Order Elec items for a 2 storey extn, + BS7671 imperfections

[JohnW2]

... unless they amended it last time round ...

Yeah - I should get an up-to-date version.

There are probably some secondhand ones around by now. I'm not quite sure why people get rid of them so relatively young (career change?!), but they do!

Is there a definition of "power circuit" ? Do lights no longer use power?

No definition that I can see/find. Table 52.3 gives you the choice of "Non-sheathed and sheathed cables", "Bare conductors" and "Non-sheathed and sheathed flexible cables" (the last begging the question as to why the first was not correspondingly qualified!!) - and the first of these is subdivided into (minimum CSA) 'Lighting circuits' (1.0mm²), 'Power circuits' (1.5mm²) and "Signalling and control circuits' (0.5mm², with a footnote allowing 0.1mm² for signal/control circuits in electronic IT equipment)). I suppose that this implies that anything which does not fall ingto the first of third of these categories must be regarded as a 'power' circuit.

Kind FRegards, John

 

[JohnW2]

Red Table 51 explicitly says that power circuits include lighting circuits...

Presumably because the minimum CSA was then the same for both, but they realised that some people might think that 'power' circuits did not include lighting ones. They could, of course, simply have written 'Power and lighting circuits'!

Kind Regards, John

 

[ban-all-sheds]

What does Green Table 51 say?

 

[JohnW2]

What does Green Table 51 say?

Have you seen, read and understood my penultimate post? (not counting this one!!)

Kind Regards, John

 

[EFLImpudence]

We did an exercise at college under the early 15th edition where we discovered it was possible to wire an immersion with 1mm2 cable within the regs. Not that you would, but you could....

That is true but no reason not to now, or 1.5mm².

Another remnant, like cookers, from 3036s when both cable sizes would have been inadequate.

 

[JohnW2]

We did an exercise at college under the early 15th edition where we discovered it was possible to wire an immersion with 1mm2 cable within the regs. Not that you would, but you could....

That is true but no reason not to now, or 1.5mm².

As per posts above, it'sno longer 'or' - BGB 524.1 (Table 52.3) now requires a minimum of 1.5mm², unless it's a lighting (or signal/control) circuit.

Kind Regards, John

 

[EFLImpudence]

If he'd said 67% that would have a known origin.

That refers to the value of the CCC to the OPD (20:30).

The 60% to which I referred was the rating of the OPD to the combined CCC of the two conductors (32:54).
I had forgotten about the previous use of 30A 3036s when the ratio was only 55% (30:54) (any lower and the 'ring circuit' would have been pointless).

Because of the 0.725 derating the CCC of the two cables had to be 41.4A which is 77% of 54A.
Therefore, with 60898s now, is there any reason why a ring circuit could not be protected by a 40A MCB?

 

[EFLImpudence]

As per posts above, it's no longer 'or' - BGB 524.1 (Table 52.3) now requires a minimum of 1.5mm², unless it's a lighting (or signal/control) circuit.

True, can't think of everything at once.

 

[JohnW2]

If he'd said 67% that would have a known origin.

That refers to the value of the CCC to the OPD (20:30).

Indeed, as you will have seen, that's what I speculated. As you will also have seen, as BAS pointed out, they had to amend 16th ed (from 67% to a straight 20A) when they realised that, although 67% was fine for a 2.5mm² ring final on a 30A OPD, it did not quite work with a 32A one!

The 60% to which I referred was the rating of the OPD to the combined CCC of the two conductors (32:54).

Interesting. Where did that one come from (I mean where, if anywhere, was it stated as a criterion?)

Therefore, with 60898s now, is there any reason why a ring circuit could not be protected by a 40A MCB?

You seem to be implying that you know the basis on which the 'dispensation' in the regs for ring finals was derived. Is that the case, or are you just doing some 'extrapolation'?

Kind Regards, John

 

[EFLImpudence]

The 60% to which I referred was the rating of the OPD to the combined CCC of the two conductors (32:54).

Interesting. Where did that one come from (I mean where, if anywhere, was it stated as a criterion?)

Well, nowhere really.
As I added with 3036 fuse it was 55% or 77% with the derating factor.
40/0.725 = 55 so the two cables (54A) would be too low so, I presume, 30A had to be used.

Therefore, with 60898s now, is there any reason why a ring circuit could not be protected by a 40A MCB?

You seem to be implying that you know the basis on which the 'dispensation' in the regs for ring finals was derived. Is that the case, or are you just doing some 'extrapolation'?

