Cables in non-insulated stud walls

[JohnW2]

Either I'm missing something or else I'm amazed that I haven't noticed this before.

Earlier today, a friend asked me what (BS 7671) 'installation method' applied to a cable installed in an internal stud wall (wooden studs and plasterboard, with NO insulation). I answered 'Method C', essentially because I have always 'taken it for granted' that such was the case.

He then asked me where BS 7671 said that, so we had a look at the book together. Whilst there are some (per Table 4D5) of cables in insulation within stud walls, we could find nothing about the situation when there is no insulation.

Did we both miss it somewhere? Do folk agree that it is Method C (which, as above, is what I have always believed)?

Kind Regards, John

 

[robinbanks]

F

 

[JohnW2]

F

What makes you say that? In fact, even if it were Reference Method F, that wouldn't really help, since none of the tables in Appendix 4 give CCCs for T+E using Reference Method F.

I don't really understand the difference between Ref Methods E and F (most references are to "E or F"), but I wonder if you are assuming that within an uninsulated stud wall would qualify as "in free air". Ref Method E (which includes "in free air", as does Method F) does exist in Table 4D5 but would, to my considerable surprise, then give the CCC for 2.5mm² T+E as 30 A, rather than the 27 A (Method C) that I think we all seem to assume it is.

 

[EFLImpudence]

If the measurements are greater than those stated, does that mean it is Method C?

 

[JohnW2]

View attachment 382198

If the measurements are greater than those stated, does that mean it is Method C?

Does that mean that you think that a stud wall qualifies as a "building void"?

If so, then most cables in a stud wall would satisfy those figures for Method B - but that would mean, for example, that the CCC of 2.5mm² T+E in such a location would only be 20A. Do you really believe that? (as I've said, I've also presumed Methoid C, hence 27A)>

 

[EFLImpudence]

Does that mean that you think that a stud wall qualifies as a "building void"?

Only if the dimensions are smaller as those given.

What I was meaning is that larger dimensions do not mean Method B so what else is there? Method C or E?

 

[JohnW2]

Only if the dimensions are smaller as those given.

I'm not sure what you mean. I don't know what to take as the 'diameter' of 2.5 mm² T+E. The (my ) table in the wiki says that it's about 10.3 x 6.0 mm, so let's take the average, 8.15mm. That means that the stated conditions for Method B would be satisfied for stud walls with 'voids' anything between about 12.2 mm and 163 mm - which I imagine would encompass virtually all stud walls.

So, if one regards a stud wall as a "building void" (and I'm not at all sure about that!), then Method B it is, hence 20 A CCC for 2.5 mm² T+E - which is contrary to what I have always thought/believed. What do you regard as the CCC?

What I was meaning is that larger dimensions do not mean Method B so what else is there? Method C or E?

As above, I'm not sure what 'larger dimensions' you have in mind, since I think the stated conditions for Method B (in a 'building void' would almost certainly be met in any real-world situation.

For decades I hadn't realised this anomaly, but it seems amazing that what must be a very common installation method is not covered explicitly by any of the Installation Methods in the regs

 

[EFLImpudence]

Put some numbers in for me in case I have it wrong.

Assume D = 10mm:

 

[JohnW2]

Put some numbers in for me in case I have it wrong. Assume D = 10mm:

That's what I did in my last post, assuming D = 8.15 mm. As I wrote:

That means that the stated conditions for Method B would be satisfied for stud walls with 'voids' anything between about 12.2 mm and 163 mm - which I imagine would encompass virtually all stud walls.

with D= 10mm, the 'void' would have to be between 15mm and 200mm wide

 

[EFLImpudence]

with D= 10mm, the 'void' would have to be between 15mm and 200mm wide

Not wide but high. There is no mention of width.

Would a stud wall not have more than 200mm. between noggins?

 

[JohnW2]

Not wide but high. There is no mention of width. ..... Would a stud wall not have more than 200mm. between noggins?

Ah, we were interpreting differently

It doesn't actually say whether it's talking about height or width, but I suppose the "V" is a bit of a clue! I was assuming that the cable was travelling vertically, not horizontally, and the "V" was therefore width!

In that case, the question is answered, isn't it - as you imply, a cable in a stud wall would never satisfy that requirement, so we seem to be back to my original point - that, seemingly amazingly (for such a common situation), the regs seem totally silent about the CCC of a cable in an uninsulated stud wall.

