Electic Wires


Postby matty555 » Sun Nov 22, 2009 9:41 am

Hi

I have just totally renovated our bedroom and fitted a new wooden floor on top of a chipboard floor, part of which was already in place.

I decided to put loft insulation between the joists of the floor as this bedroom has always been chilly underfoot. I replaced one of the junction boxes to the sockets as it looked faulty and have placed all wiring on top of the insulation that I fitted, as recommended. All of the cables are standard twin and earth 2.5mm x 16.5.

My concern is the overheating of the wires because of the chipboard floor, underlay and wooden floor, even though the wiring is on top of the insulation and there is adequate space around the junction boxes.

Can someone advise me whether my concerns are justified please - or do I need to take down the kitchen ceiling and remove all insulation?

Many thanks

Matty
matty555
Rank: Labourer
Progress to next rank:
50%
Posts: 3
Joined: Sun Nov 22, 2009 9:12 am

Sponsor

Simply Build It

Postby moggy1968 » Sun Nov 22, 2009 5:35 pm

so if I understand you correctly the cables are on the insualtion, not sorrounded by it, so thats ok. even if they run surrounded by insulation it's not necessarilly a problem, but you have to calculate the load on the cables with that in mind to see if larger cable needs to be used. Part of planning an installation is calculating the cable size which takes into account the method of fixing. thats where an electrician earns his money!!
more problematic is that junction boxes shouldn't be used under floors, but as this was already there your probably ok on that one as well. the other thing should be that your cables go through holes drilled in the joist, not accross notches cut on the top of them. the rules on this are quite involved and you should check them but basically your cables need to be at least 50mm from the floorboards so they don't het a nail hammered through them, especially in a wooden floor is being laid over the top.
you also need to be sure on this type of work that you don't fall foul of the part P legislation. You should check out the requirements of this. it sounds as though you are ok, but you should check in case tere is something you haven't covered in your post.
moggy1968
Rank: Ganger
Progress to next rank:
98.7%
Posts: 198
Joined: Sat Feb 21, 2009 11:32 am

Postby ericmark » Sun Nov 22, 2009 6:56 pm

First cable size 2.5mm² twin and earth cable has an outside size of between 5.4 to 5.7 by 10.2 to 10.9mm according to if solid or stranded cable the size you state does not fit into any of the sizes given by Batt cables for twin and earth so I have no idea on what cable you have.

Pictures are not allowed on this forum so I can’t show you how it is worked out but there are 4 methods shown in the BS7671:2008 as to use of twin and earth cable.
#100 Installation method for flat twin and earth cable clipped direct to a wooden joist above a plasterboard ceiling with a minimum U value of 0.1 W/m2K and with thermal insulation not exceeding 100 mm in thickness.

#101 Installation method for flat twin and earth cable clipped direct to a wooden joist above a plasterboard ceiling with a minimum U value of 0.1 W/m2K and with thermal insulation exceeding 100 mm in thickness.

#102 Installation method for flat twin and earth cable in a stud wall with thermal insulation with a minimum U value of 0.1 W/m2K and with the cable touching the inner wall surface.

#103 Installation method for flat twin and earth cable in a stud wall with thermal insulation with a minimum U value of 0.1 W/m2K and with the cable not touching the inner wall surface.

From your description you have either 102 or 103 and for 2.5mm cable this will allow 21 or 13.5A. The former is OK for standard ring main but the latter would require de-rating or change of cable.

So if the cable is pressed against the ceiling or the floor you are OK but is surrounded by insulation or clipped mid way down beams you have a problem.

The cure is really a cable change either to thicker cable or to higher temperature cable I have given figures for PVC with Ali-tube or other XLPE (Cross-Linked Polyethylene) or LSZH (Low Smoke Zero Halogen) cables the values are higher.

From what you say it seems you are OK but I am wary of just saying it is OK in case something has been missed in the telling. Plus no idea what the cables are since sizes don’t match what Batt cables say they should be.
ericmark
Rank: Project Manager
Posts: 1796
Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.

Postby matty555 » Sun Nov 22, 2009 10:30 pm

Many thanks for your responses. Just to clarify, the cables are threaded through drilled holes in the joists, the junction box replaced some very badly fitted terminal blocks and all wiring sits on top of the insulation and not pressed up against the flooring.

Looking at the cable, it appears to be 2 x 2.5 + 1.5 to a BS6004 6242YH which was the cable that was already under the floor. With regard to the power consumption, it's only a TV/DVD player that are plugged into these sockets and these are turned off at the sockets when not used.

I think from what you're advising that we should be OK and my worries are not necessary.

Many thanks again

Matty
matty555
Rank: Labourer
Progress to next rank:
50%
Posts: 3
Joined: Sun Nov 22, 2009 9:12 am

Postby ericmark » Tue Nov 24, 2009 12:51 am

It is the B or Y at end of 6242 that is important and Y = PVC
The sockets or items connected to cable don't matter it is the size of supply fuse or MCB that matters.
As you say likely OK. But I would never say it is OK without seeing and this forum does not allow pictures.
The whole idea is to get rid of heat so if not sure clip or tie rap to lengths of cable tray or similar to radiate heat.

The course to inspect and test is quite intensive and however much I would like to tell you all the points to check would be impossible.

However I think you are likely OK.
ericmark
Rank: Project Manager
Posts: 1796
Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.

Postby matty555 » Tue Nov 24, 2009 7:59 pm

Hi

Thanks for your reply ericmark, the cable has a Y at the end of the 6242 number.

matty
matty555
Rank: Labourer
Progress to next rank:
50%
Posts: 3
Joined: Sun Nov 22, 2009 9:12 am

Display posts from previous
Sort by
Order by



  • DIY How to Project Guides

  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!



 


  • Related Topics