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Making a domestic ring main
locoboy - 20/6/12 at 07:36 PM


I am an electrical novice and before you all start beating me with part P reference books or any kind of electrical bible I am asking a question in order to confirm my own theories.

I have attached a sketch of my current and proposed garage wiring.

At present I have no ring main in my garage, the garage is attached and has its own consumer unit, albeit an old one with the pull out fuses.

The garage has a 'spur' arrangement of ...
3 double sockets on one side
1 double socket 'spur' arrangement on the other side
1 double socket 'spur' close to the consumer unit,

I'm sure the term spur is not correct because it is not 'spurring' off an existing ring main but I'm sure you get what I mean.

I have built a partition wall to split the garage and make it into one third storage and 2 thirds workshop.

On the partition I would like to include 3 more double sockets.

I am aware that the current setup is not ideal at there is no loop back to the consumer unit.

In terms of what is fused with what.....
The 3 sockets on the 9 meter run are on their own fuse
The 1 socket 50cm away is on it's own fuse
The 1 socket 9m away is on it's own fuse.

Am I right in that I need to add the wiring in the PINK box in order to make the existing 3 sockets and my 3 new sockets into a ring main and make them a lot safer?

Am I right in thinking that the other 2 sockets that are not linked into any kind of ring main are pretty much ok as they are? after all the whole lot has been fine for the last 5 years of use!

For those concerned I would not be doing this myself I just don't want to sound stupid when talking to the guy that will be doing it!
garage wiring
garage wiring

big-vee-twin - 20/6/12 at 07:48 PM

Your 'spur circuit is called a Radial circuit, and should be wired in 4mm2 cable and fused at 20 amps.

Therefore you need to wire the second set of three sockets in 4mm2 and yes the pink bit too and then connect them back into the same fuse.

Having said this there is nothing unsafe about a Radial circuit and it can have a maximum of 6 sockets installed onto it.

I would highly recommend you change your consumer unit to MCB type and also use an RCD to protect your socket circuits you can get individual ones to do separate circuits.

A ring circuit is usually wired on 2.5mm2 and fed from a 32 amp fuse/MCB. So if your original wiring is indeed 2.5 then you will make things a little safer.

And yes I am an electrician

Stott - 20/6/12 at 07:50 PM

Yes you're right.

The correct term for those sockets in their current arrangement is 'radial' for info.

If you wish to change to a ring and thus uprate the capacity of the circuit then you need to complete the last socket on the radial back to the CU as you have drawn.

Providing the sockets are currently wired with 2.5mm which they should be, then you can complete the ring and uprate to a 30A rewirable fuse.

You should ensure continuity of the ring including the earth conductor, easily tested from the CU, this is to guard against a split ring whereby it is effectively 2 radial circuits but being fed off a 30-32A protective device which the 2.5mm cable can't take, but I'm sure the electrician installing it will know this


I predict a Riot - 20/6/12 at 08:31 PM


Firstly you have missed out the most important issue - what is the nature of the distribution circuit to your garage consumer unit?

Also what type of earthing system is installed and are the main earth bonds in place and of the correct size to suit your main incoming supply?

You must not do anything until you know all this

A circuit coming directly from a fuse or circuit breaker is indeed called a radial circuit and no it does not need to be 4mm cable only if you want to fuse it at 32amps a 2.5mm radial is fused at 20 amps, there is NO limit to the amount of socket outlets you can install, a radial is limited by the area it covers, a max of 50 metres squared.

If you are not using armoured cables or protecting a PVC cable in conduit or trunking you MUST install a 30ma RCD to every circuit although one can be used for all circuits.

You do not necessarily need to convert you existing wiring to a ring main, 20amps is more than sufficient for most workshops unless you intend to do arc welding. Where this is the case I usually install a separate circuit just for the welder and fitted with a type c mcb to cope with the inrush current.

If you re in any doubt call a couple of local electricians most provide a quote for free.

Yes this job would be notifiable under Part P.

I run my own electrical contracting company and have been an electrical engineer for 25 years so please take this advice seriously.



matt_claydon - 20/6/12 at 08:33 PM

Ring circuits are actually arguably less safe than radials, as with a ring you will never know if a break in the wiring develops somewhere (without specific testing) as all sockets would continue working, but as two radials on the wrong size cable. A fault in a radial will become immediately apparent as outlets beyond the fault will stop working. Ring circuits were only invented to save on copper during the war; you won't find hem in many other countries in the world.

