long as I have the lounge lights on?

Yes, thats correct

If it should, then at least this would prove that it is the wire I'm looking for to activate the outside light.

Connecting cables that you haven't properly identified isn't a particulary wise stratedgy, things sometimes go bang

 

[Mendip]

Thanks to everyone who has replied with such useful help and information. One final question - if the cable that is exposed to the outside has deteriorated over the 26 years, would this apply to the whole cable?, only there is at least 30cms of cable slack which has not been exposed, sufficient to make a new connection to the new outside light.

Thanks again.

 

[Steve]

use the slack - this bit should be ok. PVC tends to crack and deteriorate over years outside - sunlight and water get to it.