replace loft wiring

[dave08]

Hi - what a great forum!

I want to replace the wiring in my loft as I need to get some insulation up there. I plan to get the house rewired within a couple of years but can't take going on uninsulated till then.

The loft is about 40 square metres, theres just 1 lighting circuit and no ring main or other cables running up there, and the joists are 100mm high and only a few have notches in already. None are drilled and most of the wiring is just chucked over the joists. There's not enough give in some of the wiring to get insulation underneath safely, and I'd like to put some boards down once I've laid insulation below the wiring.

I'm familiar with the circuit and happy to follow all the same routes as are there now, but don't know whether I should notch and plate or drill to feed the replacement cables round the loft. I can pick up the incoming supply at an existing junction right near the loft hatch so don't feel inclined to pull a new feed all the way from the board.

I think it might be a good idea to replace the switch runs and hardware as the existing switch backboxes are substandard, but wouldn't want to tackle the two way landing/hall wiring yet as I'd have to carve up half the house.

I'd really appreciate some advice, on the joists in particular - thanks

 

[Chri5]

Since you have no need to board all the way in to the eves, this gives you an area at the front and back of the loft where cables can be laid over the joists and shouldn't cause any problems.

As you may know insulation v cabling is a touchy subject and the two should be avoided, unless you want to derate the capacity of the circuit.

I would suggest that all lighting cable is 1.5mm TE (not 1.0mm!!) and where possible you try to have a primary cable route between a designated pair of joists which will have 200mm (50% of the space between the joists) without any insulation material.

I would tend to avoid any joist notches and go for 20-25mm drilled holes, these holes would normally be 50mm or so below the top of the joist to prevent nail and screw damage when fixing floor materials over. Again it might be better to have these near the top of the joist and use plates to prevent the potential for such nail damage.

My logic for having the holes near the top is to simply avoid the cable being smothered with insulation.

Depending on the build type of your property, you may find that all the internal walls at 1st floor are stud walls in which case changing the switch cables will be simple and should require anything other than tying on to the old and pulling through.

 

[RF Lighting]

I'm no expert on these things, but I would advise against any notching or drilling of ceiling joists.

100mm is not all that substantial, and in new build properties the timberwork is calculated to allow the minimum amount of wood, any notching / drilling may weaken this too much.

You could lay 1" wooden battons ontop of the joists at right angles to the joists. This allows a bit of a gap for cables etc, without any possible weakening of the structure.

 

[ColJack]

the problem with that is the way the boards would have to go down is then paralell to the joists.. and you're making the weight bearing area 2x1 inches where the battens cross the joists.. ( red area in diagram below. )

the best way is to run the cables clipped to the beams ( no idea what they are called but they usually run perpendicular to the joists in onld roofs.. ), then clipped to a joist to get where it needs to go..

 

[dave08]

Since you have no need to board all the way in to the eves, this gives you an area at the front and back of the loft where cables can be laid over the joists and shouldn't cause any problems.

As you may know insulation v cabling is a touchy subject and the two should be avoided, unless you want to derate the capacity of the circuit.

I would suggest that all lighting cable is 1.5mm TE (not 1.0mm!!) and where possible you try to have a primary cable route between a designated pair of joists which will have 200mm (50% of the space between the joists) without any insulation material.

My logic for having the holes near the top is to simply avoid the cable being smothered with insulation.

Depending on the build type of your property, you may find that all the internal walls at 1st floor are stud walls in which case changing the switch cables will be simple and should require anything other than tying on to the old and pulling through.

Hi Chri5,
Thanks for the reply which clarified things and has helped me reach a plan.

As the loft space is so small I'll replace cables so the supply cable runs round the periphery and can lay on top of the Fibreglass. As all the switch positions are in a line above one wall parallel to the loft joists I can leave a 'trench' in the insulation above there, and route the switch wires round the periphery and along the trench.

The returns that come in to the light points can then run between joists and all be clipped to joists - near the top - for the reason you suggest, or maybe it would be better to simply lay them on the insulation - they'll be partly covered with boards nearer the centre of the loft.

That about covers it except that the walls are solid and I'll have to do some new chases for the switch wires, in pvc as the old steel tubes are too thin for 1.5TE.

I'm looking forward to getting the insulation in!

Thanks for your ideas

Dave08

 

[dave08]

the best way is to run the cables clipped to the beams ( no idea what they are called but they usually run perpendicular to the joists in onld roofs.. ), then clipped to a joist to get where it needs to go..

Hi ColJack,

I think you may have reopened my question about cable runs between joists. If I've got a couple of cables running to a ceiling rose I can imagine clipping them, but if I've got many cables in a 'trench' between two joists with no insulation laying on them and a board covering them is fixing necessary?

And should cables round under the eaves where they're pretty inaccessible be clipped? - I hope not as that'll take some fancy hammer work.

Dave08

 

[ColJack]

clipped to a joist stops the cable being surrounded with insulation on all sides..

1.5mm cable can take 15A if installed to reference method 15 ( as above.. ) so feel free to put insulation against the cables as even de-rated that's still 10A.. ( 0.5 x 20A ref method 1 capacity.. )

given that most domestic lighting circuits are protected at 6A then that's fine..

the de-rating factors only apply if the cable is surrounded by insulation.. if it's clipped to the beam then it's in contact with a thermally conductive surface..