Electric cables and flat roof insulation‎

[Thomoose]

Hi,

I am currently renovating my kitchen and am planning a rewire. I have a flat roof and the ringmain runs inside the roof and comes down to the outlets. The ceiling is being replaced but I am unsure what to do about running the cables through the roof void with the insulation. Currently the cables are clipped to the wooden joists and the void is packed with insulation - Is this OK or should the cables run in a conduit of some type?

I have read that cables should not be covered by insulation but what are the rules when laying in a flat roof? I have also read that it is OK because they are clipped to the joists but as with all these things there is so much conflicting information on what is the right way? The roof is not vented so there is no air void - hence why the void is packed with insulation.

The rewire is straight forward - We basically want more outlets on the ringmain and we want two lights instead of one and the switch will be moving from one side of the room to the other. The only major change is that we will be installing a cooker point because we currently only have a gas point and want to have an electric oven in the new kitchen. This cable will also run through the flat roof so what will I need to do about the insulation and this cable or can I run through a conduit?

Any advice will be very much appreciated and I hope what I am asking makes sense.

 

[wingcoax]

Are you aware of "PART P" in the building regulations?

 

[Thomoose]

I am aware of Part P but even with these regulations how come so many people undertake DIY electrical work themselves?

I plan to disconnect the old circuits from the consumer unit myself before carrying out the work but will have them reconnected and checked by a competent person. I am able to design and install a circuit myself but the finer details such as running it trrough a flat roof I am not to sure about. I rewired the remainder of the house myself about 3 years ago and only paid for a sparky to come an fit a new consumer unit and sign the work off. It cost me about £450 to do this rather that the £1400 I was indicated for a full rewire. the kitchen was left because it was always my intention to redesign it once I had the funds to install a new kitchen.

 

[ban-all-sheds]

I am aware of Part P but even with these regulations how come so many people undertake DIY electrical work themselves?

Most of them mistakenly think they know what they are doing.

I plan to disconnect the old circuits from the consumer unit myself before carrying out the work but will have them reconnected and checked by a competent person.

And when that person checks them, and finds that they are no good, what then?

I am able to design and install a circuit myself but the finer details such as running it trrough a flat roof I am not to sure about.

That's not a "fine detail", it's a fundamental aspect of circuit design, and getting it wrong could cause the house to burn down.

You are not able to design and install circuits yourself.

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• //www.diynot.com/wiki/electrics:books[/list]
  
  

  I rewired the remainder of the house myself about 3 years ago and only paid for a sparky to come an fit a new consumer unit and sign the work off.

  What do you mean by "sign off"?
  
  Did you end up with a BS 7671 EIC saying that somebody competent had done the design & construction?
  
  Did you end up with a completion notice saying that it complied with the Building Regulations?

 

[Steve]

Instead of going down to each socket, run the cable horizontally between them, then up or down from the last sockets. Make sure the cables follow safe zones.

 

[Thomoose]

What do you mean by "sign off"?

Did you end up with a BS 7671 EIC saying that somebody competent had done the design & construction?

Did you end up with a completion notice saying that it complied with the Building Regulations?

All circuits tested and issued electrical installation certificate compliant with Part P. Only remedial work needed were gas and water eathing. House still not burnt down three years later.

I thought this would be quite a simple job and my only concern/question is really what to do about running cables through insulation/flat roof. A ring main is a ring main or is it not and a cooker point is also a single cable straight from the mains or is it not? I only want advice on how to run the cables through the roof correctly and not to be told about told about the building regs because I will getting the work checked and certified.

 

[ban-all-sheds]

All circuits tested and issued electrical installation certificate compliant with Part P.

So was that by applying for Building Regulations approval in advance and submitting a PIR to Building Control because that's how they worked and what they asked for? I'm hoping that you're just being inaccurate with your terminology, because you would not legitimately have got an Electrical Installation Certificate, either from the electrician or the council.

I thought this would be quite a simple job

It's not rocket science.

and my only concern/question is really what to do about running cables through insulation/flat roof.

Is that your only concern/question? Or is it that it's the only one you think you don't know, and you've not asked about other things you don't know because you have no idea you don't know them?

A ring main is a ring main or is it not and a cooker point is also a single cable straight from the mains or is it not?

