Running cable in loft conversion

[marsaday]

BAS's i will find the correct solution and use the 2.5mm. dont worry yourself too much.

 

[chapeau]

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

The IEE obviously don't think so or else they wouldn't have put 101 in which has different figures from 103 and which also almost exactly matches the installation method described whereas 103 doesn't.

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.

Yes, but this is still method 101#......... Table 4A2, BS7671, page 265

You may not like it, but that's what the book says, what are the qualifications of people on this thread to say the IEE are wrong? I shall start, I don't have them.

 

[electronicsuk]

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

The IEE obviously don't think so or else they wouldn't have put 101 in which has different figures from 103 and which also almost exactly matches the installation method described whereas 103 doesn't.

I think you have misunderstood me. The point I am/was trying to make is that there is no installation method specifically for flat twin and earth cables, above a ceiling, with cable not touching an inner surface. Therefore, if the OP plans to install the cable between the existing insulation with another 200mm on top, he will need to use method 103. Although the regs speak of stud walls, it seems to be the most applicable to this situation.

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.

Yes, but this is still method 101#......... Table 4A2, BS7671, page 265

You may not like it, but that's what the book says, what are the qualifications of people on this thread to say the IEE are wrong? I shall start, I don't have them.

Where exactly did I state, either directly or implied, that the IEE were incorrect?

 

[ban-all-sheds]

Reference Method 101# - the one I used

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

Ah - but when I started writing my reply to you, marsaday had not written this:

If i remove the old insulation currently between the ceiling joists the cable will be sat on the ceiling below.

so the method which applied was not 101#

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?

It's in a ceiling/under a floor, but the results are the same. Look at para 3 in 523.7 if you don't believe me.

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."

That was only half the story - earlier on he said:

it will be sat on the ceiling joists and insulation between these joists. Above it will be the new floor filled with insulation.

i.e. it was going to be sitting on insulation with more above it.

chapeau correct. bas wrong.

Only if you conveniently ignore the fact that when you wrote this the derating factor which applied was 0.5.

If, on the other hand, we judge the correctness of your reply given the information you had at the time....

 

[ban-all-sheds]

BAS's i will find the correct solution and use the 2.5mm.

No you won't.

dont worry yourself too much.

I'm not the one who needs to worry - I'm not going to be at risk because of your incompetence.

 

[ban-all-sheds]

The IEE obviously don't think so or else they wouldn't have put 101 in which has different figures from 103 and which also almost exactly matches the installation method described whereas 103 doesn't.

Do you need a picture drawn for you to be able to understand:

"it will be sat on the ceiling joists and insulation between these joists. Above it will be the new floor filled with insulation."

?

 

[chapeau]

I think you have misunderstood me. The point I am/was trying to make is that there is no installation method specifically for flat twin and earth cables, above a ceiling, with cable not touching an inner surface.

Well i don't want to get into an argument with you here simply because the board pedant/thug tried to prove how clever he is and failed.

Again

However, my reading of the OP first post was that he was putting a new room in the loft. The existing joists are there and have insulation between them, below them is (assumed) plasterboard which forms a ceiling. Above the joists, along the top of the joists so to speak,and clipped to the joists, and so above the existing insulation, he will be running his cable. Above this plane, he will be putting additional joists (as is common in loft extensions) which will have insulation between them. Therefore the cable is now between two layers of insulation > 100mm thick and clipped to the existing joists.

This is almost exactly as shown as in 101# on the page I referred to earlier.

What is interesting, is that the current carrying capacity differs from 103#, but mine is not to reason why. I wouldn't have bothered pointing out the difference myself, or defend others, but then perhaps that's just me.

But the end result is as you said, if you want to use 2.5mm, the best way is to put in on a 16amp protective device and run it as a radial.

I think I shall leave it at that. Cheers.

 

[electronicsuk]

I think you have misunderstood me. The point I am/was trying to make is that there is no installation method specifically for flat twin and earth cables, above a ceiling, with cable not touching an inner surface.

Well i don't want to get into an argument with you here simply because the board pedant/thug tried to prove how clever he is and failed.

I don't see there's anything to argue about?

However, my reading of the OP first post was that he was putting a new room in the loft. The existing joists are there and have insulation between them, below them is (assumed) plasterboard which forms a ceiling. Above the joists, along the top of the joists so to speak,and clipped to the joists, and so above the existing insulation, he will be running his cable. Above this plane, he will be putting additional joists (as is common in loft extensions) which will have insulation between them. Therefore the cable is now between two layers of insulation > 100mm thick and clipped to the existing joists.

This is almost exactly as shown as in 101# on the page I referred to earlier.

I agree entirely that method 101 is correct if the cable is clipped to the joists. However, if the OP simply covers the existing wiring (which, let's be honest, is unlikely to be clipped) with 200mm more insulation, it will be completely enclosed. Without clipping or moving the existing insulation to sit the cable on top of the plasterboard, we must use method 103. If it's clipped, 101 is fine. The OP didn't state his intentions until replies had already been made, hence the confusion.

