Running Electrical Cables in Lofts

[Raistlin]

Am I right in thinking that it's ok to drill holes through the loft joists that you would stand on (the ones that the ceiling below is nailed to)? I need to re-route some cabling in the loft and don't want it trailing across the beams.

I take it that the maximim size I should drill out is going to be about an inch or so? I don't think I need much more than that for the cabling anyway.

Thanks

[pabscars]

Ive seen this done before, especially for pipework but I would say it depends on the thickness of your loft joists.

mine are only about 4 inch by 1.5, so I might be a little concerned drilling any bigger than half inch or so, also it might be important where you drill ie through the centre as apposed to grilling at the very top or bottom.

just my opinion, I,m no builder just done a bit of DIY in my time

[Raistlin]

Yeah, that's pretty much what I was thinking.

Worryingly I've just started looking at running cable through/under loft insulation and discovered that I've probably got issues there as well. The joists run left to righ across the space, but I need to run the cable front to back (hence drilling through the joists) - looks like I can't though because I need to either un the cable ruls down the lengths of the joists (if I'm covering with insulation) or not cover with insulation (not an option in this space).

Might have to find a better way of running them..... I guess I could clip the cables in the apex where the floor/celing joists meet the actual roof joists. This gets them out of the way and also prvents me having to drill the joists.

I might edit this thread, I think it's going to be more about the cabling than the joists :S

[pabscars]

Originally Posted by Rob M

Yeah, that's pretty much what I was thinking.

Worryingly I've just started looking at running cable through/under loft insulation and discovered that I've probably got issues there as well. The joists run left to righ across the space, but I need to run the cable front to back (hence drilling through the joists) - looks like I can't though because I need to either un the cable ruls down the lengths of the joists (if I'm covering with insulation) or not cover with insulation (not an option in this space).

Might have to find a better way of running them..... I guess I could clip the cables in the apex where the floor/celing joists meet the actual roof joists. This gets them out of the way and also prvents me having to drill the joists.

I might edit this thread, I think it's going to be more about the cabling than the joists :S

Cables are the bain of my life, Ive spent countless hours doing unseen work just to hide them from view, so i know what a dilemma it can be.

I might be tempted to run the cables as you've suggested, but for piece of mind hide them in some trunking with a lid you can detach when you need to add more, which may help guide the cable to the correct location.

Ive used it before and it looks a pucka job.

If your planning on running multiple cables, get some pieces of string running through the conduit so you use them to pull your cables through without having to get on your hands and knees all the time.

---------- Post added at 13:55 ---------- Previous post was at 13:50 ----------

Re the insulation, you could cut through half the thickness of the insulation, lay the cable; then flop in back in position,

just a thought

[Raistlin]

The problem with putting the cables in trunking, and under/within insulation, is that it can stop them from correctly dissapaiting heat. This in turn can lead to them overheating and burning through.

The problem in this location is that in the past there have been rats up there that have chewed through the cables, so I'm now replacing the damaged runs and trying to make the area usable (ie for storage).

I'm putting a couple of strip lights in, then want to insulate and board the loft out so that we can chuck things like the Christmas tree and decorations up there and out of the way.

I'm quite happy with the circuit side of things, it's the 'best practice' bit that eludes me (having not had much practice). I think I'll stick with pinning them to the diagonal joists down the length, rather than drilling through the joists. This removes the possibility of them being covered and gets them out of the way. Across the loft the regulations allow for the cables to be covered provided they're clipped to the beams (which then help them shed heat), so I'll do it that way.

Thanks

---------- Post added at 14:05 ---------- Previous post was at 14:04 ----------

Originally Posted by pabscars

If your planning on running multiple cables, get some pieces of string running through the conduit so you use them to pull your cables through without having to get on your hands and knees all the time.

As for being on hands and knees.....This is a downstairs loft, so it's tight to say the least (it's a loft above a single story extension at the back of the house), which isn't great 'cause I'm over six foot and.....shall we say.....'stocky'

[heero_yuy]

From the current building regs:

joist_drill.JPG

[pabscars]

Originally Posted by Rob M

The problem with putting the cables in trunking, and under/within insulation, is that it can stop them from correctly dissapaiting heat. This in turn can lead to them overheating and burning through.

The problem in this location is that in the past there have been rats up there that have chewed through the cables, so I'm now replacing the damaged runs and trying to make the area usable (ie for storage).

I'm putting a couple of strip lights in, then want to insulate and board the loft out so that we can chuck things like the Christmas tree and decorations up there and out of the way.

