Adding an exterior light

30 Mar 2015
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United Kingdom
I'd like to add an exterior switch operated (perhaps PIR in the future) light. I only have a socket nearby which is likely to be accessible. The light switches and ceiling pendants are further away and less accessible.

This socket is on a ring. Can I add a 3AMP FCU to this existing socket and then wire a light into this?

Assuming yes, I have a few specific questions please:
- Do I take a 1mm or 2.5mm cable from the existing socket into the new FCU?
- Should the FCU situated next to the socket and at that level or next to the light switch?
- I was planning to take the live from FCU to the light switch and then a single cable outside to the light fitting. Is this recommended?

Thanks in advance.
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No problems with any of what you are suggesting. Cable from existing to FCU needs to be 2.5mm. From the FCU to switch and light can be 1.0mm or 1.5mm. I.5mm T&E is generally used, because it is more robust.

You can place the FCU generally where you like, but have you considered using a switched FCU? - No need for a separate switch then.

If you really meant that you were placing the switch outdoors, then you would need a special outdoor switch.

The PIR can either be in parallel with the switch, or in series with the switch. First method means the light would come on when triggered, irrespective of switch position. Second method only allows light on, when the switch is on.
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More than one way to achieve what you want.
If donor socket is part of a ring final circuit ,you can spur from it in 2.5²mm Twin and Earth cable to a switched FCU. That can then feed the outside light and its switch used to operate the light.
Is the socket circuit protected by an RCD or RCBO ?
I would suggest using triple and earth from switch to lamp to allow for future PIR operation with full ON-OFF-PIR function from the switches as in this diagram.

Also provide a two pole isolater ( Live and Neutral ) to provide a means to fully isolate the lamp in the event it gets damaged and/or waterlogged. The FCU will allow isolation of the Live from the lamp but a Neutral to Earth fault in the lamp will still trip the RCD protecting the socket circuit you are taking the supply from.
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No need to use 1.5mm from FCU to light. It is sometimes done due to ignorance.

Bernard I understand modern switched FCUs use Use double pole switches.
No need to use 1.5mm from FCU to light. It is sometimes done due to ignorance.

Bernard I understand modern switched FCUs use Use double pole switches.
Aye that- all the FCUs i've bought the last 5 years have been DP.
OP- you're usually better off using flex from the FCU to the lamp- T & E doesn't work well with round glands. You can get 4 and 5 core 1mm flex (to give you the switching options above)
Thanks for all the response.
To clarify, the light switch will be situated indoors and the light fitting will be outdoors. Based on the above, I plan to use a switched FCU as my means to switch the light on/off.

I did some digging today and the plot has thickened...
Please note that this is a Victorian property and here is an image of the Consumer Unit:

When I unscrewed the socket in question, I noticed that it only has one set of 2.5mm T&E cable.

Below the floorboard I noticed a junction box which has the other end of this cable along with a couple of other cables. One cable appears to be heading towards another socket and the remaining cable, I believe is a spur from another socket.
I unscrewed a different socket in the house and noticed that this does have 2 sets of 2.5mm T&E cables. None of this is my doing I should add!

Would it be possible for me to add another 2.5mm T&E into this junction box to feed my Switched FCU to operate this external light?
You will need to establish, by testing, if the other two cables in the JB are two ring final cables.
Do you have a multimeter ?
You will need to establish, by testing, if the other two cables in the JB are two ring final cables.
One of the other cables is definitely another socket as I also unscrewed that to confirm that it has a single cable. I am therefore confident that the 3rd and final cable is the live cable from another circuit - presumably on a ring.

Do you have a multimeter ?
No. But I do own a 2 Pole Tester with a Voltage Reading display
Does your two pole tester have a continuity function ?
If a socket has only one cable it is a spur ,or possibly the circuit is a radial circuit and it is the last socket.
Yes, it has a continuity function.

I suppose it could be a radial and perhaps the socket I found two sets of cables in, is the one that feeds the JB....
Is the circuit protected by that 32 amp MCB marked sockets ? That would be unusual for a radial circuit wired in what appears to be 2.5mm² cable.
It's quite possible that originally it was a ring final ,and over the years has been added to ( badly) and that can produce some spidery legs and spin offs !!!
You may have opened a can of worms !!
Yes. the 32 amp MCB protects all the sockets in the house. My parents have owned this house for over 30 years and aren't aware of any electrical changes.
Is that a seperate RCD enclosure next to the consumer unit ,that feeds the consumer unit and protects all its circuits ?
You need to do some investigating ,and testing . Starting at the consumer unit.

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