Can cables go at angle relative to roof joists?

[frediaz]

There is one thing I cannot find in the 17th edition of the wiring regulations about permitted cable zones. In order to reduce 'cable mileage' (and voltage drop) I am thinking of running some cables in the empty roof spaces of my house above the joists and at an angle, like from corner to corner of the ceiling.
Will Building control be able to find any objections to this?

 

[RF Lighting]

There isn't a regulation that actually prohibits it, it's just a bit gash.

 

[sparkwright]

You would normally run the cables parallel and at right angles to the joists in a tidy fashion if the loft space is accessible, and if the length of the cable is too high a larger size should be considered.

That said, if the cable is not prone to damage it should be ok. You could consider fitting some timber diagonally across the tops of the joists, and securing the cable to this.

 

[bernardgreen]

You could consider fitting some timber diagonally across the tops of the joists, and securing the cable to this.

This makes a very tidy installation with cables clear of insulation. If the bearers are supported on pillars then at a later date more insulation can added with affecting the cable.

 

[seasickstevie]

I know the traditional approach has been to keep everything 'square', but with high load cabling particularly such as >9kW showers, taking it diagonally in a large house can save several meters, and as such a reduction in volt drop.

Good idea to suspend/support a board above insulation to clip to.

 

[ricicle]

Rough as a badgers ar$3 and a pain for anyone wishing to board the loft

 

[frediaz]

Thanks all for the feed-back. I like the idea of running a strip of wood over the joists and then fixing the cable to it.

About preventing a future owner from boarding the loft, this is exactly what I would want to do. If somebody wants to board the joists it means they want to use the roof to store heavy stuff and walk all over it. And these trusses are not going to take the load. Plus you don't board roof joists where you need space for 270mm of insulation.

 

[seasickstevie]

Rough as a badgers ar$3 and a pain for anyone wishing to board the loft

You can't board over 6" of insulation without building up the depth of the joists at least. And anyone doing a proper loft conversion, a cable that needs diverting is the least of their problems.

 

[Chri5]

Rough as a badgers ar$3 and a pain for anyone wishing to board the loft

You can't board over 6" of insulation without building up the depth of the joists at least. And anyone doing a proper loft conversion, a cable that needs diverting is the least of their problems.

Still rough, and with respect cable in 150mm of insulation WILL BE a problem.

Use a bigger cable

 

[frediaz]

Rough as a badgers ar$3 and a pain for anyone wishing to board the loft

You can't board over 6" of insulation without building up the depth of the joists at least. And anyone doing a proper loft conversion, a cable that needs diverting is the least of their problems.

Still rough, and with respect cable in 150mm of insulation WILL BE a problem.

Use a bigger cable

I am already using a bigger cable, instead of 1mm clable for the lighting circuits, I am using 1.5mm. Also, it is widely accepted that fixing the cable to wood runs helps in reducing the problem of cable in modern thick insulaton.

I guess the alternative would be to lay the cable above the insulation, fixed to the trusses, in the form of cobwebs all inside the roof space.

 

[ban-all-sheds]

I am already using a bigger cable, instead of 1mm clable for the lighting circuits, I am using 1.5mm.

1.0mm² can carry 8A with the most deleterious installation method in the regs.

it is widely accepted that fixing the cable to wood runs helps in reducing the problem of cable in modern thick insulaton.

Surely then you'd want the cables running along the joists for as much of the time as possible?

I guess the alternative would be to lay the cable above the insulation, fixed to the trusses, in the form of cobwebs all inside the roof space.

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• nrwilliams
• 22 Nov 2011
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• 22 Nov 2011
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[RF Lighting]

 

[frediaz]

I am already using a bigger cable, instead of 1mm clable for the lighting circuits, I am using 1.5mm.

1.0mm² can carry 8A with the most deleterious installation method in the regs.

it is widely accepted that fixing the cable to wood runs helps in reducing the problem of cable in modern thick insulaton.

Surely then you'd want the cables running along the joists for as much of the time as possible?

I guess the alternative would be to lay the cable above the insulation, fixed to the trusses, in the form of cobwebs all inside the roof space.

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• nrwilliams
• 22 Nov 2011
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• nrwilliams
• 22 Nov 2011
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• nrwilliams
• 22 Nov 2011
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• nrwilliams
• 22 Nov 2011
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That is exactly the kind of mess I am trying to avoid!

No, now I think that the best way is a combination of the shorter cable runs at an angle with being fixed to the wood 'guides'. Also, the wood guides will keep the cables above the first layer of insulation and the wood guide will create a small air gap for the cable, whereas fixed to the joists, the insulation will press against it.

 

[frediaz]

This is neat, shame that the cable tray would squash the insulation on modern installations. I guess the solution would be to fix wood spacers on the trusses to raise the trays above the insulation.

 

[Robin0577]

As per RFs pic above, use cable tray rather than wood, that's what it's made for but make sure you use a wide enough tray to accommodate all your cables without having to bunch them together or you'll have another derating factor to worry about instead of your insulation...

You could make a fairly neat job of suspending the tray from the rafters with a little thought, keeping it completely clear of the insulation.