Confused over electrical cable length and size

[Cavester]

Hi,

I've been reading up on ring circuits for domestic installations and typically it is stated that 2.5mm t&e is used. However on learning how to calculate this properly with voltage drops and cable length etc. It's coming out at 4mm for me.

I assume the cable length is the total length of cable right around the perimeter of the property including the drops down to the sockets and eventually leading back to the consumer unit? For me this is approx 80 metres.

The next question is, is it therefore better to split the sockets into 2 rings i.e. ground floor left and ground floor right.

This is ground floor only by the way.

Thanks.

 

[EFLImpudence]

I've been reading up on ring circuits for domestic installations and typically it is stated that 2.5mm t&e is used. However on learning how to calculate this properly with voltage drops and cable length etc. It's coming out at 4mm for me. I assume the cable length is the total length of cable right around the perimeter of the property including the drops down to the sockets and eventually leading back to the consumer unit? For me this is approx 80 metres.

A 2.5mm² ring is effectively 5mm² for half the length so the actual resistance of the cable is a quarter of that of its actual length.
Also the loading is taken as 20A at the centre; not the 32A of the OPD.

The next question is, is it therefore better to split the sockets into 2 rings i.e. ground floor left and ground floor right.

It's up to you but two circuits would be better.

Two 32A radials using 4mm² could be used if the circuits were not actually in a ring formation.

 

[flameport]

For voltage drop, length is the longest part of a circuit, so typically to the end of it.

However a ring is not the same, as each point has two conductors back to the origin, so the length is effectively half of the total loop length.
Another way to think about it is that as each point has two conductors, the actual effective size is 2x, so with 2.5mm² conductors, the effective size is really 5mm².
That is also why it's permitted to use 2.5mm² cable with a rating of 20A or so with a 32A circuit breaker.

Even more considerations are that the load on a ring is not located at the end of the circuit, as there is no end. Further, it's usually assumed that the load is distributed around the ring which also changes the calculation for voltage drop and other things.

The maximum length for a 2.5mm² 32A ring is about 106 metres.

The next question is, is it therefore better to split the sockets into 2 rings i.e. ground floor left and ground floor right.

Dispose of rings entirely and install radial circuits.

 

[flameport]

total length of cable right around the perimeter of the property including the drops down to the sockets and eventually leading back to the consumer unit?

If that's really how you are installing it, you are doing it wrong.

The original intents of ring circuits were that:

• there was one circuit for the entire house
• the ring covered the shortest route between outlets in the central part of the building
• outlying individual sockets such as those on external walls were wired with a single cable as a spur from the ring.

Hence the 'cable saving' deal which is often mentioned. Done like that it really did save cable compared to previous installation methods which used several different types of socket outlet each on their own individual radial circuits.

Installing multiple rings in one house and extending the ring with twin cables to each and every part of the property is total madness.

 

[Cavester]

Thanks for the explanation guys, that's solved the confusion I had on the cable length.

Flameport, I'm now a bit confused at your second post (forgive me)...

As I understand it there would typically be 2 rings for the sockets in a domestic, one for upstairs and one for down. The reason I say around the perimeter of the property is because the downstairs to which I was referring is essentially 2 large rooms and all sockets are generally against the wall in each corner and middle of the rooms for TVs etc. Therefore the shortest distance is around the perimeter.

Going from the following, the ring would need to go around the perimeter in other to go through each socket and then back to the consumer unit...

https://www.diydoctor.org.uk/projects/ringmain.htm

I also read that if designing from scratch you should not design in spurs for sockets and to leave this design to where future alterations or extensions may be required, i.e. adding a new socket from one within the ring. Also no more than one additional socket to be spured from a ring socket. Is this incorrect?

Cheers.

 

[Cavester]

Dispose of rings entirely and install radial circuits.

I was also under the impression that rings were better due to load balance etc. and that radial were less common/preferred in the UK?

 

[EFLImpudence]

I also read that if designing from scratch you should not design in spurs for sockets and to leave this design to where future alterations or extensions may be required, i.e. adding a new socket from one within the ring.

Some do say that but it is up to you.
There is nothing wrong with a spur to one socket.

Also no more than one additional socket to be spured from a ring socket. Is this incorrect?

One double socket as a spur using 2.5mm² is alright; two double sockets is not because the cable might be overloaded -

look at it. If you want to add one socket to the ring you will have to go from socket ring to spur ring and back again along the same route.

You could join these two cables and use in parallel, so therefore you could have used 4mm² as it cannot be overloaded on a 32A circuit - this may attract criticism from traditionalists.
So, why not use 4mm² radials and you can add as many branches/spurs with as many sockets as you like.

There is no logical reason to use a ring nowadays unless the circuit actually is in that shape.

 

[EFLImpudence]

I was also under the impression that rings were better due to load balance etc.

No, quite the opposite (although not really a factor).

and that radial were less common/preferred in the UK?

Does the fact that they are not preferred elsewhere tell you something - apart from British must be best.

They are less common because people do what peole have always done because that's what's done so that's what they do.

There was a good reason for introducing the ring when it was devised after WW2 for saving additional cable and rewireable fuses.

None of these reasons applies today with MCBs so the ring is virtually pointless but, as above, British tradition is hard to break.

 

[flameport]

As I understand it there would typically be 2 rings for the sockets in a domestic, one for upstairs and one for down.

That is (sadly) a common installation method, but it was not the original intent of ring circuits.
Even worse is where people put in one for each floor and another for the kitchen - mostly pointless but still a common thing.

