ring main question / explanation

[MarkTravis]

Since buying my new house, that needs a fair amount of renovation, I've been doing a lot of reading up on DIY projects and there is one thing I just don't get regarding the way in which houses are wired.

Wall sockets are wired on a 'ring' and as I understand it you start with a wire coming from the fuse in the consumer unit to the first (nearest) socket, then next, then the next and so on final this daisy-chaining ends with the wire going back into the same fuse.

Lighting, however, starts at a CU fuse then goes to the first light connection, then the second, third etc etc - again in daisy-chain but the lighting circuit terminates with the last light fitting i.e. it does not return to the fuse

Why?

Why is a ring main a good idea for your 13amp sockets but not for the lighting circuit?

 

[EFLImpudence]

Why is a ring main a good idea for your 13amp sockets but not for the lighting circuit?

Some would say it's not.

You can find the history on Google - the correct term is Ring Final Circuit.
Peculiarly British it was introduced after the war allegedly to save copper.

Fundamentally it allows smaller, easier to work with, cable to carry twice the current (for safety because of load positions actually only 120%) that a single run would allow.

Lighting circuits use very little and so it's not necessary.

 

[MarkTravis]

Save copper? I think someone was misguided with that idea. in my house upstairs the shortest run I could possibly make means that about 35ft of wire needs to run from the last socket back to the consumer unit!

 

[Spark123]

Yep, save copper. A ring circuit is usually wired with 2.5mm live conductor cable. An equivalent radial (as you correctly say lighting is usually wired with) would require a larger live conductor size.

 

[JohnW2]

Save copper? I think someone was misguided with that idea. in my house upstairs the shortest run I could possibly make means that about 35ft of wire needs to run from the last socket back to the consumer unit!

Yes, it certainly depends on the layout of the sockets. If they are positioned in a straight line radiating away from the Consumer Unit, then a ring final circuit will certainly use more copper than a radial circuit - although the radial circuit would have to use 4mm² cable instead of 2.5mm² cable (a ratio of about 1.6 **), the 'ring' would use twice as much length of cable, and would therefore 'lose', copper wise. However, more commonly, particularly on the ground floor, the arrangement of sockets is such that the 'return leg' (from last socket back to the CU) may be quite short, hence enabling less copper with a ring.

There are various other pros and cons of radial circuits vs. rings - it's not all about copper/price.

Kind Regards, John
** P.S. since I know there are plenty of pedants around here, the actual ratio (with present-day cables) is 1.46, since the CPC/earth is smaller than the other conductors (and 1.5mm² for both cable sizes)

 

[ericmark]

In the days before the 1/3 rule on drilling a joist the ring main used less copper. But with pairs of cables going to each socket it is no longer clear as to if it saves cable or not. It also allows better loop impedance because of twin feed but again because of 1/3 rule this may no longer be valid. The whole system relies on having a 13A fuse in each plug and the fact that europe use 16A does cause some problems with items like ovens. Also the re-design of the plug with plastic around first part of live pins has de-rated the plug from orignal design and now they can only carry an intermittent load of 13A. And add to this the problem of DIY work causing the ring to be split into two radials and the safety aspects more and more radials are being used instead of rings.

Using 20A radial circuits allows more independent circuits so earth leakage faults are easier to trace and there is less cumulative effect. It does mean a larger consumer units and costs more as a result. It also stops people replacing the fuse with a nail to run welding sets and the like.

The 32A radial using 4mm or 6mm has two problems. One can't spur off as not enough room to add a third cable. Also mistakes are made where people think it's a ring and add to it with 2.5mm cable.

Many countries do not allow the ring system for example USA. And it is something for debate should one stop a system because people don't follow the rules.

It would be like lowering the speed limit from 40 to 30 because of the amount of accidents caused by people doing 50. Clearly accidents are not dependent on a speed limit when it's not obayed.

Personally I like the ring but accept they is a case for the radial.

 

[pyounger]

When we had out house re-wired a couple of years ago, the electrician bored me silly over what design of circuit I'd like - he was very good at talking 'electric' but not so good at explaning. but from a cost point of view we went with a 'ring' design however the kitchen was put on it's own ring using 4mm. The garage and shed which is on a radial is on at least 4 possibly 6mm.

I'm pretty sure that within the techie-babble he mentioned that a ring was easier to add to, but don't quote me on that

 

[ban-all-sheds]

the kitchen was put on it's own ring using 4mm.

Interesting.

Does the cable run through thermal insulation, or is it just plastered/tiled over?

I'm pretty sure that within the techie-babble he mentioned that a ring was easier to add to, but don't quote me on that

Actually that's the one thing it's definitely not.

 

[EFLImpudence]

Save copper?

I would just add that when it was introduced there were very few sockets installed so you could have the whole house on one short circuit (pun?) - in length, that is

 

[JohnW2]

I would just add that when it was introduced there were very few sockets installed so you could have the whole house on one short circuit (pun?) - in length, that is

Indeed. My recollection of my parents' house in the 50s is that what very few sockets there were tended to be 'back-to-back' in adjacent rooms (hence very short cable runs between them) and, at least originally, only one socket per room - and, again 'initially', none at all in the kitchen (even the fridge, when we got one, was a 'gas fridge' - and I still don't completely understand how they work(ed)!).

Kind Regards, John

 

[ban-all-sheds]

Indeed, and it's important to remember that. i.e. that there were already sockets.

The copper comparison is not a 2.5mm² ring vs a 4mm² radial, it's a comparison of what to do about a massive need to increase the capacity of existing circuits.

All the sockets which were there were already wired in 7/.029, so rather than having to rip all that out and replace it with 7/.036, it used less copper at that stage to connect the last socket back to the board with a run of 7/.029.

 

[pyounger]

the kitchen was put on it's own ring using 4mm.

Interesting.

Does the cable run through thermal insulation, or is it just plastered/tiled over?

I'm pretty sure that within the techie-babble he mentioned that a ring was easier to add to, but don't quote me on that

Actually that's the one thing it's definitely not.

1. The cable is under floorboards the rises through conduit - which is plastered over - then in to the back of the sockets.

2. In retrospect I think he actually just pointed out that if I wanted another socket is was relatively easy to add a spur at a later date

 

[ban-all-sheds]

Did he give you a 4mm² ring just so you could easily add spurs in the future?

 

[ban-all-sheds]

(even the fridge, when we got one, was a 'gas fridge' - and I still don't completely understand how they work(ed)!)

http://en.wikipedia.org/wiki/Gas_refrigerator

 

[xr4x4]

I don't understand for what reason you would need a 4mm ring final in a standard domestic house?

And what mcb would be protecting it.