Spur socket below lightswitch

BusyMartin · Aug 13, 2023

Hi there - good site.

Mrs wants a new socket directly below a light switch on the ground floor (says it will look neater vertically inline) spurred off the
downstairs ring, which all go up into that above ceiling/below floor space.
I gather I'm OK to do this as a (very experienced) DIYer? Or are we totally no-can-do these days?

Cable would be coming down the wall from up above, as is the cable to a four switch plate on an earthed metal backbox.
The wall is very thickly plastered brick (Victorian terrace - very thick party wall), and the four switch cables are in two 20mm oval conduits, with enough space between them (I think) that I could get a third 20mm conduit in between.
I've seen on another forum topic here that some might run that new spur cable down through the backbox - but some debate as to whether that was good practice (even with grommets).
One guy says he'd just run such a cable tightly around the outside of backbox, but that's not technically then in the permitted zone, and I don't like that idea.

I can't find an example of it anywhere, but I wondered about lifting the switch backbox out, gently chasing behind it into the brick (between the two screw holes and running the new cable in a 20mm conduit down the middle at the same depth as the two exiting conduits, but diving deeper into the brick just behind the switch backbox, and then coming back out to the normal depth below it until the socket backbox.

Is this a legit way to do this, or am I better to go through the switchbox with grommets?

Cheers for any advice here.

littlespark · Aug 13, 2023

Personally, I wouldn’t run a power cable through a light switch box. Different circuits are permitted, but just doesn’t seem right.

Come round the side… Permitted zones extend horizontally from a switch or socket as well as vertically.

You don’t actually need conduit to run a cable in, although you will need to check that the sockets are rcd protected.

You might find an easier route going down to floor, rather than up to ceiling. Old house… wooden floor? I wouldn’t be surprised if there was a huge cellar.

BusyMartin · Aug 13, 2023

Much appreciated.
Concrete floor, so down is no good (hopefully no mineshaft underneath!)
Going around the side of the switch box (and maybe I'm being over-worrisome) will it not stray just outside the permitted zone at the corners where it bends?
Also, does it matter if it's tight to the box or better to come away a cm or two from the box?
It seems wrong to be physically touching it - that's why I was considering conduit.

littlespark · Aug 13, 2023

It’s fine touching the box, as close as possible.

I don’t fancy the idea of taking more brick out behind the box… might lose the bit where the fixings are.

No one will be likely to stick nails in the wall so close to a light switch.

I’ll mention again about RCD protection. Any new work on circuits must have this.

An old house, the sockets might also be in conduit. Has the place ever been rewired?
I’m thinking for ease of getting a new cable to a socket for spurring off.
(You don’t want hidden joint boxes under floors if you can help it)