Electrical wiring regs

[Anonymous]

My daughter has just received building regs drawings for an extension that she's building (timberframe, extrenal weatherboard cladding). One of the numerous notations to the drawings states: "no cables, other than electricity meter tails, are to be located in the cavity of an external wall".
I'm just interested to know why this stipulation is made. Does anyone know?

 

[Anonymous]

Not certain, but one possibility that I know of is that some of the insulation products that get blown into cavities can degrade the electrical insulation. They are supposed to check before installing cavity insulation but apparently they usually dont.

Also the ieee regs for cable size/current assume that the cable is not heat insulated, if it is then the current allowed is reduced.
When you insulate a loft you are supposed to carefully lift any power cablesand ensure that the insulation id fitted under the cables not on top of them.

I'll see if I can find my 16th edn and check if it says anything obvious.

 

[Anonymous]

meter tails because those are the only ones that are deemed unavoidable.

damp problems and the degrading of cable plus unsupported spans / damage by unfinished surfaces plus not inspectable at all.

this shouldent apply to a timber extension though, only to the original building where you may have a double skin brick/block wall

nothing against running cables in the cavity of a timber building/frame so dont think for one second it means she has to trunk it all up the walls!

just make sure its on the right side of any membrane and that its not pierced - i.e. use plasterboard boxs instead of cutting through membrane sheet.

as for iee regs, they give calculations you can use to derate cables. any manuf rateing will be assuming a free air usualy.