cables in loft- topping up insulation

doningtonphil

hello

we have insulation in our loft between joists. I want to top up with 170mm insulation layed crossways over joists. We have various cables (upstairs lighting, shower cables currently laying over top of joists. Can the top-up insulation be laid over the top if these or do I need to ease cables up and lay over top of new insulation

keystone

You should not cover cables with insulation as doing so will reduce their current carrying capacity and thus they may well overheat.

However, lighting is frequently cabled in 1.5mm TC&E which means it is well underrated for the application so you could get away with leaving it underneath but it would be preferable to get it on top. Doesn't mean either method will comply with BS 7671 and both are bad practice really. In truth your cables should be properly clipped to the ceiling joists and the situation therefore not arise.

Shower and immersion heater cables are frequently running at close to maximum capacity so should NEVER be buried.

Cheers

Gloomendoom

keystone wrote: »

In truth your cables should be properly clipped to the ceiling joists and the situation therefore not arise.

I'm a bit confused here. How does clipping the cables to the ceiling joists prevent the situation arising?

keystone

Because you put the insulation between the joists not over the joists.

Cheers

Gloomendoom

keystone wrote: »

Because you put the insulation between the joists not over the joists.

Cheers

Ah! I see what you mean, but these days the insulation required is nearly three times as deep as the joists so it is difficult to avoid covering any cables.

Raggie

keystone wrote: »

You should not cover cables with insulation as doing so will reduce their current carrying capacity and thus they may well overheat

Would you kindly expand on this for me, I would like to learn why when covered in insulation the current carrying capacity of a cable is effected?

Thanks

keystone

Gloomendoom wrote: »

Ah! I see what you mean, but these days the insulation required is nearly three times as deep as the joists so it is difficult to avoid covering any cables.

Only because the global warming stasi say so. LoL.

Cheers

keystone

Raggie wrote: »

Would you kindly expand on this for me, I would like to learn why when covered in insulation the current carrying capacity of a cable is effected?

The heating effect of a current - remember your school physics? If you pack insulation round the cable heat dissipation is substantially reduced (by up to 50%) so the current that the cable can carry before it overheats is lower than if it were in free air. As mentioned this is not necessarily a problem with lighting cables which are generally well underated with core csa or 1.5mm. Immersion heater cables and shower cables a frequently near maximum so it is dangerous to cover these at all.

HTH

Cheers

doningtonphil

keystone wrote: »

Because you put the insulation between the joists not over the joists.

Cheers

You will notice I stated the top up insulation will go across the joists not between them. Does your answer about clipping cables to joists still apply?

keystone

doningtonphil wrote: »

You will notice I stated the top up insulation will go across the joists not between them. Does your answer about clipping cables to joists still apply?

Actually I missed that but if thats the case you are going to cover them up anyway so the direct answer is no but you still shouldn't bury them. For lighting you should be OK if its 1.5mm TC&E as stated but thou shalt NOT bury thine shower cable.

Cheers