Add an electric socket to an existing light circuit (230v AC)

Question

I have an existing single light controlled by a single switch in understair storgae space. Now I need an additional electric socket there as its an ideal location to power my security alarm. (No heavy load, its just a 12v adaptor to be plugged into the 230v socket).

I tried to open the existing switch and it has a single chorded cable with 3 wires in it. With the line (Brown) goes through the switch and remainig neutral(Blue) and earth (Green/Yellow) goes directly to the bulb.

Can I safely add a socket to this circuit ? Please kindly help with the connection details.

Many thanks in advance. This is for house located in UK.

Here is the connection diagram, I am planning to get the result. Is this is fine without any issues ???

Peter Green · Accepted Answer

The first thing you need to do is find out where the light is fed from.

fed from a lighting circuit:

Putting a 13A socket on a lighting circuit is not expicilty prohibited. However there are a couple of things to bear in mind.

Lighting circuits are normally on a 6A breaker, plugging in a large appliance my trip the breaker.

Sockets intended for general use with a current rating must be RCD protected. Unless the consumer unit was replaced in the last few years the lighting circuit is unlikely to be RCD protected.

If it's possible to get a cable back to a socket circuit without unreasonable effort then that is what I would do. If not I would fit the socket on the light circuit but clearly label it with something like "burgler alarm only, fed from lighting circuit".

If you do go for the socket circuit option remember to follow the normal rules for that (which depend on exactly what type of socket circuit it is)

fed from a socket circuit via a fused connection unit:

This is similar to above except you will be more likely to have RCD protection in place.

fed from a socket circuit without a fused connection unit:

You need to add a FCU to procide overcurrent protection. Cable from the socket circuit to the FCU should be at least 2.5mm2. I would advise fitting a 5A fuse though you may be able to fit a 13A one depending on the type of light fitting and the size of any wiring.

TomS · Answer

Basically you can, of course. You have everything you need. But I STRONGLY suggest asking someone with deeper knowledge in 230V house installations. Honestly speaking, if you don't know how to wire that, it does probably make no sense to do that yourself.
I don't know the rules in the UK, but in Germany there are some rules to follow. For example, you might need to use special sockets if there's high humidity in your basement. 
You should know which cables to use, I am not sure which rules you have to follow, and if you don't know either, please ask someone who does the job for you.
Even though you are only connecting a 12 V power adaptor, it still is a full 230 V socket which someone else might use to connect some heavier devices.

Sorry for the negative answer, but please see this as friendly advise.

Transistor · Answer

It sounds as though you have phase, neutral and earth available to you. So the initial answer is yes but there are two snags:

UK regulations require an RCD/ELCB (earth leakage protection) device on all socket circuits.
Someone could plug in a high current device into the lighting circuit.

Best way out of it would be to tap off L, N & E through a low current RCD and to a 13 A socket. That would address both problems. I had a look for low current (2 A) RCD but can't find any in RS or Farnell catalogues.

AndyT · Answer

The live cable will be a radial from the consumer unit, hence you do indeed have a live and a neutral. Lighting cable in the UK is normally 1.5mm diameter, suitable to carry 15Amps. Cable for electric sockets is normally 2.5mm diameter, suitable to carry 20Amps. All single sockets must be capable of taking 13Amps. Therefore a single socket on a 15Amp cable would take up most of its capacity. Sockets must also be protected by a Residual Current Device (RCD). It depends on how recently your consumer unit has been installed as to whether your lighting circuit is protected by RCD or not.

Hence I think it unlikely that you could safely (i.e. within regulations) add a socket.

As a possible alternative: It is possible to wire things directly into a lighting circuit, rather than using a socket; bathroom fans are an example of this. But you won't be able to do this if you need an adapter, as you are unlikely to be able to wire the adapter in directly. (I guess it might depend - my laptop adapter has a normal plug, and the transformer itself is separate, hence I could cut the plug off and wire it in. My mobile charger has the transformer built into the plug; hence I wouldn't be able to cut off the plug and still have the transformer.)

For anyone reading this question who may have a slightly different wiring setup:

If there is only a single cable into the box, where the brown goes into COM on the switch plate and the blue goes into L1, the blue is switched live and is NOT neutral. If this is the case you most definitely cannot safely add a socket.

Add an electric socket to an existing light circuit (230v AC)

4 Answers

fed from a lighting circuit:

fed from a socket circuit via a fused connection unit:

fed from a socket circuit without a fused connection unit:

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