Ring spurs

[JohnW2]

This has 'clipped direct' - 23A: https://www.electrical-contractor.net/PC/IEE1966_T3.JPG

I've just noticed that the CCCs in that table relate to situations in which there is "coarse extra-current protection" and that one can multiply them by 1.33 to get figures for situations in which there is "close excess-current protection".

On that basis, with "close excess-current protection", the CCC for 'clipped direct' 7/0.029" would be 30.67A - similar to the 30A figure I 'deduced' from the table I linked to.

However, can anyone tell me what "coarse" and "close" excess-current protection were, or do I have to ask Mr Google?

Kind Regards, John

 

[JohnW2]

Not all of the 'surface' area is on the surface and there are many air gaps.

Indeed so. In fact, I would not expect the 'outside surface area (available for conduction of heat away from conductors) would be appreciably different with stranded and solid conductors of the same CSA.

Flex has many strands but the CCC is always less than T&E.

Yes, we've discussed that before, and have speculated about the reason. As above, I frankly doubt that it is primarily (if at all) related to the exposed surface area. One possibility is that, give the sort of stresses that flex is likely to be subjected to, it might include an allowance for a few strands getting broken. Another possibility is that because, unlike fixed wiring, there is no certainty about the surrounding environment (c.f. "installation method"), the 'CCCs' might take into account the possibility that it could be put under carpet, under a pile of laundry or whatever. However, like all the other ideas, those thoughts are pure speculation.

Kind Regards, John

 

[ban-all-sheds]

"Coarse" = 3036?

 

[JohnW2]

"Coarse" = 3036?

That was also the first thing which occurred to me. However, if "coarse" was a 3036, what would have been "close" in (it seems, from filename) 1966? Were MCBs around then?

 

[ban-all-sheds]

Your "however" was the first thing that occurred to me after I posted...

 

[securespark]

This has 'clipped direct' - 23A:

https://www.electrical-contractor.net/PC/IEE1966_T3.JPG

That's not the 13th, though....
That scan is from the 14th Ed., 1st October 1966.

 

[EFLImpudence]

I didn't know which edition it was. I just searched for imperial cable current capacity.

Does it matter?

 

[securespark]

This is from 13th Ed., 1st September 1955:

 

[securespark]

I thought we were talking specifically about the 13th. Sorry.

 

[Harry Bloomfield]

All I can find give its CCC as from 20A to 23A although it is equivalent to 2.9mm².

Would that be because it is stranded? Perhaps a reason for making solid cored conductors.

You are thinking of the 'surface effect' at RF frequencies I think, which doesn't apply at 50 hertz .

 

[securespark]

can anyone tell me what "coarse" and "close" excess-current protection were

 

[securespark]

These definitions appeared in the 14th Ed.

 

[JohnW2]

This is from 13th Ed., 1st September 1955: ...

I don't quite get that. It gives a CCC of 20A for 7/0.029", but says that it relates to

(i) In conduit, troughing or casing
(ii) In free air or an open trench

Only the one figure (20A) is given, but it surely does not apply to both those installation methods, does it?

Kind Regards, John

 

[JohnW2]

... These definitions appeared from the 14th Ed.

Thanks - so BAS's and my 'first thoughts' were correct. As I said, I was doubtful that MCBs were around back then, but clearly they were!

So, these figures of 20A - 23A we're seeing for 7/0.029" correspond to 26.6A - 30.67A with MCBs - similar to (maybe fractionally higher than) 2.5mm².

Kind Regards, John

 

[securespark]

Dunno...I did actually learn the 14th and did a limited amount of work to it, but never familiarised myself with the 13th!