Hi Just doing first fix on for an old house re-wire and therefore will be having Building Controls first visit to inspect circuits and cable runs etc. when they are all done. (BC final visit will be at commissioning for Testing and sign off) House was built 1950. Roof space only has 3" x 2" ceiling joists and x2 large purlin's and no struts! and will be eventually insulated with 200mm insulation, so obviously all lighting cabling to upstairs lights will be buried. I Plan on wiring upstairs lighting circuit in 1.5mm2 - clipped as per regs but to side of ceiling joists. I don't want to clip to top of joists as might fit loft legs for boarding purposes later - so can't cable on top of ceiling joists as regs suggest, but conscious of over heating and derating factors etc. What would the a good solution? Someone said run cabling in 25mm Adaptaflex flexible conduit as will maintain an air gap between insulation and cabling - but this seems over kill and expensive. Hope you can help
I don't have a copy of the regs with me, but I don't recall the derate factor being that different between clipped direct and covered and conduited and covered
First there is no need for 1.5mm2 cable. 1.0mm2 is perfectly adequate, cheaper, and saves the worlds valuable resources of copper. It is rated at 16 amps max so after derating in 200mm of insulation will be around 10 amps, so perfectly OK for a 6amp lighting circuit.
I would be interested to know if you plan to use plastic cable clips, as they seem to be outlawed under 18th edition regs, in order to comply with cable collapse due to fire regulations.
I agree that using 1mm is fine (despite some people arguing "it must be 1.5mm, we've always done it that way"!) but are you sure it's rated to 16a? I'll have a look in the book later, but from memory 1.5mm t&e is rated to 16a, 1mm is rated to 10a
If the cables are within the roof structure the house would have to fall down before the cables become an entanglement hazard.