Cables in non-insulated stud walls

[EFLImpudence]

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

No, obviously I meant no restriction - reduction - on the maximum CCC of a particular cable.

... 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'

Yes, but if the basic tabulated CCC of a cable is, say, 27A then do or can the regulations list every circumstance that does not reduce that CCC?

Perhaps we need a better definition of 'free air'.

 

[JohnW2]

No, obviously I meant no restriction - reduction - on the maximum CCC of a particular cable.

"Reduction" from what? From what you go on to write, it sounds as if you are talking about 'reduction' from the CCC "in free air" (Ref Method E or F - as I asked before, what's the difference between them?) - is that correct?

Yes, but if the basic tabulated CCC of a cable is, say, 27A then do or can the regulations list every circumstance that does not reduce that CCC? ... Perhaps we need a better definition of 'free air'.

As I wrote earlier, within an uninsulated stud wall is (in common sense terms) not far off "in free air" - but, as you say, an actual definition would be nice. Whilst, as you say, one can't expect the regs to comprehensively list every circumstance that qualified as "in free air", I remain fairly amazed that the regs do not list what must be one of the most common installation methods (probably more common than the Installation Method 40 that you have been talking about!).

If you actually believe what you are now saying, is it also your belief that the CCC of 2.5mm² T+E entirely within an uninsulated stud wall is actually 30 A, rather than the 27 A which I think that nearly all of us assume?

 

[martygturner]

Or is it a reverse definition, if none of the following conditions apply then it is in free air.... there is some arcane rule of law around presumption on innocence which sets this premise and is followed in general scientific theory, the null and alternate hypothesis, whereby the alternate can only be accepted if there is measurable divergence from the null ( status quo), ie null - cables shall be derated if c exist and then only not derated if the conditions of the null are not found.

 

[JohnW2]

Or is it a reverse definition, if none of the following conditions apply then it is in free air....

That is seemingly what EFLI is implying - and, as you go on to say, would not be an unreasonable approach.

However, if so, that begs the question as to why Reference Methods E and F exist (and I still don't know the difference between them!) - since, if they didn't, the default "in free air" would still apply?

 

[martygturner]

why not ask them the basis for the tables, as its a scientific approach the source material will be freely available. Is there not a reference to the source in the book ?

 

[securespark]

I've always taken 2.5 as 20A in installation methods without insulation or other factors.

 

[EFLImpudence]

I've always taken 2.5 as 20A in installation methods without insulation or other factors.

Why?

Surely 'without insulation or other factors' must mean the maximum CCC for the particular cable.

 

[EFLImpudence]

There is this:

Goodness knows where that leaves us.

 

[JohnW2]

I've always taken 2.5 as 20A in installation methods without insulation or other factors.

Why -particularly for what is probably the most common installation method (Method C), for which we all seem to accept that (as indicated in the regs) it is 27 A?

 

[JohnW2]

There is this: ....
Goodness knows where that leaves us.

I think it leaves us where I've been saying all along - that, for whatever reason, the regs don't seem to have considered what is probably one of the most common installation methods.

 

[JohnW2]

why not ask them the basis for the tables, as its a scientific approach the source material will be freely available. Is there not a reference to the source in the book ?

Not that I've noticed - but, there again, I may well not have looked for it!

There undoubtedly was some sort of scientific basis originally, but most of the tabulations of CCCs have been unchanged for decades, so the original sources are probably ancient.

There do, of course, also seem to have been considerations other than scientific ones at work - in particular the 'fiddling' which seemed to happen which resulted in the creation of Table 4D5, seemingly to allow 2.5mm² T+E in walls/ceilings with insulation to be used for ring finals (which, at least in some cases, would not have been allowed by the very-long-standing 4D2A)!

 

[securespark]

Why?

In practice, I have only used 2.5 to supply:

1. A radial via a 16 or 20A breaker.

2. A ring final via a 32A breaker.

 

[JohnW2]

In practice, I have only used 2.5 to supply:
1. A radial via a 16 or 20A breaker.
2. A ring final via a 32A breaker.

Have you never used it to supply a double socket as an unfused spur (which. despite all the arguments about 'ratings', is theoretically a possible 26A load) from a ring final? If so, how can you be happy using a cable you regard as having a CCC of 20 A to supply a possible 26A load?

 

[securespark]

Only run a single, and I wasn't happy under the 14th, where it said you could run two singles from a unfused spur, I was only happy running one. But in those days, if your boss overruled you....

I sometimes put a triple in, when they were available.

 

[JohnW2]

Only run a single, and I wasn't happy under the 14th, where it said you could run two singles from a unfused spur, I was only happy running one.

Is that because you believed the CCC of 2.5mm² T+E was only 20A?

I sometimes put a triple in, when they were available.

Fuse-wise that's obviously no different from one single.