Just going on the numbers.

 

[JohnW2]

The 60% to which I referred was the rating of the OPD to the combined CCC of the two conductors (32:54).

Interesting. Where did that one come from (I mean where, if anywhere, was it stated as a criterion?)

Well, nowhere really....

Therefore, with 60898s now, is there any reason why a ring circuit could not be protected by a 40A MCB?

You seem to be implying that you know the basis on which the 'dispensation' in the regs for ring finals was derived. Is that the case, or are you just doing some 'extrapolation'?

Just going on the numbers.

Fair enough - but, as I implied, without knowing what was going on in the IET's mind when they produced (and modified) the regs relating to ring finals, one can't be sure that extrapolation from those numbers can be done. In any event, when you ask whether a ring final could be protected by a 40A MCB, you presumably mean 'electrically speaking' - since we know that, per current regs, the only permitted ring final (with CCC of one cable <In of MCB) is one with a 30/32A OPD. If you wanted to use a 40A MCB, you would therefore presumably have to produce your own justification 'from first principles', that it was acceptable, without any reliance on (or reference to) what the regs say about 30/32A ring finals.

Kind Regards, John

 

[EFLImpudence]

Oh yes, just theoretical.

However, since BaS implied that the ring is allowed as it is only because of 'tweaking' and was originally permitted on a 30A 3036 when that is only 3A more than the CCC of one leg.

Presumably it was because of the 0.725 derating when even a 20A radial was/is not allowed with 2.5mm².
It would have to have been two 15A - same difference.

That derating no longer applies and it is possible to have two 25A radials.
Therefore why not a 40A MCB for a ring?


even the old reason of 'saving copper' doesn't seem to stand because two 25A radials would be more useful and the routing likely would use less copper because part of the 'ring' would no longer be required.

 

[JohnW2]

Oh yes, just theoretical. ... However, since BaS implied that the ring is allowed as it is only because of 'tweaking' and was originally permitted on a 30A 3036 when that is only 3A more than the CCC of one leg. ... Presumably it was because of the 0.725 derating when even a 20A radial was/is not allowed with 2.5mm².
It would have to have been two 15A - same difference. ... That derating no longer applies and it is possible to have two 25A radials. ... Therefore why not a 40A MCB for a ring?

Sure, one can play these theoretical extrapolation games. Given that the current regs explictly allow a 30A ring with a cable CCC of 20A, one can, if one wishes, 'pro rata' that to a hypothetical 40A ring with a cable CCC of at least 26.67A - hence just OK, by the skin of it's teeth with 2.5mm² clipped direct.

Whether such pro-rata extrapolation is acceptable is a different matter. For example, the process (assuming that there was a formal design process) which led the IET to the currently-allowed ring final presumably will have relied upon assumptions about the precise distribution of loads around a maximally-loaded ring. That's were the whole thing becomes iffy, because one could probably get a wide range of answers depending on what distributional assumptions one made - and, in practice, such assumptions cannot necessarily be exactly 'pro-ratad'.

even the old reason of 'saving copper' doesn't seem to stand because two 25A radials would be more useful and the routing likely would use less copper because part of the 'ring' would no longer be required.

As I've illustrated on a number of occasions, the 'saving copper' arguments were always pretty trivial, and not necessarily even true with some circuit layouts

Of course, as we've often discussed, there seem to be so many 'safety margins' built in to some of the figures we work with (particularly CCCs) that many things which would be 'totally unthinkable' in terms of the regs would very probably be pretty safe in practice, particularly given that socket circuits will very rarely be 'full loaded' (i.e. right up to the In of the OPD, or even beyond) for a significant continuous period of time - so you could, if you so wished, probably dream of things like a 32A (or maybe even 40A) 1.5mm² radial

Kind Regards, John

 

[ban-all-sheds]

What does Green Table 51 say?

Have you seen, read and understood my penultimate post? (not counting this one!!)

Yes.

What does Green Table 51 say?

 

[JohnW2]

What does Green Table 51 say?

Have you seen, read and understood my penultimate post? (not counting this one!!)

Yes. What does Green Table 51 say?

Apologies - I (subconsciously) assumed that your 'Table 51' was a typo, relating to the other table we had been discussing.

Green Table 51 does, indeed, still have a footnote indicating the (for the purpose of that table) "Power circuits include lighting circuits".

Kind Regards, John