connecting a new ring main


Postby nft29 » Sun Aug 07, 2011 10:01 pm

I have created a new ring main in my garage and have no spare ways on the MCB fuseboard. As the new ring only consists of 3 twin 13A sockets is it possible to connect to an existing ring main by directly connecting to another ring main within the fuseboard rather than via 30A connectors?
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Postby ericmark » Mon Aug 08, 2011 11:11 am

Possible yes likely no.

With a ring main there is a maximum length of cable permitted with RCD protection and a B32 MCB that is about 106 meters which sounds a lot until you realise how much the cables go up and down walls and across and return on the ceiling to comply with 1/3 rules in drilling timbers.

Job one is to measure the loop impedance with a B32 MCB that is a max of 1.44 ohms. With such a low figure the multi-meter is no good to measure this and either a special low ohm meter with at least 200ma draw or an earth loop impedance meter are required.

This produces a second problem in hire cost of £75 and to buy about £750 and coupled with Part P charges which may also apply it can very easy cost more to DIY than to get job done by a scheme registered electrician.

You don't really give enough information to give a good answer. Do remember all new sockets will need RCD protection so if not already provided with consumer unit then this could prove costly.

It may be better to use a RCD FCU and reduce total supply to 13A with garage on a radial rather than a ring.

I am uncertain myself about using one B32 MCB to supply two rings. The problem is it may be regarded as a figure of 8 system which is not allowed however you can take a spur from the consumer unit.

House and garage design will also have an impact. My garage is part of the house and contains the consumer unit so distance would not be an issue. But if the garage is detached then there are a whole load of other problems in complying with laws and regulations.

I would get a quote. He will likely say what he intends to do. Once you know both what can be done and price not to DIY you may well think it's not worth doing the DIY route.

With hire charges and LABC charges the DIY route can add another £200+ to the materials bill for whole job so often just not worth it.
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Postby kbrownie » Wed Aug 10, 2011 11:18 am

It would be best to extend your existing ring main.
If you have already routed the cable back to the board this should not be to difficult.
But I personally would not have installed a RFC for a new three socket circuit, I would have gone with a radial.
But as stated above more information is needed.
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Postby nft29 » Fri Aug 12, 2011 12:37 am

Thanks for the response. I agree the radial solution sounds simplest. The consumer unit has RCD protection so no issues there. The garage is integral to the house and only a matter of a few meters from the new sockets so extended one of the existing ring mains was my original thinking. I was just trying to avoid some cosmetic issues adjacent to the CU which is in a frequently used storage cupboard.
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Postby kbrownie » Fri Aug 12, 2011 6:30 pm

Can you break in to the circuit near the CU, either by the use of junction boxes or the addition of conveniently placed socket outlets. or combination of both.
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