Running cable in loft conversion

M

marsaday

Can i run my lighting cable in the rafter? It is 100mm and will have 75mm of celotex in there. then there is a 25mm breather gap and then you have the bare slate. So is it ok to run the cable in this 25mm breather channel, basically up against the slate?

next i plan on running the power cable over the current ceiling joists. Above this will be laid a new 8" joisted floor. insulation will go between these joists. Is there anything i need to think of in terms of protecting the cables?
 
Sponsored Links
Joined
7 May 2007
Messages
3,316
Reaction score
275
Location
Lincolnshire
Country
United Kingdom
Can i run my lighting cable in the rafter? It is 100mm and will have 75mm of celotex in there. then there is a 25mm breather gap and then you have the bare slate. So is it ok to run the cable in this 25mm breather channel, basically up against the slate?

I don't see any problem with that.

next i plan on running the power cable over the current ceiling joists. Above this will be laid a new 8" joisted floor. insulation will go between these joists. Is there anything i need to think of in terms of protecting the cables?

The current carrying capacity of a cable must be derated if it runs through thermal insulation. If the cable is to be totally enclosed by insulation, it's very likely that you will need to use a larger conductor size than you would normally for said circuit. If you can keep the cable touching a solid surface on one side, such as plasterboard, this will help in dissipating heat, and although correction factors will still have to be applied, it's likely that you wont have to use an increased conductor size.

If you can tell us the circuits in question and the installation method, we can give more specific advice.
 
M

marsaday

This is the ring for the attic. it is separate from the other house rings. there will be 4-6 double sockets on it.

it will be sat on the ceiling joists and insulation between these joists. Above it will be the new floor filled with insulation.
 
Joined
27 Aug 2003
Messages
69,780
Reaction score
2,884
Location
London
Country
United Kingdom
When you submitted your application for Building Regulations approval, what did you say would be the way you would comply with Part P?
 
Sponsored Links
M

marsaday

it hasnt gone in yet. wiring up a loft is pretty straight forward i thought. i will be using a sparky, i just want to check out some stuff here.

Can anyone then tell me what i need to think about in wiring up my new loft (bed room and en suite). i thought it was pretty straight forward.

When i wired the house 2 yrs ago with the sparky we took up a feed for the new light and power rings. These are not connected into the terminal obviously.

the only thing i need to think about is the power supply. the current lighting supply is laid under the current loft insulation. is this really a big consideration? if it is then i will lay the new ring on top of the new insulation.

i would prefer to have it below the new floor however, so it is out of harms way.

Can anyone clarify this for me. thanks.
 
Joined
27 Aug 2003
Messages
69,780
Reaction score
2,884
Location
London
Country
United Kingdom
it hasnt gone in yet.
OK - so what will you say? I'm not being awkward, but it is of crucial importance because if you say one thing and then do another you will have one hell of a mess to sort out when you ask for your Completion Certificate and they say "No".


wiring up a loft is pretty straight forward i thought.
But do you only think that because of your unknown unknowns?


i will be using a sparky
Then let him make all the technical decisions - he will be the one signing to say it was all properly done, so he will do what he wants anyway.

If you tell him you don't want surface wiring, he will find a way to do that and will design the circuits in a way that makes it safe to do that.


Can anyone then tell me what i need to think about in wiring up my new loft (bed room and en suite).
You need to think about what lights, switches and sockets you want where.

Then you tell your electrician and he gets on and makes it happen in ways which are legal.


the current lighting supply is laid under the current loft insulation. is this really a big consideration?
Why don't you tell us, as you think it's all so straightforward?

I'm not poking fun, but the fact that you don't know how big a consideration thermal insulation is in circuit design shows that it isn't all straightforward for you right now.

http://www.tlc-direct.co.uk/Book/4.3.1.htm


I would prefer to have it below the new floor however, so it is out of harms way.

Can anyone clarify this for me. thanks.
1) Your electrician will not install cables in harms way.

2) If you want them below the floor he will put them there, and design the circuits in a way that makes it safe.
 
M

marsaday

can anyone answer what i have asked for. Is it a no no to lay the ring main under the new joisted floor which will have 200mm of insulation between it.

how can i protect these cables so they dont over heat. i know i can lay them on top of the insulation, but would prefer to have the cables out of the way.

please no replies which offer a lecture. There is little point in having an electrics forum if all you say is refer to your electrician.
 
Joined
2 Oct 2007
Messages
2,280
Reaction score
200
Country
United Kingdom
It's quite straightforward and electronicsuk has pretty much answered your question though not with precise figures so allow me just to finish off his helpful answer.

Rings are normally 2.5mm2 cable which is rated to carry 27 amps if fixed to a surface which is in free air. If you surround this cable with insulation > 100mm thick then 2.5mm2 is derated to carrying 17 amps which ain't enough.

4mm2 cable, normally 37 amps, is derated to 22amps in such insulation.

6mm2 cable, normally 47 amps, is derated to 27 amps in such insulation.

10mm2 cable, normally 64 amps, is derated to 36 amps in such insulation.

So it can be done, just increase the thickness of copper. If you want to check then this is found in Table 4D5, BS7671, page 282.
 
