Loft insulation - what to do about some cables?

[Patroller]

Hi all,

I am insulating my loft this week and have begun the big job of clearing all of the rubbish and old useless insulation out.

I have read that electric cables should not have insulation placed ontop of them - this is obviously very difficult to avoid because many lofts are probably like mine - cables going in every direction.

Although it isnt possible to avoid them all I am going to ensure that the large power cable serving the electric shower is not buried under insulation. What is the best way to do this - some kind of conduit, or simply leave a channel insulation-free for it to run in?

Thanks for all replies.


EDIT - UPDATE -

Pictures added in last post.

 

[PrenticeBoyofDerry]

With lighting circuits that are protected by a 5 amp fuse or 6 amp MCB, it should not be an issue.
With socket circuits and other power circuits such as showers it is.
The reason being that the current carrying capacity is effected by the cover of thermal insulation, it will not allow heat to be disapated satisfactory and then becomes a fire hazard.
Up grading the cables to a larger size will help but not often practical.
If there is enough slack on the cable, to have it laying above the insulation that will help.
But it is best to know your cable sizes and the size of the protective device(fuse) that is on the circuits to give better advice.

 

[Patroller]

Thanks for your post. I think that just by coincidence the shower power cable has enough slack to just sit ontop of the insulation, but the same cannot be said about the majority of the wires. Until this week much of the joists and cables were buried under a mountain of old felt for what has probably been at least 20 or so years, without a problem.

Thanks for your advice.

 

[PrenticeBoyofDerry]

If your shower cable is above the insulation that is good.
If it's a loft and you don't life in a one storey property, generally the socket circuits don't come in to the loft area, they are run in the floor/ceiling void.
But you lighting circuits will, if they are protected by 5 amp fuse or 6 amp mcb they should be safe as your lighting circuit cable will normally be 1.00mm and can safely carry the current.
But if the fuse rating is any higher will need to do some calculations.
If your socket circuits do run in to the loft area, they will need to be checked out and a safe installation method or an upgrade of cable sizing made.
This applies to cables that are totally surrounded by insulation and heat can not be disapated safely.

 

[geraint]

With lighting circuits that are protected by a 5 amp fuse or 6 amp MCB, it should not be an issue.
With socket circuits and other power circuits such as showers it is.
The reason being that the current carrying capacity is effected by the cover of thermal insulation, it will not allow heat to be disapated satisfactory and then becomes a fire hazard.
Up grading the cables to a larger size will help but not often practical.
If there is enough slack on the cable, to have it laying above the insulation that will help.
But it is best to know your cable sizes and the size of the protective device(fuse) that is on the circuits to give better advice.

disagree... the lighting circuit could have no end of lights... so cannot differenciate between power and lighting...

 

[r.bartlett]

Can someone explain this

If a loft is fully insulated and the cables are below the insulation in the summer they would be protected from the 40+ deg heatbuild up a hot summer can induce...

but If they were laid on top they would be left to the extremes of the loft space whilst laying on top of insulation..

yet they are supposed to be above to stop them overheating...How's that work?

 

[PrenticeBoyofDerry]

disagree... the lighting circuit could have no end of lights... so cannot differenciate between power and lighting...

They can have no end of lights, but it is normal procedure in domestic installation, to have no more than 12 light fittings per circuit or 1380 watts per circuit.
This would allow a 6A MCB to offer the suitable current for the circuit to operate satisfactory.
As most domestic lighting circuits are installed with cable that is equal to or exceeds 1.00mm2 T&E. We can assume that the cable will safely carry 16Amps, with ref method C installation (clipped direct), which is again normal practice in domestic installs and is more than often found in lofts.
So you will find that a 1.00mm2 T&E cable, if surrounded by thermal insulation, with the highest factor of 50, derating the cable will still allow 8Amps to be carried safely by this cable, as it is protected by a 6Amp breaker.
As with power circuits the way they are designed and the cable and fuse ratings factored in, it is very unlikely that these circuits would be safe to use if covered by thermal insulation, without further consideration to the installation method or the upgrading of cable sizing.
For example a standard radial circuit for sockets, would be designed to be protected by a 20A MCB and the cabe sizing would be 2.5mm2 T&E.
2.5mm2 T&E can only carry 27A, when using ref method C.
So if this cable was then subjeted to being surrounded by thermal insulation at a factor of 50, the cable could then only safely carry 13.5A, but the fuse would be offering 20A, so we now have a problem, so you then need to re-route cable or upgarde cable sizing.

