Our garden featured a delightfully crap concrete fountain thing that ran off a little 11w/240v pump powered by a cable that emerges from an outside wall of the house. There is a separate switch for it inside the house.

I’m planning to hook up a considerably more useful 2-gang outdoor socket with RCD (this one) instead.

Question is: What sort of gauge should the cable be to handle that socket?

Ideally I’d like to use the existing cable as it’ll be an utter ballache chasing it all out and replacing it.

It is three-core stranded double-insulated. Not sure of the cross-section, but it does look a bit weedy (like appliance cable rather than wiring) and I’m not overly keen on it bursting into flames the first time I draw a decent amount of power.

For a 2 gang socket you’ll be looking at 2.5mm CSA for the cores.
I’d be having a ggod look at what the cable you have in is. If it is armoured, then great, if it is standard flex, I’d be inclined to pull it out.If it is flex, then you really need the RCD at the house end, as if you cut through the cable with your spade, you are going to get a good belt off it.

It’s not armoured but it doesn’t need to be because I’m mounting the socket directly on the wall where the cable comes out – so no part of the cable will be exposed anywhere.

No way it’s a 2.5mm cross-section though. Looks like I’m replacing it. That’s going to be a pig of a job 🙁

Has the switch inside the house got a fuse in it? Something like this?

If so, and it comes from the socket circuit in the house, then 1.5mm cable from the fcu to the socket would be fine.

If the circuit you have the switch on is on an RCD in the main board you don’t need an RCD socket outside, a normal waterproof socket is fine.

Rich.

Hmmm it might be 1.5mm – I’ll get the callipers out when I get home.

Think the switch is fused – but it is inside a cupboard so kinda hard to see.

Yeah I know the RCD is probably overkill – but it was only a couple of quid more than the non-protected one and if it saves tripping the main board RCD if/when rain gets into the socket then all the better. Doesn’t do any harm does it?

The main rcd could still trip. It’s not the closest that trips first, it’s the most sensitive. You may find the main board will trip anyway.

Rich.

Hmmm it might be 1.5mm – I’ll get the callipers out when I get home.

1.5 is the cross sectional area of the conductor

Yeah I know the RCD is probably overkill – but it was only a couple of quid more than the non-protected one and if it saves tripping the main board RCD if/when rain gets into the socket then all the better. Doesn’t do any harm does it?

It just means you don’t know which one it will trip; could be either.

So further inspection reveals it is 0.75mm H05RN-F cable.

That’s not going to handle the load is it? Bugger.

No, 0.75 only carries 6A so that will have to be replaced.

First, confirm that the switch is on a socket circuit or something appropriately sized. It is unlikely but it may have been taken from a lighting circuit or something that isn’t suitable for an outside socket.

Find out which mcb in the consumer unit it’s powered from. Should be 16A minimum.

The cable may be able to be pulled through the wall by attaching a piece of 1.5 or 2.5 FTE (flat cable) with electrical tape.

The fuse in the FCU is likely to be only 3A so that will need to be changed to a 13A, Alternatively, my preference would be use 2.5mm cable and then ditch the FCU as that will allow you to use both sockets to 13A per side rather than 13A total. Probably won’t ever need to but may as well do it properly whilst you’ve got the chance.

Rich.

Thanks Rich, that’s all very helpful and welcome advice.
Cheers