2.5mm or 4mm?


Postby Karungle » Tue Jan 01, 2008 3:25 am

The ground floor of my house has eleven sockets on a ring mail circuit. I'm going to renew the cable as some of casing on the old cable is in less than perfect condition. Should I use 2.5mm or 4mm twin core and earth cable for the replacement? At present it is protected by a 32A RCD.
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Postby ericmark » Tue Jan 01, 2008 11:58 am

Standard ring mains use 2.5mm cable. If wiring as spur the 4mm is required to allow a 32amp feed. But on a ring main with 2.5mm you can fit a third cable and take a spur but 4mm is normally too thick to allow a third cable to be fitted. If one needed a longer run then 4mm ring could allow this. As a rule of thumb 100 sq meters per ring is used but really this is too much in any normal rewire to come within the 4% volt drop or 9.2 volt. About 60 meters of 2.5mm cable is the limit and by time one goes up and down walls and 1/3 into room before drilling beams one can quickly use up 60 meters I have not calculated volt drop with 4mm cable but you can use more. With most sockets being feed through an earth leakage device the earth loop impedance is rarely an issue R1 and R2 are only measured to enter them on paperwork but with the Part P regulations this must be completed. It is a gray area as to replace a cable due to damage does not require Part P but even like for like a consumer unit does. the problem comes where on installation work you can't sign off other peoples work and you need at least the 16th Edition qualification to sign it off. One could take the course and exam it tests one ability to read a book more than electrical know how it's the C&G 2391 that is the real test but the government do not seem to realise that so only ask for 16th Edition (BS7671:2001) this is due for upgrade soon.
Also consider when an electrician rewires he will tend to use special tools to cut neat channels down your wall. Very long drills to drill behind skirting board on a two up two down one man about one to two weeks but DIY you will take months to do same work. Some firms will blitz a house in one or two days with 4 or 5 men. It's not the wiring that causes mess but all the plaster and lifting floor boards etc. Think hard before going down DIY line it may be more trouble than worth. When a few years ago my son did house rewires he spent a couple of thousand pounds on grovers drills and test rigs worth it to do 15 houses a year but not to do one.

All best Eric (Links cannot be published)
ericmark

Postby sparx » Tue Jan 01, 2008 9:09 pm

Ericmark,
please help a newcomer to electrics(sic) i've only been in the game since 1983 and until last year I have never heard of using 4mm for spurs from ring. Where did this piece of misinformation come from? Niceic I would guess as they are good at it....I have been unable to trace such a requirement in any regs book form 14th edition to proposed 17th.
Also people saying can only spur from socket, must fit 'pigtails' to back boxes even when using twin/cpc cables, & so on, is someone out there rewriting regs unofficially or what?
Presume new 'Domestic Installers-part-P' must carry some blame,
IE ' last week I couldn't spell Lectrishun-now I are one!'....
happy New Year, thank God only 5 years to retirement, can't wait,
regards to you & all others prepared to help others,
SPARX
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Postby Karungle » Tue Jan 01, 2008 10:08 pm

This is interesting. I had expected a pretty straight answer of either to use the 2.5mm or the 4mm cable. Maybe I need to say a little more.

I shall go underneath the floor before I begin and make myself a little plan of the existing wiring setup. I am guessing that there is a single ring but I may be wrong. I reckon that 50m of cable should easily cover the whole distance though. We have a modern consumer unit which was put in when the house lighting was rewired a few years ago.

There is plenty of space beneath the floorboards to crawl around so access shouldn't be a problem. I do not intend to raise the sockets from where they are as I am quite happy with their present locations. Raising them would create an unacceptable mess anyway and I have no intention of redecorating the whole house. I will put in new sockets though.

