De-rating cables within insulation. Best route?

[Hannah77]

Hi, I am running the lighting and ring circuits between external studs. The holes need to be positioned in the middle of the stud depth. I am installing insulation batts in the full depth of the stud so in theory the cables will be in the middle of 140mm insulation. If I routed the cable to the rear of the stud between each hole I would increase the length of wiring but then they would be only covered by insulation on one side and next to a ventilated cavity. The build up on the outside of the stud is 11mm OSB, membrane, studs and render board. Has anyone got any advise on experince with a similar set up? Not sure about how to work out de-rating.

[Hannah77]

Pretty please! 😭

[Mr Punter]

If you clip the cable to the stud it will help, as timber is not as insulative as insulation.  I think this is only an issue if you have high loadings through the cables.  In the past we had to use 4mm cable for power circuits but it seemed really OTT.    Your sparky is the one responsible for this call.

[andyscotland]

You've probably solved this now but thought I'd add a couple of details for anyone that finds it in future.

 

First, I'd echo @Mr Punter's recommendation that you should discuss this with your spark - once you start departing from the standard circuits, cable sizing & de-rating can be complex and depends on a lot of factors (e.g. supply earth arrangements, circuit lengths, protective device selection) so the right answer will depend on the specifics of your installation.

 

The starting point in BS7671 is 523.9 which says that cables "shall, wherever practicable, be fixed in a position such that it will not be covered by the thermal insulation". So it will usually be better - for any loading of cable - to use a service void, or notch studs rather than drilling the centres. If that's not possible for structural / mechanical protection reasons then your suggestion of clipping the cable out to the edge of the stud, across the face of the insulation, and then back in to the hole in the next stud is the next best. That does assume you can safely extend the length of the circuits without causing knock-on issues for things like voltage drop or loop impedance.

 

If you can achieve that, that gets you into the territory of installation method 102 for T&E cable in a stud wall where the cable is touching the wall surface (though there are some caveats depending on the exact materials). Table 4D5 gives the current carrying capacity for those methods. That allows 13A for 1mm², so a 6A lighting circuit should be fine. For 2.5mm², the rated current in the table is 21A - assuming the ring you mention is a standard 32A socket ring final, regulation 433.1.204 states that the cables must have a current carrying capacity of not less than 20A so again that's just OK.

 

So long as the loops into the stud centre and back out are touching the stud, and are short, I would not count them as being "totally surrounded by thermal insulation" and therefore they don't require additional de-rating.  I don't think 7671 actually differentiates between an interior or exterior wall surface, but working from first principles I'd say if you can take the cable along the outside wall (especially if you can clip them to the OSB) that would give you more of a safety margin as both the stud and the surface will obviously be colder than the inside wall, giving the cables more chance to dissipate any heat that builds up on the short loops into the stud centres.

 

If you can't go in and out, and have to run through the centre of the insulation, then you would be looking at installation method 103 - T&E in a stud wall not touching the surface. Table 4D5 gives 8A for 1mm² so the lighting would be OK, but 2.5mm² can only handle 13.5A in that setup and you would have to go to 6mm² to meet the 20A requirement for a 32A standard ring final. At that point I would probably look at alternate circuit designs / protective device sizing as although theoretically you can get 2 x 6mm² into most socket terminals it is a real pain.

 

If at any point the cable is totally surrounded by insulation then you would need to de-rate for that - by 50% if it's surrounded for more than 0.5m, or on a sliding scale for shorter sections.

 

Again, this is really intended for background information to assist the discussion with your spark.