Ring mains and spurs

[dave]

For all you electrical experts out there. I believe I am allowed to take a spur from a socket which is on a 2.5 mm sq cable ring main, and have one socket on that spur. If I take another spur from a seperate socket in the same room, can I join these two spurs as a seperate ring and connect say 3 or 4 sockets to the joined spurs. In effect I end up with another subsidiary ring of the same current carrying capacity as the main ring, it seems there shoudn't be any overload problem, or is this against the wiring regs.

Dave

[tinnews]

Having a ring with parallel bits as you describe isn't one of the 'normal configurations' therefore to conform with the wiring regulations you'd have to do all the loading calculations from first principles. That's a non-trivial task.

The advantage of staying with one of the normal configurations is that you are (effectively) allowed to assume that the calculations have been done for you already and that as long as you don't do really silly things everything will be OK.

You might be able to get the effect you want by simply splitting the existing main and inserting a new length of 2.5sq mm T&E between the ends you have thus created to make a new, longer ring. There's no rule that says you can't run the ring back on itself or whatever.

[ARWadsworth]

New longer rings may result in voltage drop being too large. Table 7.1 OSG

A parallel ring near the start/end of a circuit may overcome this problem but this is a non standard design.

Adam

[dave]

Thanks for the replies. Having thought a bit more I can see that there could be a potential difference generated between the two spur sockets that would cause a possible high circulating current in the parallel ring, probably not a good idea. I was really trying to avoid splitting the old ring as its position a and wire lengths mean junction boxes under the floor which I don't really like even though its legal at a pinch. Is there anything that stops me using suitable sized chocolate block connectors inside the socket wall boxes to split and extend the ring, that is if they will fit in????>

Dave