I should mention that I live in Scotland so I do not think that there would be a problem with me undertaking the work.
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Postby sparx » Tue Jan 01, 2008 11:44 pm

oops typo in my post 1983 should have been 1963, did myself out of 20 years experience...if only!
Scotland at present escapes dubious benefits of part-p & a lot more that our mainly Scottish Gov. has endowed on the rest of the UK, enjoy while you can....
Volt drop sums are irrelevent as long as one ring for each 100m2 or part, so if house total area is 101m2 then must have 2 rings with load spread as evenly as practicable across them. The IET have already done the sums for us & state catagorically in the regs book that if we follow the rules for rings then 'No Further Calculations Required!'
Look up A1 ring circuit for confimation, also see A2 radial 32A MCB-4mm2 cable under 50m2, A3 radial 20A MCB-2.5mm2, under 20m2. all domestic only standard circuits, all other circuits must be 'designed' and calcs made available with test sheets for later ref. @ Periodic Inspections,
hope this helps, what you propose seems fine to me,
regards SPARX
Last edited by sparx on Wed Jan 02, 2008 12:01 am, edited 1 time in total.
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Postby 333rocky333 » Tue Jan 01, 2008 11:59 pm

Also keep reading about this 4mm spur from 2.5 ring !
Dont think its relevant in your case though.
You sound like you need a 2.5 ring 32amp mcb, or if you desire create two if you have enough spare ways, you should not need to wire any so called spurs.
Check part p first I know it covers wales so maybe scotland
******************
Everyone seems to interpret the regs differently
The 17th is completely rewritten, with new lay out and numbering so may be clearer.

To the best of my knowledge and 16th regs
******************
source bs7671 regs book

definition of a spur= Branch from a ring final circuit
******************
Source iee guide to building regs

A non-fused spur is wired in the same size (csa) cable as the ring final circuit.
******************
Source iee quide drawing

Spur can be from a socket
Spur can be from a j/b (accesibility of such can be disputed though)
******************
The spur from the mcb/fuse or ring origin does not now appear though, so unsure if this still allowed,
maybe now with consumer units they expect you to have enough ways to do a seperate circuit instead rather than that.
This could now be classed as a radial circuit, Bunched in with the ring circuit if you did this, so dont know
*****************
Source iee quide
Although 2.5 mm is recognized size you are still limited to 100met sq and total cable length is limited to maybe about 70 metres to get relevant Ze for the fuse/mcb fitted usually 32amp
Therefore to be sure keep cable length less than that or form two smaller ring circuits.
This is usually ample for domestic.

There is no reason you cant do a 32amp ring totally in 4mm, but unless someone can verify you can mix 2.5 and 4mm on the same ring I personally would avoid mixing both.
In my view If you did want to work out voltdrop, ze, etc, you would have to be very clever if there was two sizes mixed.

Height of your sockets and rcbo / rcd protection may be another issue though.


SPARX
The regs used to say, NOT SURE IF STILL DOES, the Box and socket frontplate must be linked with g/y earth MINIMUM size 2.5mm, therefore the 1.5 earth in 2.5 T+E would be insufficient, I will try to find out if still applies.
The money you have stashed from all that old copper wire you should be able to retire now.
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Postby ericmark » Wed Jan 02, 2008 12:16 am

