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Extending Upstairs Ringmain

Postby Me1 » Thu Aug 04, 2011 7:55 pm

I've moved into a 3 bedroom house that is 4 years old and there is a downstairs ring main and an upstairs ring main. Off the upstairs ring main in each bedroom there are 2/3 sockets already in and a socket on the landing and also one in the loft (all are on the ring). What I'd like to have done is have a further socket put in each bedroom to connect a TV to that is mounted on the wall. As the sockets will be high up the wall my thought is that the socket in the loft (that is part of the ring main) could be used to extend the ring main round the edge of the loft floor and take the sockets of as required for each room. Would this be ok or would the size of the upstairs ring then be to big (as I'm thinking the size will be doubled)?
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Simply Build It

Postby ericmark » Fri Aug 05, 2011 10:16 am

I would replace the back box or add to the back box so I could fit two single devices. Need a duel box not twin as duel has centre bar to take screws. Try this screwfix link to see type [url][/url] Then fit a fused connection unit and from this feed the socket in loft and all other sockets as far easier with a single wire to each socket than trying to form them as part of a ring.

If you did want to extend the ring then first I would add a second socket in loft as easy to come from one socket then feed all others and return to second socket.

With the FCU there is unlikely to be a problem but with extending the ring you must measure the impedance (resistance) of the cable used and add it to the incoming loop impedance which must be less than 1.44 ohms with a ring coming from a B32 MCB. It's not simply a matter of adding sockets you need to check by doing so the impedance will not raise over the limit.

A C32 is 0.72 ohms and D32 0.36 ohms but using a FCU because of the 13A fuse it becomes 2.42 ohms and using a 3A fuse 16.4 ohms. Hence why using a FCU is likely best method.

There is a meter you can plug in to measure but at around £200 for cheapest not really a DIY thing. There are plug in testers that do the job and these between £30 and £45 for one with a loop test will do some basic test. The Martindale EZ150 Loop Check Plug tester has a series of lights for different limits and the Socket & See SOK34 Easy Socket and Earth Loop Tester is about the cheapest. As to how well they work I don't know as I use a proper meter.
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