What is interesting, is that the current carrying capacity differs from 103#, but mine is not to reason why. I wouldn't have bothered pointing out the difference myself, or defend others, but then perhaps that's just me.

It's because method 101 gives a means for heat to escape - plasterboard and joists are much more thermally conductive than loft insulation.

But the end result is as you said, if you want to use 2.5mm, the best way is to put in on a 16amp protective device and run it as a radial.

Yup, as long as method 101 is used then it's fine to do so.

 

[ban-all-sheds]

However, my reading of the OP first post was that he was putting a new room in the loft. The existing joists are there and have insulation between them, below them is (assumed) plasterboard which forms a ceiling. Above the joists, along the top of the joists so to speak,and clipped to the joists, and so above the existing insulation, he will be running his cable.

But after his first post came his second, in which he said "it will be sat on the ceiling joists and insulation ", so why did you decide that meant it wasn't sat on the insulation, but was clipped to the joists?

 

[chapeau]

It's because method 101 gives a means for heat to escape - plasterboard and joists are much more thermally conductive than loft insulation.

Method 103# demonstrates a cable clipped to a joist. The current rating capacity for any cable is different from Method 101# where the cable is clipped to a joist. If you were to look at the picture examples in 103# and 101# you would intuitively assume that for this method that the current carrying capacity of 101# was less than 103#, whereas it is the opposite.

Using either method results in the same answer in practice.

 

[ban-all-sheds]

But 101 does not show a cable completely surrounded by insulation, which is what marsaday described.

Using either method results in the same answer in practice.

101# & 103#??

 

[martinxxxxxx]

It's because method 101 gives a means for heat to escape - plasterboard and joists are much more thermally conductive than loft insulation.

Method 103# demonstrates a cable clipped to a joist. The current rating capacity for any cable is different from Method 101# where the cable is clipped to a joist. If you were to look at the picture examples in 103# and 101# you would intuitively assume that for this method that the current carrying capacity of 101# was less than 103#, whereas it is the opposite.

Using either method results in the same answer in practice.

What is the same answer in practice?
- I am a little slow I know but - I am getting a different answer when I look in my copy of the BRB.

Would I be right in assuming that the reason the cable capacity is derated further in 103 to that in 101 (even though both diagrams have a cable clipped direct to the joist) is that in the 103 method it is enclosed top AND bottom by plasterboard? (wow what a long question)

Martin

 

[chapeau]

Well, I was having a conversation with electronicsuk. He said

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.

I continued the conversation when I said

But the end result is as you said, if you want to use 2.5mm, the best way is to put in on a 16amp protective device and run it as a radial.

Two methods of installation have been proposed, it's either in a ceiling or a wall.

If you assume this is a ceiling with > 100mm insulation (which it is) then method 101 applies, current carrying capacity of cable is 17amps.

if you assume it is a stud wall (which it isn't) method 102 now applies and the current carrying capacity of cable is 21 amps.

Using either method results in the same answer in practice i.e. it's OK to use 2.5mm cable on a 16amp radial.

I do apologise to the board for continuing this pedantry started by others. I only wanted to give a simple and helpful answer which I did, hopefully the OP is better informed now and can make better decisions.

 

[martinxxxxxx]

If you assume this is a ceiling with > 100mm insulation (which it is) then method 101 applies, current carrying capacity of cable is 17amps.

if you assume it is a stud wall (which it isn't) method 102 now applies and the current carrying capacity of cable is 21 amps.

Using either method results in the same answer in practice i.e. it's OK to use 2.5mm cable on a 16amp radial.
.

You were comparing methods 101 and 103, now you are referring to 101 and 102.
What happened to your comparison with 103.

Surely 103 is the method that is actually applicable here?

As 523.7 does confirm.

And the OP has described as such UNLESS he removes the existing insulation and lays wiring direct on the plasterboard and clips to the joists. Which I don't imagine is going to be the case.

Therefore the most accurate advice is to follow Method 103 ergo 6mm is required in this area. he can keep his 2.5 run to the loft and step up to 6mm for the new ring circuit where method 103 is applicable. Or 4 mm if he chooses to have a 16A radial.

I don't think 2.5mm can be used without altering the installation method.


Martin

 

[chapeau]

Hi martin,

I've given this more thought as I dig out the vegetable patch and cut some firewood.

Heat rises. Therefore a wire in a wall, which may well be vertical, should be considered differently from a wire in a ceiling, which is almost certainly horizontal.

A horizontal wire will lose convective heat away from the wire. A vertical wire will lose heat into the same space as the wire above it, heating it up.

Makes sense to me and might explain why the current carrying capacities are different, and why using methods that specifically say 'stud wall' should not be used in ceilings.

I hope you find that thought more interesting and constructive than others on here.

And if you look at my first post I think i said use thicker wire

Cheers.