installation of a ring cicuit

[ban-all-sheds]

That's for the designer to decide,

OK - suggest a way. Since you think it would be a compliant design you must have in mind a way in which it complies.

I was merely pointing out that 433.1.5 does not "clearly and unequivocally call for the cable to be capable of carrying 20A".

433.1.5 does. Read the last sentence again.

so maybe that gives some clue as to how a designer might try to claim compliance with 433.1.1 with an Iz less than 20A.

Go on then - fire away. I'm curious to see how you dispose of 433.1.1 (ii).

Of course, the problem could be solved by having an In less than 32A - but, unfortunately, MCBs with In between 20A and 32A are not freely (if at all) available. 25/26A would probably be ideal in the situation which was being discussed.

No it wouldn't - Iz is 16A remember, and you've cut yourself off from the deemed to satisfy 433.1.1 by not being compliant with 433.1.5.

 

[ricicle]

OK - suggest a way. Since you think it would be a compliant design you must have in mind a way in which it complies.
.

It doesn't have to comply if you are willing to sign a deviation to say that you have designed the circuit in such a way that the cable will not take more than 16A or you can prove that the cable will not suffer thermal damage because of load cycling.

 

[ban-all-sheds]

How are you going to do either of those for a domestic ring final circuit?

 

[JohnW2]

http://www.fastlec.co.uk/hager-25a-mcb-type-single-pole-p-3201.html

Thanks. Fair enough; as I implied, I hadn't seen one. ...so that could be one potential solution (the designer could probably simply argue pro-rata scaling to the 433.1.5 deemed-to-satisfy RFC) - technically, of course, provided they had a CU whose manufacturer recommended that particular MCB.

Kind Regards, John

 

[JohnW2]

I was merely pointing out that 433.1.5 does not "clearly and unequivocally call for the cable to be capable of carrying 20A".

433.1.5 does. Read the last sentence again.

Which last sentence? The last one of 433.1.5? The one which starts "Such circuits are deemed to meet the requirements of Regulation 433.1.1 if ...."?

so maybe that gives some clue as to how a designer might try to claim compliance with 433.1.1 with an Iz less than 20A.

Go on then - fire away. I'm curious to see how you dispose of 433.1.1 (ii).

As I implied before, by using judgement in relation to the anticipated loading (and distribution of loading) in the particular circuit in question. This is the designer's prerogative.

Of course, the problem could be solved by having an In less than 32A -.... 25/26A would probably be ideal in the situation which was being discussed.

No it wouldn't - Iz is 16A remember, and you've cut yourself off from the deemed to satisfy 433.1.1 by not being compliant with 433.1.5.

(16/20)*32 = 25.6 ...then invoke an analogy of the thinking of 433.1.5

Kind Regards, John.

 

[TheGayOwl]

How about running two 20A or 16A radials instead?

I don't enjoy terminating 4mm ² into sockets...especially consideringn any future additions.

 

[JohnW2]

How about running two 20A or 16A radials instead?

Yes, that's certainly an option. It would presumably have to be 16A in the case we are discussing, since the cable has been de-rated to a CCC less than 20A. However, as I've written before, whilst I like radials, I'm not all that comfortable with 20A (let alone 16A) ones that have several sockets - since it's so easy to overload the circuit.

How I don't enjoy terminating 4mm ² into sockets...especially consideringn any future additions.

Indeed. I doubt that anyone enjoys it, and I feel sure that (even in professional hands, and certainly otherwise) there is a potential safety downside (as one struggles to get everything back into the box).

Kind Regards, John.

 

[TheGayOwl]

Sorry typing thid from phone now so will be short as fiddly!

Indeed John, would prefer occasional nuisance trips though rather than overheating cable. What is likely to be on cct(s). If upstairs then possibly not much chew on them.

Regards.

 

[JohnW2]

Indeed John, would prefer occasional nuisance trips though rather than overheating cable.

