Board upgrade and new feed tails

[apprentice87]

433.3.1(iii) & 434.3(iii)

basically if DNO don't agree (i.e over 3m, or whatever they say) then 7671 requires overload / fault protection

Not sure if it matters?? What i mean is, the DNO might not like you relying on their fuse, but a fuse IS a fuse, regardless of who owns it, so the cable IS protected as the regs require??

john..

 

[Andy™]

depends how you interpret it. BS7671 requires overload / fault protection unless certain criteria are met to omit it. one of those being using DNO's fuse. if DNO say no then imo that exemption hasn't been met therefore will require a fuse fitted to comply

ultimately,the DNO fuse will still protect the cable the same way as your own fuse would so electrically safe just non compliant on a technicality

 

[Captain Kirk]

It sounds like your electrician has the best of intentions but limited knowledge or experience of the DNO and BS 7671 requirements.

1/ The DNO does not allow their fuses to protect the consumer tails or any submain. There is a dispensation for DI PVC tails up to 3m adjacent to the cut-out and not liable to be hidden or damaged (meter box or immediately through the wall) since 16th Ed 1980 and ESQCR.

2/ Consumers unit and switchgear in excess of 3m distant or cables buried in building fabric require protection/isolation via HRC switchfuse - the recommended domesctic MEM proprietary unit is 80A.

Tails from meter to switchfuse to be DI PVC sized on consumer unit main switch module - 100A - 25mm tails - Earthing lead from DNO terminal to MET - 16mm PVC copper.

No RCD protection of submain supply to consumer unit. That is why the Split consumer unit was implemented to prevent the whole of the installation being disconnected by one RCB - particularly lighting.

3/ Submain to consumer unit can be concealed SWA as phase conductors are protected by earthed concentric sheild for burying less than 50mm. Consider the R2 value of the SWA cpc or use a 3 core SWA with incorporated copper cpc core.

PVC DI/single PVC cpc or PVC T&E can be buried if protected by earthed concentric shield - steel conduit/trunking - earthed metal capping is not recommended.

4/ If the DNO fuse requires up grading - this will be chargeable and could be excessive if the cutout has to be renewed. A 60A fuse cannot be upgraded to 80A in a 60 A outcut, although 80/60A were installed. A 100A fuse can only be fitted if the DNO supply cable is 25mm copper or 35mm Al.

A formal application indicating maximum demand requires to be submitted to the DNO, but they will almost certainly enforce the 3m rule

 

[unphased]

I looked more deeply into the so called 3m rule some time ago and have posted my thoughts on the NAPIT forum if anyone is interested. At some stage in proceedings somebody in the DNO misinterpreted a Reg. and decided to limit their tails to 3m to meet the Reg. They don't want to admit this so they kept it as one of their requirements. They used to refer to it as a BS7671 requirement but not sure they do that now.

 

[MARK MCCANN]

Ok, So far as i know, there are no rules, regulations, laws, or anything else that says that if you have tails longer than 3 metres then they have to have an OCPD.

Yes, so far as i can remember the regs do state that a cable over this length has to have an OCPD all of its own, if the cable requires it, [being of a lesser current carrying capacity than the cable that feeds it]

No idea of the cable size that feeds your house or its capacity, but you could argue that the board fuse protects your cable, BUT, the DNO do not like the idea of THEIR fuse being used as protection for the cable, so you can have any tails you like really, how are the DNO going to know?? At the end of the day, a fuse is a fuse and the cable will be protected..

Having said all that, Mr Electrician sounds like he wants to do everything PROPERLY and how it should be. This is no doubt for TWO reasons; 1, he sounds like a good electrician, and; 2, he wants to do it all by the book as it were to prevent future problems..

As for increasing the size of the tails, why not, especially as they will be plastered into the walls...

Finally, how much more would 25mm tails cost?? Errm, not much.. Now [and there is another thread on here that talks about this sort of thing] since the I2R losses are going to be lower using the 25mm cable, there will be a saving to be made over the long term as you will not be wasting money heating the cable up [in theory at least] For tails about 3 meters long and a constant current of 30A you will save about 1 watt so if my maths are right at current [hahaha] prices if you drew 30A 24 hours a day you would save about 0.7 of a penny a day.

