I want to run a new radial circuit length 21m to 8 double sockets. It's for 2 bedrooms, with computers etc.
Will 4mm cable be OK?
Part will be fixed to a joist, part buried in plaster, part in a groove in kingspan behind OSB.
Also a similar radial to one storage heater: will 2.5mm do for that.
I've got the red 17th edition On Site Guide, on page 48 Cable size mm
4.0/1.5 is the 1.5 the area of the earth wire? [george]
You have not said what the rating of the protective device will be...
At 20A you could use 2.5mm T&E
The last bit means you are using installation method 102
See appropriate column in table:
OSG table 7.1 suggests 31m is your length limit on both TN-S and TN-C-S.
If you need 32A protection, then you would likely have to go to 6mm^2 T&E because of that section of 102 installation. 4.0mm^2 will only go to 27A in the circumstance.
Yes - the same size as in 2.5mm^2 (one of the reasons 4mm^2 is a bit limited in application at times)
At that sort of length with that number of outlets I'd usually make it a spur to a mini-CU and then ring circuit the sockets and radial the heater from from that. You haven't mentioned the rating of the heater.
jgh
But the storage heater will need an off-peak supply; 32A ring + 20A storage heater = 52A submain (ignoring diversity).
That's heavy cable, probably SWA as you wouldn't want it on an RCD as a heater fault would take out the sockets and v.v., plus the cost of a CU.
Alternative is 2 x 2.5mm 20A radial circuits off existing RCD protected boards. Easy.
Owain
Yeah, two radials, but I'd still put an isolator at the far end so you're not having to make loads of 42m round journeys when working on the outlets. That's how my workshop is wired which is about 40 feet or so from the CU.
jgh
As an amateur rewiring my house I like to test sockets often with a cheap neon test plug which gives 3 red lights. But they do not indicate many faults in ring circuits, wires can be cut but all the sockets will still work as two radials. That's why I dont want any ring circuits in my house.
[george]
Its as easy (in fact easier) to test a new ring circuit though[1]. What is more, they are more fault tolerant after installation.
[1] Although harder to test an unknown ring circuit where you are trying to establish if any liberties have been taken.HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.