I presume you are talking about tripping of the MCB due to overload. However,as we know, an MCB will allow appreciably more than its In to flow for moderate amounts of time. Admittedly, the regs implictly deem that degree of overload to be acceptable (and hence presumably safe), but I would be more comfortable if I believed that exceeding In (hence Iz, if designed to be no greater than In) was going to be rare - and (obviously depending on circumstances) it would often be very hard to be reasonably confident that this was going to be the case with a 16A radial which had several double sockets.

Put more formally, rather in terms of 'comfort', the designer is required to work on the basis of the anticipated load, and ensure that this does not exceed the In of the protective device. Again, I would think that it would often be hard for him/her to be very confident that the load on a circuit with several double 13A sockets was not expected to exceed 16A.

Kind Regards,John.

 

[ban-all-sheds]

Which last sentence? The last one of 433.1.5? The one which starts "Such circuits are deemed to meet the requirements of Regulation 433.1.1 if ...."?

Yes, the one which then clearly and unequivocally continues with "the current carrying capacity (Iz) of the cable is not less than 20A..."

As I implied before, by using judgement in relation to the anticipated loading (and distribution of loading) in the particular circuit in question. This is the designer's prerogative.

This is a ring final circuit in a domestic dwelling.

I genuinely do not believe how anybody who was neither mad, homicidal or utterly deluded about their competence could claim that they had exercised reasonable skill and care and that to the best of their knowledge and belief such a circuit was not less safe than one which complied with Chapter 13 Part 4.

(16/20)*32 = 25.6 ...then invoke an analogy of the thinking of 433.1.5

IHNI what you mean by that.

You can no longer take account of any of the provisions in 433.1.5 because you do not comply with its requirement for Iz.

You are left with the problem of justifying a departure from 433.1.1 (ii) for a final circuit in a domestic dwelling in a way which shows that is not less safe than one which complies.

 

[ban-all-sheds]

and (obviously depending on circumstances) it would often be very hard to be reasonably confident that this was going to be the case with a 16A radial which had several double sockets.

But not hard at all, according to you, for a 32A ring final.

Again, I would think that it would often be hard for him/her to be very confident that the load on a circuit with several double 13A sockets was not expected to exceed 16A.

Unless, of course, it was your 32A ring final where you had exercised your designer's prerogative of deciding that you were confident.

 

[JohnW2]

Which last sentence? The last one of 433.1.5? The one which starts "Such circuits are deemed to meet the requirements of Regulation 433.1.1 if ...."?

Yes, the one which then clearly and unequivocally continues with "the current carrying capacity (Iz) of the cable is not less than 20A..."

Thanks confirming. In that case, you presumably understand that it is not saying (at all, let alone 'clearly and unequivocally') that this is the only acceptable design.

As I implied before, by using judgement in relation to the anticipated loading (and distribution of loading) in the particular circuit in question. This is the designer's prerogative.

This is a ring final circuit in a domestic dwelling. I genuinely do not believe how anybody who was neither mad, homicidal or utterly deluded about their competence could claim that they had exercised reasonable skill and care and that to the best of their knowledge and belief such a circuit was not less safe than one which complied with Chapter 13 Part 4.

I don't know about you, but I am not a professional designer of electrical installations, so I'll leave it to those who are to comment on whether they feel that, in appropriate circumstances, it would be reasonable/acceptable for them to exercise judgement such as we are discussing. I think what you may be overlooking to some extent is the question of 'design current'. If a designer believes that the anticipated load is low, then I think such judgements might become more reasonable.

(16/20)*32 = 25.6 ...then invoke an analogy of the thinking of 433.1.5

IHNI what you mean by that.
You can no longer take account of any of the provisions in 433.1.5 because you do not comply with its requirement for Iz.

I'm talking about designer's judgement, and it would not IMO be unreasonable for him/her to base their judgement on the same, or similar, reasoning as the IET appear to have applied in arriving at their deemed-to-satisfy design. That's obviously not the same as claiming compliance with 433.1.5.

You are asking me questions which only an experienced designer of electrical installations can really answer, and it would be good if some of them could comment. I have merely been making the point that they do have the option of basing a design on their own knowledge,judgement and calculations, and do not necessarily have to restrict themselves to blindly sticking with deemed-to-satisfy designs in the regs.