Wops did I say 4mm spur from 2.5mm ring not what I intended to say I was talking about 4mm spur from consumer unit sorry to mislead. I have got three 2.5mm into socket but not a 4mm with two 2.5mm and unless a very deep box fitting block connectors is very tight. As to only allowing a spur from a socket that's not true you can use junction boxes as long as they are accessible. Although can't really see how boxes under floor boards are accessible? And yes I tried to put Karungle off DIY I did not want to just answer use 2.5mm which would have answered his question then find there was something about his house I had not foreseen. I have not seen any 32Amp RCD's RCBO's yes but 40 amp seems to be norm for smallest RCB and 80amp more normal. I would expect Karungle was thinking of a MCB but I took it as in indication he may be giving duff info. I wanted him to enter into the project eyes wide open and not be one of those who having realised they should have should have registered the job under part P after already completing it and worrying about what to do next. He did not say he lived in Scotland which I think makes it a lot easier for him. You were a late starter like me I started in 1969 as an auto electrician and only in 1979 did I start with low voltage electric systems (50 - 1000ac is low voltage to other I know Sparx knows) Having spent about 10 years of that overseas you have probably more knowledge than me. Especially on domestic most of my time was industrial. And I made my fair share of mistakes especially on paper work knowing to put EEBADS in the Method of protection against direct contact. And method 1 to 20 in Installation method 1 being the normal for a house was often entered incorrect. I for many years recorded the earth loop impedance without a clue as to what was the pass level the same with R1 and R2 reading best is no one ever picked me up for them being wrong and I know I used a whole role (100 meters) of 2.5mm on the ring main of one house and I never doctored the results so someone should have picked up on the R1 R2 reading being too high. But then no one sent results to clearing house or council. As I moved up I realised all my mistakes and I also realise any DIY man is bound to make the same mistakes as I did. Although now I have 2391, 16th Edition, Inspection and testing of in service electrical equipment and management of same plus a foundation degree in electrical and electronic engineering this was not always the case. I originally trained as an auto electrician and made some real bummers on the transition. Why should I expect any DIY not to make the same mistakes as I did. And yes you are wright again I should pay my dues to the IEE or the new name what ever it is and become updated in all electrical practices but in 2004 I was involved in an industrial accident and have not been able to work since and quite frankly can't afford their fees.

All best Eric
ericmark

Postby ericmark » Wed Jan 02, 2008 10:09 am

Sorry used the word spur where I should have said radial my fault. Dyslexic rules KO.
ericmark

Postby sparx » Wed Jan 02, 2008 7:44 pm

Hi again rocky & others sorry to go on about one item but regs have never required back boxes to be pigtail earthed from front plate!
Reg # 543-02-07 is the only one I know to mention such bonding & that is if using conduit or cable sheath(MICS) as CPC, when it is required to bring the earth from back box to plate, not vice-versa.
Those who say you can't use conduit as CPC should be refered to reg 543-02-05/04.
It would be very helpful if anyone refering to regs could quote relevant number to avoid confusion, (please note NICEIC say what they would like regs to be not what they are!!)
I await delivery of the new 17th RED BOOK, more fun on the way....
best wishes SPARX
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Postby BLAKEY1963 » Wed Jan 02, 2008 11:17 pm

[quote="sparx"]Hi again rocky & others sorry to go on about one item but regs have never required back boxes to be pigtail earthed from front plate!
Reg # 543-02-07 is the only one I know to mention such bonding & that is if using conduit or cable sheath(MICS) as CPC, when it is required to bring the earth from back box to plate, not vice-versa.
Those who say you can't use conduit as CPC should be refered to reg 543-02-05/04.
It would be very helpful if anyone refering to regs could quote relevant number to avoid confusion, (please note NICEIC say what they would like regs to be not what they are!!)
I await delivery of the new 17th RED BOOK, more fun on the way....
best wishes SPARX[/quote]

SPARX
i read all this with intrest AND CANNOT wait for 17th red book
as well

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Postby sbelectrical » Mon Jan 07, 2008 5:37 pm

I love my job being a sparx but guys please dont get stressed the man only asked quite a simple question, not who on here knows the regs more hehehe.
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Postby thedoctor » Mon Jan 07, 2008 6:11 pm

Would somebody be able to answer the guys question?
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Postby kbrownie » Tue Jan 08, 2008 12:28 pm

Hi,
Simple answer; if the ring main is protected by a 32MCB and the area of installation does not exceed 100m2. 2.5 twin and earth is fine.
But this must be done by part p qualified electrician.
Best of Luck
KB
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Postby Karungle » Fri Jan 18, 2008 8:59 pm

Thanks for your help. I used 2.5 twin & earth. The work does not have to be done by a part p qualified electrician though as I live in Scotland. Anyway it has now been checked over and passed by a fully qualified electrician so I'm a happy bunny. :) Thanks again.
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