Anyway, sounds to me like you have found a very good electrician, so my advice is to let him get on with it..

john..

Thanks for your reply...he's done the work and im very happy. Went with the larger tails (25mm) and he put a large earthed hd galv cover full length of cables, with plenty of rawlplugs and screws to secure it (he bent this over at top, to avoid sharp edge) and put on some of left over cable insulation around the tails where they exit out the galv cover into the ceiling void. He's also fitted a DP isolater with fuse in the meter cupboard outside. All the work looks of high quality and is very neat. A full Rcbo is also now in as well.

 

[Andy™]

hope its a steel plate and not capping

 

[sirsparkabit]

I have never seen the point of galvanised capping ,never bought any never will .
Seem to remember something about 3mm thickness for steel plate ,however I may have dreamt it .
As long as op is happy with job hey ho

 

[unphased]

Well, all I can say is thank you to all those sparks who do use it else house rewires would be twice as difficult. Being able to pull new wires in using old wires dragged through the capping, especially on lighting circuits, is great.

 

[sirsparkabit]

Well, all I can say is thank you to all those sparks who do use it else house rewires would be twice as difficult. Being able to pull new wires in using old wires dragged through the capping, especially on lighting circuits, is great.

I use oval for that , a far better job

 

[unphased]

Why is it a better job? Its just a different way of doing it. Its not as quick having to thread the wires through oval and getting two 2.5mm2 down an oval is awkward unless you keep them side-by-side. Capping has been used for donkey's years. Use it all the time.

 

[UNG]

Why is it a better job? Its just a different way of doing it. Its not as quick having to thread the wires through oval and getting two 2.5mm2 down an oval is awkward unless you keep them side-by-side. Capping has been used for donkey's years. Use it all the time.

You are using the wrong size oval conduit if it is awkward to feed 2 T&E's down it

 

[sirsparkabit]

I will explain my thought on it , After pulling out cables and seeing the damage you get where they have been nailed to rough walls and new ones get snagged sometimes ,I prefer the oval purely for the all round protection .
My original post was relating to galvanised capping which serves no better than plastic .

 

[unphased]

I bow down to your superior methods. Lol.

 

[sirsparkabit]

I bow down to your superior methods. Lol.

Nothing superior about it , Just my preference for the type of jobs I do .
Everyone to their own .
Sure if I did jobs in new builds ect full of breeze blocks I may use plastic capping .

 

[unphased]

Plastic capping! Yeuch!

 

[RichW]

Nothing superior about it , Just my preference for the type of jobs I do .
Everyone to their own .
Sure if I did jobs in new builds ect full of breeze blocks I may use plastic capping .

Good enough is good enough. The installer makes his choice unless the client has an opinion on it. I think I used plastic capping in my house back in 1992 but I’ve never had to replace any wires and the house includes a basement. I was rather keen back then and installed the ring circuit with no cut wires, which I haven’t done since. I usually clip straight to bare wall these days as one can assume that in 40 or 50 years it’s all going to be different anyway.

 

[unphased]

Jeez...what's this a must have last word contest? I really don't care!

 

[Andy™]

Jeez...what's this a must have last word contest? I really don't care!

yet here you are

 

[Richard-the-Ninth]

Gentlemen, you are all adults, I don't really want to lock any thread, but if I have to.........

 

[UNG]

1/ The DNO does not allow their fuses to protect the consumer tails or any submain. There is a dispensation for DI PVC tails up to 3m adjacent to the cut-out and not liable to be hidden or damaged (meter box or immediately through the wall) since 16th Ed 1980 and ESQCR.

The 16th edition was first issued 1991 and the 15th edition was first issued in 1981 so I'm a bit confused by the detail in your comment

4/ If the DNO fuse requires up grading - this will be chargeable and could be excessive if the cutout has to be renewed. A 60A fuse cannot be upgraded to 80A in a 60 A outcut, although 80/60A were installed. A 100A fuse can only be fitted if the DNO supply cable is 25mm copper or 35mm Al.

I've seen 100A fuses fitted on 16mm² copper DNO supply cables, in fact most of the old local network infrastructure that is still in use in my local area is only the equivalent of 4 core 16mm²