I'm quite happy with the circuit side of things, it's the 'best practice' bit that eludes me (having not had much practice). I think I'll stick with pinning them to the diagonal joists down the length, rather than drilling through the joists. This removes the possibility of them being covered and gets them out of the way. Across the loft the regulations allow for the cables to be covered provided they're clipped to the beams (which then help them shed heat), so I'll do it that way.

Thanks

---------- Post added at 14:05 ---------- Previous post was at 14:04 ----------



As for being on hands and knees.....This is a downstairs loft, so it's tight to say the least (it's a loft above a single story extension at the back of the house), which isn't great 'cause I'm over six foot and.....shall we say.....'stocky'

I follow ya buddy, carrying a bit of timber myself ,

Firstly if your cables are getting hot, this suggest's an issue in itself, eg your pulling too much juice through them, so I would obviously ensure they are of the correct type and thickness to carry the required amps you intend to pull.

When I wired in my shower I had to purchase wire much thicker than anything I had used in the house previously, but it was of the correct type for a 10.5 kW shower,

Apologies if it sounds like I,m trying to teach you how to suck eggs.

Re Rats
1, get some traps up there or borrow my air rifle
2, can you get galvanised metal trunking to stop the little suckers chewing through it and run it on top of the boards you intend to put down.

[Raistlin]

Originally Posted by heero_yuy

From the current building regs:

Attachment 18930

Nice find, thanks

---------- Post added at 15:01 ---------- Previous post was at 14:56 ----------

Originally Posted by pabscars

Firstly if your cables are getting hot, this suggest's an issue in itself, eg your pulling too much juice through them, so I would obviously ensure they are of the correct type and thickness to carry the required amps you intend to pull.

Nope, nothing getting hot yet - just saying that was why I didn't want to bury them in the insulation.

2.5mm twin + earth should be fine for the power, and 1.5mm twin + earth for the lighting/switches, just trying to work out the runs

Bloomin' ratties..... Not just their fault though, the moron that wired it last time didn't allow any slack and ran three cables right over the loft hatch. You have to lift it 3 inches and then slide it under the cables so you can get it out of the way. Then you have to climb through the cable run to get into the damn loft..... Hence at least part of the reason for the re-wire

Also quite nice to be doing something constructive again for a change, always like electrical wiring for some reason - think it's because it's simple, but it's got to be right, if that makes sense. Kinda like the actaul concepts are all straight forward, and provided everything's linked up you'll end up with power somewhere, but getting it right and making sure everything's safe takes a bit of thought and planning. I'm also quite sad in that I take great pleasure in having very neat connections and runs

[pabscars]

Originally Posted by Rob M

Nice find, thanks

---------- Post added at 15:01 ---------- Previous post was at 14:56 ----------



Nope, nothing getting hot yet - just saying that was why I didn't want to bury them in the insulation.

2.5mm twin + earth should be fine for the power, and 1.5mm twin + earth for the lighting/switches, just trying to work out the runs

Bloomin' ratties..... Not just their fault though, the moron that wired it last time didn't allow any slack and ran three cables right over the loft hatch. You have to lift it 3 inches and then slide it under the cables so you can get it out of the way. Then you have to climb through the cable run to get into the damn loft..... Hence at least part of the reason for the re-wire

Also quite nice to be doing something constructive again for a change, always like electrical wiring for some reason - think it's because it's simple, but it's got to be right, if that makes sense. Kinda like the actaul concepts are all straight forward, and provided everything's linked up you'll end up with power somewhere, but getting it right and making sure everything's safe takes a bit of thought and planning. I'm also quite sad in that I take great pleasure in having very neat connections and runs

Nothing sad about wanting to do a pucker job, its what makes us blokes who we are, we like to stand back and admire our work for all eternity and when seeking praise from her indoors, get granted with tut and a sarcy comment like "its just a bloody wire, now shut up and peel the spuds"

[LondonRoad]

Originally Posted by pabscars

Nothing sad about wanting to do a pucker job, its what makes us blokes who we are, we like to stand back and admire our work for all eternity and when seeking praise from her indoors, get granted with tut and a sarcy comment like "its just a bloody wire, now shut up and peel the spuds"

Yup I know the feeling well. I took the opportunity while our garage was getting converted to run some network points behind the walls and under floors. Now I have neat hard wired sheilded Cat6 network points. Happy as the proverbial pig in ***** me. She who must be obeyed said "It seems an awfull lot of bother when it works fine wirelessly"

[jamiefrost]

Just in case you were still lookinmg into this ,

De-trating cables buried in insulation

Table 4.5 - Derating factors (CI) for cables up to 10mm² in cross-sectional area buried in thermal insulation.

Length in insulation (mm) Derating factor (CI)
50 0.89
100 0.81
200 0.68
400 0.55
500 or more 0.50

JJ