Joined
27 Aug 2003
Messages
69,780
Reaction score
2,884
Location
London
Country
United Kingdom
can anyone answer what i have asked for. Is it a no no to lay the ring main under the new joisted floor which will have 200mm of insulation between it.
No. Although why you want them that deep beats me.


how can i protect these cables so they dont over heat. i know i can lay them on top of the insulation, but would prefer to have the cables out of the way.
By making sure that they are large enough not to overheat when carrying their design load.

Did you not read that book chapter I pointed you at? It's pretty straightforward.


please no replies which offer a lecture. There is little point in having an electrics forum if all you say is refer to your electrician.
Please explain the point in you trying to do the circuit design work if you are going to be using an electrician?

As I keep saying, you will be able to have the cables there if that's where you want them, so what more do you need to know?

Tell your electrician that you want them there and he will be able to design the circuit so that the cables don't overheat. He'll probably ask you why you want them that far below the floor.

But he has to do the design work, not you, because he , not you, has to sign a declaration which says (with my emphases):

I being the person responsible for the Design, Construction, Inspection & Testing of the electrical installation (as indicated by my
signature
below), particulars of which are described above, having exercised reasonable skill and care when carrying out the Design,
Construction, Inspection & Testing, hereby CERTIFY that the said work for which I have been responsible is to the best of my
knowledge and belief in accordance with BS 7671:2008...



If your plan is to install the cables yourself, put the floor down, and then get an electrician to lie to LABC for you it won't work.
 
Joined
28 Jan 2010
Messages
641
Reaction score
75
Country
United Kingdom
you won't need a 32amp ring for a bedroom.

Just run a 16amp radial - you should then be fine to use 2.5mm for this and clip it to the ceiling joists.

Hope this helps mate.
 
M

marsaday

thanks Chapeau i understand what to do now. However i have brought 2.5mm cable up to the loft area and would prefer to work in this.

i think i have my solution !

If i remove the old insulation currently between the ceiling joists the cable will be sat on the ceiling below. Above this i will have my 8" new floor which will have the insualtion between it. if i use netting the gap between cable and insulation is likley to be 3".

does this sound like a solution.

i will be doing most of the work myself and the sparky will be wiring up the plugs etc. he is happy to work like this. this is why i am working out how to go about this task.
 
Joined
27 Aug 2003
Messages
69,780
Reaction score
2,884
Location
London
Country
United Kingdom
Rings are normally 2.5mm2 cable which is rated to carry 27 amps if fixed to a surface which is in free air. If you surround this cable with insulation > 100mm thick then 2.5mm2 is derated to carrying 17 amps which ain't enough.
13.5A, actually, which isn't enough for a ring final.


4mm2 cable, normally 37 amps, is derated to 22amps in such insulation.
18.5A. Still not enough.


6mm2 cable, normally 47 amps, is derated to 27 amps in such insulation.
23.5A


10mm2 cable, normally 64 amps, is derated to 36 amps in such insulation.
32A.


So it can be done, just increase the thickness of copper. If you want to check then this is found in Table 4D5, BS7671, page 282.
He's laying the cables on top of existing insulation and then covering them with another 200mm. It's Ref Method 103#.
 
Joined
2 Oct 2007
Messages
2,280
Reaction score
200
Country
United Kingdom
Reference Method 101# - the one I used

"above a plasterboard ceiling covered by thermal insulation exceeding 100 mm in thickness"

Reference Method 103# - the one ban_all_sheds used

"in a stud wall with thermal insulation with cable not touching the inner wall surface"

So is it in a ceiling or in a wall?

marsaday said "Is it a no no to lay the ring main under the new joisted floor which will have 200mm of insulation between it."

chapeau correct. bas wrong.

Again.
 
Joined
27 Aug 2003
Messages
69,780
Reaction score
2,884
Location
London
Country
United Kingdom
thanks Chapeau i understand what to do now. However i have brought 2.5mm cable up to the loft area and would prefer to work in this.
So it wasn't straightforward enough for you to get that right, was it?

You just thought it was so straightforward, and that thermal insulation was such a non-issue that you didn't need to check first.


does this sound like a solution.
No, it sounds like a bodge done by someone trying to extricate himself from a problem situation he has gotten into by his incompetence and ignorance.

And it still won't allow you to use 2.5mm² for a ring final.


i will be doing most of the work myself and the sparky will be wiring up the plugs etc. he is happy to work like this.
Oh well - a guy prepared to falsify certificates and lie to the authorities must be trustworthy and worth using, mustn't he?

If only you had explained that that was what you meant by "i will be using a sparky" - it could have saved a lot of time.


this is why i am working out how to go about this task.
I hope your family appreciate you putting their lives at risk.
 
Joined
7 May 2007
Messages
3,316
Reaction score
275
Location
Lincolnshire
Country
United Kingdom
OP, if you run in a 16A radial in 2.5mm with the cable touching the plasterboard ceiling on one side and insulation atop, this should be OK assuming no other correction factors need be applied. If the cable is totally enclosed in insulation all the way around, then you'll be needing 4mmsq conductors for a 16A radial.

So yes, it would be best for you to remove the existing insulation and lay your new cables directly on top of the plasterboard ceiling of the room below. Whether or not it is wise for you to do this work without an electrician is another question - you may not like what B-A-S is saying, but you may regret not heeding his advice when it comes to the time when a completion certificate for the building works has to be issued.

With regard to method 103, although it does mention plasterboard walls, it makes little difference whether it's a wall or ceiling.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Top