Not saying rings shouldn't be used - they are still pretty much the standard here, but it's wrong to suggest that turning a radial into a ring makes it safer. If your currents radial is wired using 2.5mm2 cable but fused at 32A, then of course things are slightly different!

Also, radials are sometimes more efficient on use of wire where the sockets do not form a natural 'loop' leading back to the CU. An example would be a corridor with sockets down one side, (or a garage with sockets on only one or two walls).

[Edited on 20/6/12 by matt_claydon]

SteveWalker - 20/6/12 at 08:34 PM

What is the consumer unit fed from?

In my case, the fusebox in my garage (which also feeds the shed next to it) is fed from a 20A MCB (and an RCD), so there is no point in it being wired as a ring in the garage itself. There is no discrimination (it should be arranged so that a fault in the shed should blow the fuse for that circuit, leaving the MCB set and still feeding the garage), but I'm not bothered by that as losing both circuits at once is no problem.

SteveWalker - 20/6/12 at 08:35 PM

Originally posted by big-vee-twin
Your 'spur circuit is called a Radial circuit, and should be wired in 4mm2 cable and fused at 20 amps.

2.5mm2 for 20A and 4mm2 for 32A isn't it?

daniel mason - 20/6/12 at 08:47 PM

this is where it gets messy. dont take advice from anyone who is NOT an electrician.
the post about the earthing system is what you need to be looking at. you dont need to wire a 20amp radial in 4mm as 2.5 mm can cope with 20 amps unless its running through hige levels of insulation.
what sort of supply/wiring type feeds the garage?
is it rcd protected from the house?
is there a 10mm earth to the garage?
are the main bonds done in the house?
what size cable and fuse size feeds the garage?

just make sure you do all of the above correctly before even considering wiring it yourself, (which you shouldnty do)
i always cringe when i see these threads on here. but on the other hand, if your a competant guy and get decent advice then you 'should' be ok!

BenB - 20/6/12 at 10:25 PM

What kind of earthing system have you got? That also makes a difference....

MikeR - 21/6/12 at 11:18 AM

This has got me thinking (thread sort of hijack).

I've got when i bought the house a 13amp fused spur from my house into my garage (done i assume when the house was build in 1989). That then feeds a single socket and the lights. I've got plugged into that a four way and then an extension, the extension then has two more four ways plugged into it. My logic being I only ever use one device at once so i'm fine having lots plugged in. Could I reasonable extend the one socket into more 'radial' sockets to save having lots of wires draped around or would that need part p?

SteveWalker - 21/6/12 at 11:51 AM

Yes, you can add more sockets to the radial and this is much better than using multiple trailing sockets. Daisychaining trailing sockets can quickly add to the earth loop impedence and if it becomes too high, the fault current can be too low to blow the fuse or trip the breaker in the required minimum time.

big-vee-twin - 21/6/12 at 12:19 PM

Originally posted by SteveWalker
Originally posted by big-vee-twin
Your 'spur circuit is called a Radial circuit, and should be wired in 4mm2 cable and fused at 20 amps.

2.5mm2 for 20A and 4mm2 for 32A isn't it?

In the current IEE regs circuit arrangements are shown as you say - 20 amp 2.5mm2, if the wiring is fairly old it is most likley to be 4.0mm and 20 amp fuse as that was the traditional 'standard arrangement' for a radial.

If you are going to use 2.5mm for a radial I would advise a full set of calculations to determine disconnection times etc..For a DIY installation I would still go with 4mm and 20 amp MCB.

Bottom line is get someone in who is qualified and he/she will identify the age of the wiring and adopt a safe solution to suit.

JoelP - 21/6/12 at 07:30 PM

An electrician would know all these things. However, on the off chance that you intend to do this yourself i would stress that the RCD is the most important thing you need to fit. I would argue that having an rcd is more important than using the correct sized cables, and a league beyond how important correct sized earth bonding is. An rcd can mitigate a multitude of sins, including dodgy earths and exposed conductors. If you do nothing else, make sure you get the rcd fitted. Then check your earthing is ok (main earth first, then bonding to services). Then worry about turning it into a ring.