Yes, but you do have to ensure that the former has an Iz of at least 20A, and the latter complies with Ib ≤ In ≤ Iz, with (in both cases) Iz taking account of the installation method and the type of protective device.

I only want advice on how to run the cables through the roof correctly

Those links I gave you tell you that, but you should also get a copy of the Wiring Regulations or On-Site Guide to get info on the installation methods recognised.

and not to be told about told about the building regs because I will getting the work checked and certified.

So you've notified, and the inspection arrangements are at the behest of LABC?

 

[LoveRocket]

Banal, take it easy with the part-p stuff, you've boasted about how you ignore it, so it's a bit rich of you to continue attacking new posters over it.

Thom, you really need to keep those cables away from the insulation. To be able to use cable on a 32a ring final circuit, the cable has to be able to carry 20a. 2.5mm cable installed to ref method 100 (clipped to a joist surrounded by LESS THAN 100mm of insulation) is good for 21a, so you'll just get away with it, more than 100mm of insulation and you're down to 17a, and this is a no no.

You really can't mess around with this mate, a ring circuit is a terrible design anyway, especially on a high-load situation like a kitchen, and now there's thermal insulation involved. You should be looking at running several radial circuits, and go out of your way to keep the cables away form the insulation.

PS. A copy of the On-site guide would be a great investment .

 

[ban-all-sheds]

Banal, take it easy with the part-p stuff, you've boasted about how you ignore it, so it's a bit rich of you to continue attacking new posters over it.

Will you please explain how this exchange:

Are you nofitying this work?

No.

can in any reasonable way be construed as "boasting"? If, as I'm pretty certain, you won't be able to come up with a logical explanation which even makes sense to you, do you think you might tell me when I can expect you to stop your ridiculous behaviour?

And I wasn't attacking anybody, as I've often said, I don't care if people in a non-professional capacity ignore the notification requirements of the Building Regulations, although I do think they should only do so in an informed way. But if they are labouring under a misapprehension of how notification and certification works, or if they are the victim of, or conspiring with, a corrupt electrician then these are all different situations.

 

[bongos]

Thomoose, purchase this before you start work. At least you can make sure you know where you're at with circuit desgin and why circuits are spec'd like they are.

You never know, it might come in handy if you do more work at a later date...

 

[Thomoose]

LoveRocket - thanks for sticking up for me - I am new to this and thought this forum was somewhere I could go for help and not to be preached about building regs.

Ban-all-sheds - I have to agree that you are boasting. If I didnt need help I would not have posted on the forum - To quote your good self "It is not rocket science" but you are trying to baffle me with science. I am looking for tips and pointers not chapters and verse about right and wrong and questioning prevoius work I have had done. From what you are say - everytime I have an electrician do any work on my house they would have to replace everything because they did not do the original work.

I am aware of the issues with running cables with insualtion and the devaluing but as per my original post can I run them in a conduit or not. The rafters are 130mm and the full 130mm will be packed. As the roof has no ventilation the rafters have to be packed to comply with building regs and solve problems with condensation. The current ring main and lights ran in the rafters packed with insulation but 25 years ago when the extension was built this was probably OK with building regs.

 

[ban-all-sheds]

LoveRocket - thanks for sticking up for me - I am new to this and thought this forum was somewhere I could go for help and not to be preached about building regs.

Nobody is preaching to you about the Building Regulations. I was just trying to find out how you thought it worked and what you thought you had done about it.

All I asked was for clarification of "All circuits tested and issued electrical installation certificate compliant with Part P", as it didn't make sense, and confirmation that you've notified, and that the inspection arrangements are at the behest of your LABC, because nothing else works.

But if you want to proceed on possibly mistaken beliefs, or possibly be stitched up by a dishonest electrician or possibly end up one of the people we see here quite often who have gone down one of those roads and are now between a rock and a hard place with Building Control, then go for it.

Ban-all-sheds - I have to agree that you are boasting.

Ok - will you please explain how this exchange:

Are you nofitying this work?

No.

can in any reasonable way be construed as "boasting"?

If I didnt need help I would not have posted on the forum - To quote your good self "It is not rocket science" but you are trying to baffle me with science.

No I'm not - I'm giving you the help you need to get to the stage where you are able to to do the job you said you wanted to do.