That being the case, I'm inclined to stick with what I've always believed - that it is Method C. It's almost 'in free air' (E or F), so assuming it to be Method C may well be a little'conservative. What do you think?

 

[EFLImpudence]

Ah, we were interpreting differently

Ah, right.

It doesn't actually say whether it's talking about height or width, but I suppose the "V" is a bit of a clue! I was assuming that the cable was travelling vertically, not horizontally, and the "V" was therefore width!

That never occurred to me. I think it safe to assume that all the cables in the installation method diagrams are resting on the bottom surface of the various Voids for convection purposes.

In that case, the question is answered, isn't it - as you imply, a cable in a stud wall would never satisfy that requirement, so we seem to be back to my original point - that, seemingly amazingly (for such a common situation), the regs seem totally silent about the CCC of a cable in an uninsulated stud wall.

I suppose it is but does that not imply that there are no derating factors to consider.

That being the case, I'm inclined to stick with what I've always believed - that it is Method C. It's almost 'in free air' (E or F), so assuming it to be Method C may well be a little'conservative. What do you think?

I agree.

 

[JohnW2]

That never occurred to me. I think it safe to assume that all the cables in the installation method diagrams are resting on the bottom surface of the various Voids for convection purposes.

That would only work if the cables were travelling horizontally. What if, as is commonly the case in stud walls, the cables are travelling vertically - I don't think it is then "... safe to assume that all the cables in the installation method diagrams are resting on the bottom surface of the various Voids"

I suppose it is but does that not imply that there are no derating factors to consider.

What is it that you "suppose". If you're talking about the stud walls (without insulation) I'm talking about, I think it pretty unlikely that any 'derating factors' would often apply, isn't it?

I agree.

Thanks As I've said it's what I've always thought/believed - but, after decades of having that belief, it was only yesterday that I came to realise, for the first time, that the regs do not seem to consider this (common) situation at all

 

[EFLImpudence]

That would only work if the cables were travelling horizontally. What if, as is commonly the case in stud walls, the cables are travelling vertically - I don't think it is then "... safe to assume that all the cables in the installation method diagrams are resting on the bottom surface of the various Voids"

Well, then I suppose we cannot know.

However, are there any vertical voids that would be narrow enough to warrant the stated small dimensions?
Having said that, I don't really understand the definition of V:

What is it that you "suppose".

If you're talking about the stud walls (without insulation) I'm talking about, I think it pretty unlikely that any 'derating factors' would often apply, isn't it?

Que?

Thanks As I've said it's what I've always thought/believed - but, after decades of having that belief, it was only yesterday that I came to realise, for the first time, that the regs do not seem to consider this (common) situation at all

Perhaps if they don't mention something then there are no restrictions on the CCC.

 

[JohnW2]

Well, then I suppose we cannot know.

Quite - that's what I've been saying from the very start, much to my surprise I find that the regs appear silent ion relation to cable in uninsulated stud walls.

However, are there any vertical voids that would be narrow enough to warrant the stated small dimensions?

As I said, I misunderstood and, as you have pointed out,Installation Method 40 clearly only applies to horizontal voids/ducts, that 'V' is a vertical dimension and they don't mention (horizontal) 'width' - so 'narrow enough' does not really apply.

What I don't understand about Installation Method 40 is that is says "WHERE <dimensional requirements> use B" - so, even if it is a horizontal cable run, what Reference Method does one 'use' if those dimensional requirements are not satisfied?

Having said that, I don't really understand the definition of V:
View attachment 382272

Well done for finding that (I didn't, but will look!) - but, again,it clearly only applies to horizontal cable runs.

Que?

As with any installation method, if there are de-rating factors, then there are de-rating factors which have to be applied to the 'basic tabulated CCC' - but, as I said, I can't think of (m)any that would be likely to arise within an uninsulated stud wall. Which factors did you have in mind?

Perhaps if they don't mention something then there are no restrictions on the CCC.

I presume that your tongue in in your cheek, and that you're not suggesting that (because of this omission in the regs), it would be OK to wire my 10.5 kW shower with 1mm² cable, provided the cable ran entirely in an uninsulated stud wall

... or (looking at your last two quotes above) are we perhaps talking at cross-purposes? As I presume you understand, the reason I would like to know the Reference Method for installation of a cable in an uninsulated stud wall is obviously in order that I can look up the 'basic tabulated CCC' for a particular cable, before applying any 'restrictions' or 'de-rating factors'