Is This A ring Main?

8 Feb 2004
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United Kingdom
I'm tidying up some wiring in the garage and fitting a consumer unit in the adjacent shed and have a query.

I have 2 x 2.5 cables exiting the house and entering the garage and they are both protected by the same fuse so obviously off the same circuit.

One cable supplys the garage socket and from this socket a spur has been taken to the small CU in the shed.
The other cable goes directly to the shed CU and I assume this completed the ring main.

Is this correct and would it be better to not spur off the garage socket to the CU?


Part P is known.
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From your description it does sound like a ring as long as the terminations meet up in the CU in the shed.

From your description I wouldn't say it spares of to the shed, it just feeds the shed next on the ring.

Is this ring fed from an RCD protected supply ? If not I would seriously consider it.
What does the paperwork say? It is not minor works so it should include a "Schedule of test results" the versions shown as example in BS7671:2008 have a tick box for ring main so should be very easy to see on paperwork if it is part of ring main or not.
they might just be two spurs or two radials

what else is there on this way at the house CU?

What size are the cables, and what rating is the MCB or fuse?

Photos would be nice.
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There still should have been paperwork but without it needs some tests.

The 2.5mm cable used with most power circuits in the home is rated well below the 32A fuse or MCB which normally supplies it. So long as it is in a ring formation there is no problem. Also a single socket can be spurred from any point on the ring. Or by using a fused connection unit a radial circuit can be connected to a ring.

The big problem is where a ring has become split so you have two radials supplied from a single 32A supply.

When any electrician extends a system he has a schedule of tests he will complete which includes testing it is a ring main, and ensuring under fault conditions it will disconnect within the prescribed time, and the volt drop will be within limits.

However when the DIY man works on the system these are normally not checked.

With power completely off if one removes any socket on the ring with fuses removed of MCB off then a simple bell and battery can be used to check the pairs of wires are connected so proving the ring on line, neutral and earth. Remember main switch off both for safety and for test. You should if using a ohm meter do the test on lowest ohm setting but also between pairs you can test at highest ohm setting and so long as nothing plugged in should be over a few meg ohm.

The problem in getting anyone to do these tests is 1) they sometimes don't take safety advice and can allow others i.e. wives to turn supply back one during the test. 2) After completing the test the socket removed to do the test is not replaced correct so the very thing you have been testing to ensure has not happened you have done by doing the test.

The electrician will normally test at the consumer unit which will reduce chance of power being turned back on and reduce chance of a bad connection when reconnecting.

Once you have a "Schedule of test results" future testing is far easier as with the special meters used one can very quickly detect changes in reading and be alerted to any changes. This is why the last test results should be available to guy doing new tests. However in many places the owners think they are cleaver holding onto previous results then checking themselves to see if they match so they can catch out electrician which is really silly.

I hope you can see that although you could do some testing yourself you would be far better getting someone who has the right test equipment to do the work. Remember it is not just electric shock one is worried about but also fire. And however much we try to guide the DIY man to do his own tests he is unlikely to have either the equipment or knowledge to spot many of the faults.

35 years without smoking!
Sounds like what you have is this:


In which case it is not a ring final, as it is can't be both a ring and a final circuit.

What do the two cables connect to when they reach the garage CU?

If they both join together and supply it, then as I said you haven't got a final circuit, and only ring finals are exempt from 433.4, 434.4 and 523.8.

If they both join together and supply it then it does not comply with the rules for parallel circuits in 433.4, 434.4 and 523.8.

If they don't join together you have two radials, so the questions which then arise are:

1) What is the rating of the device in the house CU? (It'd better not be 32A)

2) Where does the cable from the socket go when it reaches the garage CU?

This was done in 2003 by a sparky that has long gone.
I'd hesitate to call him a "sparky"...
Hi Ban

I believe your drawing is correct.

What do the two cables connect to when they reach the garage CU?

Both supply cables are /were connected to the feed in on the consumer unit.

What is the rating of the device in the house CU? (It'd better not be 32A)
It is 32 MCB and I have had no problems since installation but will change it if you advise..

I did all the electrics myself under the supervision of a sparky friend who told me how to do it. Unfortunately he has moved on and I cannot contact him.
It is 32 MCB and I have had no problems since installation but will change it if you advise..
That went with the possibility of you having 2 radials, but you haven't.

It's not that that you need to change, it's how the garage CU is supplied.

I did all the electrics myself under the supervision of a sparky friend who told me how to do it.
A sparky told you to put a CU onto a ring final?
In descending order of preference:

1) From a switchfuse supplied directly from the meter tails.

2) From a dedicated MCB on the non-RCD side of the main CU.

3) If there's no RCD in the garage CU then from a dedicated MCB on the RCD side of the main CU.

With 1 & 2 you should use a cable type or installation method which doesn't require the cable to be RCD protected.

With 2 & 3 the MCB should be rated in excess of any of the MCBs in the garage CU.
why PM me to ask me to have a look..?
BAS is dealing with this one...

but if you want my professional opinion..

your "sparky" friend was an idiot to advise you to do it like this..

the garage CU should ( as BAS has already pointed out ) be fed with a suitable size cable and off either:
1. A switchfuse from the main tails ( to reduce the OCPD size to something not requiring 25mm cable to the garage CU )
2. An MCB in the main CU ( non RCD side if suitable cable or installation method is used and providing the cable doesn't travel outside to feed the garage ).

you need to install minimum 4mm cable to feed the CU from a 32A breaker, and possibly bigger depending on the correction factors and volt drop..

the socket should be fed on a radial in 2.5 minimum from nothing bigger than a 20A breaker, or in 4mm from a 32A breaker..

I'm not certain that you can't have a socket on the same breaker as the garage CU, but it's not best practice to..
Thanks for all the replys.

I am going to get a Sparky in as it is a bit too complicated for me.

Thanks again :D

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