I also read that if designing from scratch you should not design in spurs for sockets and to leave this design to where future alterations or extensions may be required

As before- just because people now do that doesn't make it correct or sensible.

I was also under the impression that rings were better due to load balance etc. and that radial were less common/preferred in the UK?

A ring can have better load balance, which is also part of the original design intent, and would be applicable if there was one per property.
However with one per floor and one for the kitchen, and even another for a futility room, any such notion of balance is thrown away.

They are only common because since they were designed in the 1940s, most people have just installed the same old thing without actually thinking about why they are doing it or whether it is actually the most appropriate type of installation.

 

[JohnW2]

There is no logical reason to use a ring nowadays unless the circuit actually is in that shape.

... other than 'CPC redundancy' - although that, if considered desirable, could be achieved without making L and N into rings.

As for "... unless the circuit actually is in that shape", the modern tendency to require some sockets on most walls of most rooms means that sockets circuits tend to be such that they could, to a lesser or greater extent, be designed as physical rings - but I don't think that, in itself, is a particularly good reason for choosing a ring circuit. Of course, if the circuit is fairly 'linear' in physical arrangement, then 2 x 2.5mm² cables (for a ring) will actually cost a fair bit more, and use more copper, than would a "1 x 4mm² radial".

Kind Regards

 

[EFLImpudence]

... other than 'CPC redundancy' - although that, if considered desirable, could be achieved without making L and N into rings.

As for "... unless the circuit actually is in that shape", the modern tendency to require some sockets on most walls of most rooms means that sockets circuits tend to be such that they could, to a lesser or greater extent, be designed as physical rings - but I don't think that, in itself, is a particularly good reason for choosing a ring circuit. Of course, if the circuit is fairly 'linear' in physical arrangement, then 2 x 2.5mm² cables (for a ring) will actually cost a fair bit more, and use more copper, than would a "1 x 4mm² radial".

I must say that that is a bunch of very strange comments.

 

[JohnW2]

I must say that that is a bunch of very strange comments.

Why?

My comment about CPC redundancy (about the only electrical argument for a ring final) was straightforward enough - if one feels the need for it, it can be achieved without an (L/N) ring.

As for the rest, a slight alteration of your words seems to indicate that you were saying that "... there is a logical reason for a ring nowadays if the circuit is actually in that shape". I pointed out that the modern desire for many sockets in each room meant that it would often be possible to wire a sockets circuits in a fairly 'ring' shape but that, nevertheless, I did not feel that that, in itself, was necessarily a good reason for using a ring.

The final bit was just a correct statement of the facts - that if sockets are arranged in a fairly 'linear' fashion, the both cable cost and amount of copper used are likely to be less with a 4mm² radial than with a 2.5mm² ring.

What is 'strange' about any of that?

Kind Regards, John

 

[EFLImpudence]

My comment about CPC redundancy (about the only electrical argument for a ring final) was straightforward enough - if one feels the need for it, it can be achieved without an (L/N) ring.

Well, you often mention CPC redundancy and while it may be true it is not a consideration for any other circuit so cannot be thought to be of any importance. That it may be an advantage is far outweighed by the real disadvantage of the same happenning to the other conductors and it is not the only reason for a ring.
Then saying "without an (L/N) ring"; I'm not sure what the brackets do but a DIYer may infer than not connecting one end of a ring circuit's L & N was alright.
Why do you not advocate (what is a form of) high integrity earthing for all circuits?

As for the rest, a slight alteration of your words seems to indicate that you were saying that "... there is a logical reason for a ring nowadays if the circuit is actually in that shape".

I meant more the reverse - if it is not the actual shape then there definitely is no logical reason.

I pointed out that the modern desire for many sockets in each room meant that it would often be possible to wire a sockets circuits in a fairly 'ring' shape but that, nevertheless, I did not feel that that, in itself, was necessarily a good reason for using a ring.

Why?
It's a better reason than running the outward and return legs in the same place.

The final bit was just a correct statement of the facts - that if sockets are arranged in a fairly 'linear' fashion, the both cable cost and amount of copper used are likely to be less with a 4mm² radial than with a 2.5mm² ring.

Yes, of course, yet people continue with rings regardless.

What is 'strange' about any of that?

Namely that most of it didn't really need saying in relation to the thread; would be of no help to DIYers and you didn't explain the reason for most of your statements.

 

[ban-all-sheds]

on learning how to calculate this properly with voltage drops and cable length etc. It's coming out at 4mm for me.

What length and current are you assuming for this ring?

And as I type this I realise that I've not yet read the whole thread, so if anybody hits this in the next few minutes bear with me in case I have to delete it on the grounds of it being redundant.

 

[ban-all-sheds]

As for "... unless the circuit actually is in that shape", the modern tendency to require some sockets on most walls of most rooms means that sockets circuits tend to be such that they could, to a lesser or greater extent, be designed as physical rings

Surely that "modern tendency" has the opposite affect? You could argue the topological efficiency of a ring if sockets were only required on the outside walls of a property, but as soon as you want them on internal walls you end up needing a cable snaking all over the place.

Of course, if the circuit is fairly 'linear' in physical arrangement, then 2 x 2.5mm² cables (for a ring) will actually cost a fair bit more, and use more copper, than would a "1 x 4mm² radial".

Grave error.

A radial circuit is not, not, NOT a single cable running from the CU through every socket.

A radial circuit looks like this:

A & B could be sockets or JBs.