Kind Regards, John

 

[JohnW2]

and (obviously depending on circumstances) it would often be very hard to be reasonably confident that this was going to be the case with a 16A radial which had several double sockets.

But not hard at all, according to you, for a 32A ring final.

Again, I would think that it would often be hard for him/her to be very confident that the load on a circuit with several double 13A sockets was not expected to exceed 16A.

Unless, of course, it was your 32A ring final where you had exercised your designer's prerogative of deciding that you were confident.

Some degree of reasonable judgement, based on the circumstances of the installation concerned, is obviously necessary in relation to the design of any socket circuits; otherwise one would have to design on the basis of 13A loads being plugged into every available socket, and therefore have a maximum of two single sockets or (without wanting to re-gnght the discussion about the rating of double sockets!) one double socket on a 32A circuit.

I would have thought it was self-evident that in exercising that reasonable judgement, a designer will often be more confident that the load on a circuit is unlikely to exceed 32A than that it is unlikley to exceed 16A.

Kind Regards, John.

 

[ban-all-sheds]

Thanks confirming. In that case, you presumably understand that it is not saying (at all, let alone 'clearly and unequivocally') that this is the only acceptable design.

I understand that it is clearly and unequivocally saying that if you want to use the provisions of 433.1.5 to design a ring final circuit deemed to comply with 433.1.1 then the Iz of the cable must be at least 20A.

You do not.

There may be ways in which a circuit not immediately in compliance with 433.1.1 can be justified by the designer, but if the Iz of the cable is not at least 20A then, clearly and unequivocally, the provision for RFCs in 433.1.5 is not one of them.

I don't know about you, but I am not a professional designer of electrical installations, so I'll leave it to those who are to comment on whether they feel that, in appropriate circumstances, it would be reasonable/acceptable for them to exercise judgement such as we are discussing. I think what you may be overlooking to some extent is the question of 'design current'. If a designer believes that the anticipated load is low, then I think such judgements might become more reasonable.

Stop wriggling.

This is a ring final circuit in a domestic dwelling.

When ricicle told the OP (who is not a qualified designer) that he could decide that he had "a properly designed ring final circuit in which the cable will share load evenly with a theoretical maximum of 16A on any one leg which is a fraction more than 50% of the current carrying capacity of 2.5mm² so is not going to be a problem bearing in mind that ring finals rarely spend very long, if ever, at their maximum allowable design current", and I challenged him on that, because of 431.1.1, YOU said "I do not believe that in any way prevents a designer producing a design which satisfies 433.1.1 without satisfying the requirements of 433.1.5's deemed-to-satisfy provision."

And you've kept on in that vein, despite all your disclaimers about not being a qualified designer and despite recognising the difficulties of establishing Ib for final circuits serving BS 1363 accessories.

The OP (who is not a qualified designer) came here for advice.

YOU supported the notion that he could install a circuit which did not comply with 433.1.5

So YOU have to justify that, not say "Oh well I'm sure that some people might sometimes be able to". Or take your ideas off to another of your dedicated eristic threads where they can safely be ignored.

You are asking me questions which only an experienced designer of electrical installations can really answer,

And I'm asking them because of your support for the idea that the OP could install a circuit which does not comply with 433.1.1 and which does not qualify for the deemed compliance provided by 433.1.5.

 

[JohnW2]

I understand that it is clearly and unequivocally saying that if you want to use the provisions of 433.1.5 to design a ring final circuit deemed to comply with 433.1.1 then the Iz of the cable must be at least 20A.

You do not.

Why on earth are you trying to put those words incorrectly into my mouth? If one wants to use the deemed-to-comply provision of 433.1.5 to satisfy 433.1.1, then one obviously has to have a cable of at least 20A CCC. That's what the deemed-to-satisfy provision says!

The rest of what you write is essentially repetition of points previously made. I have at no point suggested that the op should adopt a non-conventional design. The only reason I got involved was to point out that you appeared to be making an incorrect assertion that the deemed-to-satsify provision of 433.15 represents the one and only acceptable basis for designing an RFC.

Kind Regards, John