Adding plug socket in loft. How?

You'd need to give a lot more info, intentions, etc. to get a proper solution. My first reaction is that it isn't a good idea and "No"

I know in my house the lighting circuit is 1.5mm2 cable vs 2.5mm2 for the sockets, which is pretty standard. It would be a bad idea just in terms of breaking stuff and burning the house down but also a massive bodge.

What are your internal walls made of? It might be suprisingly easy for an electrician to put a socket up there from the proper circuit if it's hollow dry wall.

I’ve had a socket installed (by a qualified Electrican) spurred off the light circuit which is dedicated to a 50w ceiling speaker. It was done this way simply to avoid cutting the plug on the transformer and simpler for the on/off switch on the wall.

I wouldn’t plug any domestic appliances in as that would almost certainly trip the fuse.

I would have thought a fused spur off 1.5mm is ok, however I'm out of date since I last looked at the regs

It’s in the in-laws house so haven’t looked at the installation yet, all I know is they have a light up there. It’s to plug in a 5v 3A 15w power supply for a WAP. Don’t really want to start making any holes in the wall etc if poss.

Not a sparkie but I wouldn't do it myself, and I've done quite a few bits round my house over the years.

Main reason is that I'd assume the loft light is probably on the upstairs light circuit. This should be a separate fuse/RCD on your consumer unit. So if you switch it off there you'd not expect a socket to lose power. Not a massive safety issue but a bit odd.

The second reason is that there's nothing to stop someone plugging in something more powerful, like a 5 bar electric heater. This will pull too much power for the 1.5mm lighting circuit cable.

As it's for a wireless AP have you looked at a mesh system that uses your power sockets as backhaul? Discussed in the thread about ethernet cable the other day.

Hmm, not as straightforward as I thought then. I’ll get an electrician to have a look.

If it's for a plug in light in attic why not just hard wire it to a surface mount switch.

Or....

Use lighting sockets (round pin not square) and change the plug.

Stops folk plugging in daft appliances.

An isolator switch or lighting socket is the answer, the 5A are fine for this and maintain the circuit "type".

Standard three pin socket no, you shouldn't mix sockets and lights on the same circuit.

Lots of things dangle of lighting circuits - smoke alarms, speakers, networking kit etc etc.

I've just wired things via an FCU with an appropriate fuse in it.

It’s to plug in a 5v 3A 15w power supply for a WAP.

That kind of power supply is unlikely to have a cable feeding it - it will be a single unit with pins to fit a 13A socket. So other kinds of socket are not going to solve the problem. If connecting a 13A to lighting circuit, I think a fused connection with a 3A fuse in it (clearly labelled 3A fuse max), and then a 13A socket after that, might be acceptable, but I'm not an electrician.

If it's a WAP, it needs an ethernet feed, so needs a wire putting through a hole anyway. Unless it's a Powerline WAP but it doesn't sound like it because they don't have a separate power supply.

Can't you get a PoE WAP?

In the spirit of everything we need tech wise now exists, how about a WAP light bulb https://xy-connect.com/en/features/

You can get 5 amp 3 round pin plugs and sockets. They're specifically designed for lighting circuits. I'm just about to fit some so I can have side lights that can be switched on and off at the light switch on the wall by the door.

Yes, you can do it, with certain considerations.
It isnt a lighting circuit, it is a circuit, that the lights are fed off.
It will be protected by a 6 amp circuit breaker probably. So you will be limited as to how much current consumption the socket will take. The usual things in an attic are signal boosters and other such small wattage low power consuming devices.
I would have no problem at all in using the 'lighting' circuit to supply a dedicated socket for a low power device. If you are worried about someone plugging in a higher powered appliance (not that it would be unsafe, but could be a nuisance by tripping the circuit breaker), then fit a specialist 'T' pin plug, or, as mentioned above, one of the small lighting plugs, however, I'm not sure they are fused, but being as the CB is 6 amp, that shouldnt be an issue. either way, mark up the socket as 'for use of xxx only'.
The 3a 5v 15w refers to the output, 3A x 5V = 15W. It'll be a tiny wattage at 230v.
All sockets must have RCD protection, so that is another consideration.

For this use-case, a PoE injector is definitely the way to go. Also means you don't have to go into the loft to reboot the AP if required.

So the power supply to the device is indeed a moulded 3pin enclosure with a wire coming out so no option to change it for a 3 round pin socket. PoE would work, with a PoE splitter, as the power connector is USB C but it means running the ethernet cable up there which whilst possibly easier than running power is not my preferred option. The WAP in question isn’t your bog standard WiFi AP as it’s a Helium hotspot which needs to be in the loft as it is connected to an antenna on the roof…

So there is no cable needed other than power?

Get something like this, cut off the plug and wire it into an FCU on the lighting circuit?
https://www.amazon.co.uk/Adapter-Charger-Compatible-ThinkPad-Chromebook-Black/dp/B09QHJVC5Z/

That looks like a neat solution @sixtoes. Might have a look at doing that. Cheers. 👍