I am able to design and install a circuit myself.

But now you find the process of doing it baffling?

I am looking for tips and pointers

And here they are:

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• //www.diynot.com/wiki/electrics:books[/list]
  
  You said you wanted to DIY. This site is not about absolving you of the responsibility to become competent to do what you have chosen to do. It is not about doing it for you.
  
  
  

  not chapters and verse about right and wrong

  The only "chapters and verse" I have given you are ones telling you the right way to design circuits. If you aren't prepared to read them because you think it's wrong to do so then I suggest you don't try to do your own design & installation.
  
  
  

  and questioning prevoius work I have had done.

  The only previous activities of yours I have questioned (as in asked for clarification) are to do with notification and certification, but see above regarding that.
  
  
  

  From what you are say - everytime I have an electrician do any work on my house they would have to replace everything because they did not do the original work.

  What nonsense - I have said or implied nothing of the sort.
  
  
  

  I am aware of the issues with running cables with insualtion and the devaluing but as per my original post can I run them in a conduit or not.

  • http://www.tlc-direct.co.uk/Book/4.3.1.htm
    
    
  • http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
    
    
  • http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
    
    
  • 
    http://www.amazon.co.uk/exec/obidos/ASIN/0863418546
    
    
    
    http://www.amazon.co.uk/exec/obidos/ASIN/0863418627

 

[Thomoose]

Banal, take it easy with the part-p stuff, you've boasted about how you ignore it, so it's a bit rich of you to continue attacking new posters over it.

Thom, you really need to keep those cables away from the insulation. To be able to use cable on a 32a ring final circuit, the cable has to be able to carry 20a. 2.5mm cable installed to ref method 100 (clipped to a joist surrounded by LESS THAN 100mm of insulation) is good for 21a, so you'll just get away with it, more than 100mm of insulation and you're down to 17a, and this is a no no.

You really can't mess around with this mate, a ring circuit is a terrible design anyway, especially on a high-load situation like a kitchen, and now there's thermal insulation involved. You should be looking at running several radial circuits, and go out of your way to keep the cables away form the insulation.

PS. A copy of the On-site guide would be a great investment .

I have had a look into this and I think can use Celotex 80mm insulation board. Install it so it fits against the marine ply (underside of roof) thus leaving at least 50mm gap between the ceiling and insulation (at least because there is a slight pitch on the roof so at one end there will be about an 80mm gap). I will have to look into this a bit futher but I am lead to beleive that this will create a warm roof and will eliminate condensation problems.

To save time and energy I will arrange for a sparky to come round to discuss what should be done and how much of the work they will be happy for me to do. When I rewired the house a few years back I simply replaced all the cables, added 10 outlets and split the ring main in two (up and down stairs). Shower cable was upgraded to 10mm to cope with new shower and switched moved to other side of bathroom. The lights I replaced all cables, moved a couple of switches and split the loop in two again.

Thanks for all your help (including you ban-all-sheds) but I dont think forums are for me. It is easier to get someone (or two) round and have them tell you face to face.

Its is not DIY if you are not doing it yourself and these days with all the rules and regs the only things you can really is a bit of decorating or work in the garden.

 

[LoveRocket]

Hmmmm, ref method A is "enclosed in conduit in an insulated wall" and gives a CCC of 20a for 2.5mm cable - just scraping in for a 32a RFC. You must leave 45% of the conduit free, so use a big one!

Still, you really REALLY need to think hard about your design here. You are installing a cable right on its safe limits, and any unbalanced loading could push it over. A fault with ring continuity in the future WILL put it over it's limits - imagine, a fault on the ring somewhere, the washing machine, dishwasher, kettle and toaster all on, dragging 30a one leg of the ring, the little 2.5mm cable covering in insulation massively overloaded.

Whilst your kitchen is in bits, you've got opportunity to design a much better installation. Ring circuits are only still used 'because we've always used them'. It's time to move forward like the rest of the world and ditch them.

 

[ban-all-sheds]

I have had a look into this and I think can use Celotex 80mm insulation board.

Will that provide the U-value you need for compliance?

I will have to look into this a bit futher but I am lead to beleive that this will create a warm roof and will eliminate condensation problems.

You'll need a vapour control layer, e.g. foil-faced Celotex, foil-backed plasterboard.