Adding 4 Double Sockets to Existing Ring Main - Ring or Spur?


Postby Copper809 » Wed Nov 18, 2015 4:25 pm

I need to add 4 new double sockets to an existing ring main. I assume it's best to add as a ring and not as a spur line. My current plan is to take power from two existing sockets on one ring and loop the new sockets on the new ring between them. I'm doing it this way as I cannot rip up the walls to determine the existing ring wiring path. If I do this it is likely there will be parallel rings on one consumer unit fuse. Is this all doable and will not compromise loadings?
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Postby ericmark » Wed Nov 18, 2015 8:16 pm

We have debated the fig of 8 ring at length and really have not come up with an answer. So all I can do is make you aware of the problem.

The British ring final has a flaw. One can end up with too much load on one end of the ring. To reduce to probability we have some rules or guide lines, one does not have to comply with them, but if you don't then it's up to you to do all the complex maths to ensure there is no over load.

So we do not connect any permanent load over 2 kW. Items like ovens, dish washers, washing machines, tumble driers and immersion heaters should have their own dedicated circuit. I am sure you realise only the immersion heater tends in practice to follow that rule. But this would also include items like a garage or shed. Although you could run a 6mm cable from the ring to a garage or shed unless taken from the centre of the ring it could result in over loading.

The same applies to a fig of 8. If that centre connection of the figure of 8 is at the centre point then OK no over load will result. But if near one of the ends then overload could result.

So other than the physical problem of connecting the wires, there is no difference connecting a 4 or 6 mm sq cable to a socket on a ring to draw more than 13A to forming a fig of 8. Both will ensure the new cable is not overloaded and both could over load the existing cable.

I would say using a fused connection unit (FCU) is a better option. It is less likely to produce an overload. We were taught to test for fig of 8 and flag it as a fault, but there is nothing which says directly you should not make a figure of 8 in the regulations, but it does have phrases like should not produce an overload for an extended time and the fig of 8 can produce an overload for an extended time.

So I would not install a figure of 8 circuit, however to condemn an existing fig of 8 would require careful measurement of the loop impedance to show it was too near to an end of ring.

Point is of course if you have the test gear to be able to complete the minor works certificate you would also have the means to work out where the ring goes so would not need to form a figure of 8.

So I would recommend installing two fused spurs rather than a figure of 8. If having a problem connecting have a look at grid switch systems which will allow a fuse and a socket in the same double socket plate.
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