New and Independent Light Switch off Existing Wiring for Outside Lights


Postby andyguest13 » Thu Dec 10, 2015 10:37 am

Hi,
I have installed a pair of outside lights and am wondering the best way to get switched power to them without chasing loads of new plasterwork out.

Here's the setup near where the new cables enter the house, in the hall, near the front door: there is a 2-gang 2-way switch right next to the where the new pair of cables appear, which controls the hall light and the landing light. I am told (father-in-law ex-builder) that I can *somehow* run a new, independent switch off that for the outside lights - but I don't see how. Is it possible?

An alternative I suppose is to have the outside lights come on with the landing light and utilise those terminals. I really don't want to chase loads of plaster out but I am assuming the correct way is to take a whole new feed off the hall ceiling rose.

thanks
Andy
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Postby ericmark » Thu Dec 10, 2015 8:55 pm

It depends how the lights are wired. In the main neutral is not taken to light switches so it would need wiring back to the ceiling rose. Where 12 volt spot lights are used then some times you find the switch back box rather than the ceiling rose is used as a junction box and if that's the case then yes. But unless 12 volt spot lights are used likely answer is no as there is no neutral at the switch.

Be aware often the sleeving on the black or blue wire showing it is line not neutral is missing. Do assume because it's black or blue it is neutral.
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Postby andyguest13 » Mon Dec 14, 2015 1:37 pm

I assume you mean "Live" and not "Line", and that you meant to write "Do NOT assume"....

At any rate, nitpicking aside, am I right in thinking that if I connect a multimeter across the hall light terminals (red & black) it'll show 0v since there isn't both live and neutral? That's what I see, anyway.

How can I (or just "can I") use a multimeter on the 2gang 2way switch terminals to find a pair of live and neutral to run a whole new switch off? That switch has a 2 core wire going into it for the hall light (red=com, black=L1) and the other side controlling the landing light has 3 core into it (red=com, black=L2, yellow=com). Plus some earths of course.

thanks
Andy
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Postby ericmark » Tue Dec 15, 2015 12:37 pm

OK to start with any phase wire plus the neutral is considered as live, In order to separate the phase wire from neutral we call the phase wire line and the neutral neutral in a single phase supply.

The reason neutral is considered as live is should the neutral wire break it will become live and give you a nasty shock. The earth system in the house makes a huge difference and where there is no earth rod i.e. TN supply we tend only to switch the line. With a TT supply we need to be more wary.

However back to your problem if the blue or black wire is switched line rather than neutral then you need to get the neutral from the ceiling rose. There is a huge problem with borrowed neutrals as isolating a supply where the neutral has been borrowed can cause the neutral to become live.

So although it would work if you took a neutral from a socket circuit it would be dangerous.

In this case meters don't help much as they don't have any load. I would say easy way while trying to work it out is use tungsten bulbs. A dim glow on a tungsten bulb is a good indication that it's wired in series rather than in parallel.

I would be 95% expecting to find no neutral. With a two gang switch if both lights are from the same fuse/MCB then using tape of sleeve to identify cables having a link from line feed on one switch to the other should free a wire which suitable marked could then be used as a neutral swapping it in the ceiling rose.
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