CABLES IN THE LOFT


Postby cubs » Thu Aug 25, 2011 1:34 pm

DearSir/Madam,
I live in a bungalow and have two questions regarding the routing of the cables in the loft. The cables run through the loft down to the skts and lights in the rooms below.
Q1. Is it ok to drill holes in ceiling joists to run the cables through?.

Q2. I am doing a rewire and would like to know the best route to run the cables in the in the loft, so if a a loft conversion was carried out in a few years time the disruption to the cables in loft would be a minimum.


Yours sincerely,
Mr. B.Cannings.
cubs
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Postby BLAKEY1963 » Thu Aug 25, 2011 3:59 pm

Mr B. cannings

The regulations of the current 17th edition do list joist depths for cables.
derating factors for running cables exposed to loft insulation is also listed
. The rewiring of your property can be disscussed when informing local authority
building control .
They will explain to you about part p once you pay them the appropiate fee required.
The building control officer can then answer your questions where you wish to run cables regarding your joists and he can approve your cable routing b4 enclosing.

BLAKEY1963
BLAKEY1963
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Postby BLAKEY1963 » Thu Aug 25, 2011 4:10 pm

[quote="cubs"]DearSir/Madam,
I live in a bungalow and have two questions regarding the routing of the cables in the loft. The cables run through the loft down to the skts and lights in the rooms below.
Q1. Is it ok to drill holes in ceiling joists to run the cables through?.

Q2. I am doing a rewire and would like to know the best route to run the cables in the in the loft, so if a a loft conversion was carried out in a few years time the disruption to the cables in loft would be a minimum.


Yours sincerely,
Mr. B.Cannings.[/quote]
BLAKEY1963
Posts: 654
Joined: Wed Jun 13, 2007 10:57 pm


Postby sparx » Thu Aug 25, 2011 4:22 pm

Sir, you say you are doing a rewire, presumably you have notified LABC before starting and paid their inspection fee as required by law. Unless of course you are Part-P registered in which case you should know the answers already.
If you have I would respectfully suggest obtaining a copy of the 'Electrical guide to the building regulations' (ISBN 0-95-487910-4) or similar.
This states wiring routes such as along eaves or drilled in centre 1/3 of depth of joists between 1/4 &1/3 of span from each supported end etc.
Don't forget you must issue Installation Test Certificates for all work done, or not only be non-compliant with regs. but may invalidate your household insurance, and jepodise future sale.

regards
sparx
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Postby sparx » Thu Aug 25, 2011 4:23 pm

Sir, you say you are doing a rewire, presumably you have notified LABC before starting and paid their inspection fee as required by law. Unless of course you are Part-P registered in which case you should know the answers already.
If you have I would respectfully suggest obtaining a copy of the 'Electrical guide to the building regulations' (ISBN 0-95-487910-4) or similar.
This states wiring routes such as along eaves or drilled in centre 1/3 of depth of joists between 1/4 &1/3 of span from each supported end etc.
Don't forget you must issue Installation Test Certificates for all work done, or not only be non-compliant with regs. but may invalidate your household insurance, and jepodise future sale.

regards
sparx
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Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.


Postby ericmark » Fri Aug 26, 2011 3:45 am

When doing re-wiring for power cables I look for a route which will not likely be covered in insulation. For lighting there is not so much of a problem. With exception of bungalows normally the cables go under the floor rather than through the loft.

One does need to do some careful planning as it is all too easy to end up using too much cable and for the loop impedance and therefore volt drop not to comply with regulations. There are two sets of linked regulations. The BS7671:2008 is the electrical publication which the different parts of the building regulations refer to. The parts of building regulations are given letters and the main electrical bit is called "Part P" however there is also L, F and M which can also have some bearing on what is required.

The IET guide to 17th Edition (another name for BS7671:2008) includes many of the other requirements not given in the main book. For example there is a minimum number sockets for each room which varies according to it's use.

The problem with DIY is the involvement of the LABC who can demand one to follow certain safety procedure which can cause problems. In the main they often want to inspect before power can be returned to circuits which can result in no power to work with. The logistics have to be carefully planned so one can use power tools to do the work or have all battery tools or generator.

The inspection and testing is a real problem. You should do it and submit it to the LABC who can either accept your readings or re-test to verify your reading. If you don't test they can charge you when they test. The problem there for is you need test equipment.

Although the Part P regulations allow one to DIY in real terms this allows an electrician who is commercial to do the odd domestic job or work on his own house and is not really designed to allow the un-trained guy to DIY. The help offered varies from council to council and also they can refuse to let you do the work if they don't think you have the skill.

My parents needed some work doing and my son and I decided we would do it. When the LACB inspector arrived first question was who will sign the installation certificate. My son said he would and since he held his C&G 2391 and C&G 2381 (Now 2382) there was no real reason why he should not do so. However the inspector was not satisfied at which point my son said OK my dad (me) will do it, he has a degree in electrical engineering. At which point the guy backed down and allowed us to proceed. In real terms this was wrong as my son had insurance and I didn't. However we did have the full test set and he did want to see we had the instruments.

When completed the work was not checked by the LABC inspector and he accepted my paperwork without any complaint. However it was correct and I think he knew it would be.

It does vary as I have said and the building inspector does not always know the right method anyway. We were doing a bathroom and he insisted on a fan being installed in spite of the room having opening windows which according to regulations means we did not need a fan.

Fortunately we submitted a detailed plan so any items he did not like could be addressed before we started. That is far easier than having to "do it again and do it properly". This does mean some careful measuring. Although 106 meters seems a lot of cable it is easy to exceed. And also I would never wire to the limit. I would consider 80 meters to be safe maximum as if you have energised before the LABC inspector arrives the readings will be lower.

As to loft conversion normally this requires the beams to be beefed up so likely it will not matter where you run cables they likely will need completely removing or will be bridged and unless you have already had plans drawn up one can't really work out what will or will not get in the way. I have in the past routed cables up the A frame to avoid the insulation only to have to then re-route as insulation is put in the roof rather than between rafters when converted.

Which cable to use also has to be considered. Although expensive Ali-tube cable does have some plus points when compared with twin and earth.
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