Yes you can do this as long as your new extension doesn't put too much on to the circuits Earth fault loop.
If the ring is run in 2.5mm twin and earth then your spur should be in the same size cable to a suitable FCU,from there,depending on the power the new light is pulling you can run a smaller cable as long as the fuse in the FCU protects it i.e...
If your new light is 1000w then 2.5mm should be run to the FCU, from the FCU you can run either 1mm or 1.5mm cable ( for ease of terminating in new light fitting) to the new fitting and a suitable 5amp fuse placed in the FCU.
Thanks for that. What sort of FCU would you reccommend and where should I locate it. I was planning to run the spur from a socket that is on the inside of the outside wall where the light is to be placed. Should I run the spur to the FCU on the inside and then run the cable from the FCU outside to the light?
I was thinking of doing this but the light site is round the corner from the socket. Can I run the cable round outside at the top of the wall or do I need to chase it all the way up the inside and along the top of the wall to the middle of the patio doors and out? For the outside option I was wondering if I could run the cable 1.5m up the wall then inside again for the switch and then up from there?
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