 

[PrenticeBoyofDerry]

Can someone explain this

If a loft is fully insulated and the cables are below the insulation in the summer they would be protected from the 40+ deg heatbuild up a hot summer can induce...

but If they were laid on top they would be left to the extremes of the loft space whilst laying on top of insulation..

yet they are supposed to be above to stop them overheating...How's that work?

Sorry, don't quite understand the question.
Extremes of the loft space means?
So this answer may not be what you are looking for,

If the cable is surrounded by thermal insulation heat can not disapate, but if it subjected to free air the heat can be disapated.

 

[davelx]

Can someone explain this

If a loft is fully insulated and the cables are below the insulation in the summer they would be protected from the 40+ deg heatbuild up a hot summer can induce...

but If they were laid on top they would be left to the extremes of the loft space whilst laying on top of insulation..

yet they are supposed to be above to stop them overheating...How's that work?

The maximum expected ambient temperature should have been taken into account when selecting the size of cable to be installed in the first place - there are derating factors and formulae in the wiring regs for this purpose.

 

[stgeli]

For example a standard radial circuit for sockets, would be designed to be protected by a 20A MCB and the cable sizing would be 2.5mm2 T&E.
2.5mm2 T&E can only carry 27A, when using ref method C.
So if this cable was then subjected to being surrounded by thermal insulation at a factor of 50, the cable could then only safely carry 13.5A, but the fuse would be offering 20A, so we now have a problem, so you then need to re-route cable or upgrade cable sizing.

Thanks for this.

I recently replaced all the insulation in my loft with 100mm + 170mm. Unfortunately, in this instance, it is a bungalow so there are loads of all sort of cables, lighting and socket, around. I managed with only three runs buried in insulation not counting those that ran up through it to the surface.

Two were lighting so I'm not worried about those but another was a single 2.5mm2 cable on a socket circuit with about 2.5m buried. To try and help, I put the cable in some 50x30mm trunking to give it some air. I'm now concerned that this might not be enough.

Would a larger section of trunking give it the space it needs to breath? The circuit is a kitchen and utility room (but not the oven) so has the potential to be quite heavily loaded.

 

[PrenticeBoyofDerry]

What type of circuit is it, Ring or Radial?
What rating is the protective device, fuse or mcb?
and if it's a fuse is it semi-enclosed BS3036?
and what type of cable is it, ie 2.5mm2 T&E flat grey?
There are certain installation methods that can be used to lessen the de-rating.
Once we are aware of what type of installation you have, we can make a little more sense of it and a more accurate calculation.

 

[stgeli]

It's a ring circuit with nine sockets on 2.5mm2 T&E cable with 32A RCBO protection.

With the cable de-rated by 50% due to the insulation, it can only carry 13.5A which is rather a lot less than the protection.

It sounds like I need to remove the additional insulation from the area around the cable and leave it sat on top of the 100mm.

Thanks for the help here and apologies to the OP for taking over their thread.

 

[PrenticeBoyofDerry]

If you can lay the cable above the insulation, I would suggest you do.
As it is a ring final circuit, the only other safe alternative other than upgrading your cable size, would be to split the circuit in to two radials, protected by 20A RCBOs and the cable to be run in conduit, this will allow 20A carrying capicity of your cable.
A lot of extra work as two new circuits and installing conduit plus as you have to install new protective devices, it will then be deemed notifiable work to building controls which comes at a cost.

 

[stgeli]

I'll cut a nice channel in the 170mm and leave the cable on top of the 100mm and joists, possibly on a plasterboard strip. I'm happy to forgo a small amount of insulation for peace of mind. It's above the utility room which has no radiator in anyway so there's little actual heat loss.

Thanks for your help.

 

[PrenticeBoyofDerry]

Thanks for your help.

No probs, glad I could help.