Cables, new insulation and fire risk??

[CharlesLevy]

Hi,


We’ve got fed up with how easily noise travels between the ground and first floor of our little house so we’ve decided to combine stripping the boards with installing some insulation under them. We’ve gone for rockwool flexi slab (http://guide.rockwool.co.uk/products/building-insulation/flexi.aspx) but I’m not too sure how to protect the cables that run through the joists under the floor. Some I can tack up onto the joists up out of the way. Some I might be able to fit into coving. But, given the how low to the ground many of the holes in the joists are (and that the insulation is 100mm thick), its going to be hard to separate much of cabling from the insulation – I’ve put in a few photos to illustrate what I mean.
I feel like this is something I should be worrying about, but is also the type of thing everyone must have already solved. Any suggestions on what to do would be very welcome!

Untitled

• CharlesLevy
• 9 Apr 2011
• 1

 

[Chri5]

Cabling will need to be derated when in insulation.

Most 1.5mm lighting cables are good for 16 amp, so being on a 10 amp main fuse should be ok. Caveat being that if your lighting is in 1mm then fuse WILL require changing down to 6 amp.

Most 2.5mm socket ring cables are near max when on a 30 or 32 amp main fuse, they will need to be derated down to a 20 amp main fuse.

They most serious issue will be shower and high load circuits like water heaters amd electric cookers (oven and hobs). The circuits tend to be very near max when clipped direct in open air, so covering them with a blanket of insulation runs a very high risk of over heating on load.

The choice is to provide the existing cable with a 50mm open air run way not in insulation, trunk them (and some derating will still be required) or to change the cable for a larger item.

 

[CharlesLevy]

Thank you. This is probably a daft question, but how do I tell what size cables I have? Is it the thickness of the metal core that I should worry about, or the cores pluss the insulation?

Charles

 

[bernardgreen]

Not a daft question

It is the cross section area of the copper conductors that is the important dimension and measured in square millimetres but often refered to incorrectly as 2.5 millimetre.

If my maths are correct 2.5 square millimetres of conductor is about 1.7 millimetres in diameter

 

[CharlesLevy]

Thank you!

 

[TicklyT]

Details of the cable type and size are usually moulded into the outer sheath of cables at regular intervals too. A number something like 6242Y 2.5 refers to 2.5mm² cable, but sodd's law says it will be moulded on the side fixed to the joists.

 

[Chri5]

The wiki at the top of the page, UK electrics section has a table of physical cable size v core size.