Unfused spurs

[JohnW2]

Does this not come under 433.2.2?

Ah, yes, I'd overlooked that. My apologies. It therefore looks as I have to reverse my conclusion about the 'bottom line' and say that (provided that 433.2.2.(i) and/or 433.2.2(ii) is/are satisfied, which seems very probable) you are probably right, and BAS therefore wrong. However, given that (as BAS implied) you could then no longer invoke the 'deemed to satisfy' clause 433.1.5, compliance would become dependent upon your being able to demonstrate yourself that 433.1.1 would be satisfied with a 1.5mm² cable between the ring and FCU - which should be no problem!

Kind Regards, John.

 

[ban-all-sheds]

Does this not come under 433.2.2?

Not for a ring final.

 

[JohnW2]

Does this not come under 433.2.2?

Not for a ring final.

Why not? Apart from the 'deemed to satisfy' clause relating to RFCs (433.1.5), I can't see anything anywhere in Chapter 43 which distinguishes between different types of final circuit. Am I missing something?

Kind Regards, John.

 

[holmslaw]

..

 

[ban-all-sheds]

Why not? Apart from the 'deemed to satisfy' clause relating to RFCs (433.1.5), I can't see anything anywhere in Chapter 43 which distinguishes between different types of final circuit. Am I missing something?

The whole validity of a ring final rests on 433.1.5 - unless the circuit complies with it it fails in other ways as it no longer qualifies for the exemptions.

433.1.5 requires the circuit to be wired in 2.5mm² minimum, so that includes any spur cable on the supply side of an FCU.

 

[JohnW2]

The whole validity of a ring final rests on 433.1.5 - unless the circuit complies with it it fails in other ways as it no longer qualifies for the exemptions.

As I said, I really don't think think that 433.15 is anything more than a description of one arrangement of an RFC (the arrangement nearly always used) which is deemed to satisfy the requirements of 433.1.1.

I have to say that my interpretation is that the regulations do not pretend to be exhaustive and therefore permit any type of circuits, provided that the designer can demonstrate that the circuit satisfies the stated underlying requrements of the regs - in the context we are talking about, the requirements of Chapter 43 as regards overcurrent protection. As with 'deemed to satisfy' clauses in most regulations (the Building regs are full of them), I would therefore assume that 433.1.5 only exists to prevent people having to repeatedly re-invent that same wheel (the 'standard' RFC). However, I don't see that as in any way preventing a designer from undertaking his/her own analysis and calculations to demonstrate that a proposed circuit (ring final, or anything else) satisfies the general requirements of the regs - provided, of course, they have the knowledge and ability to do that. That's the way regulations generally work.

Having said all this, it wasn't me who intoduced this issue and I think it's essentially academic. Whilst the risk associated with using a very short length of 1.5mm² cable from a ring to a FCU is vanishingly small, I imagine that almost everyone would wire it in 2.5mm² cable, anyway.

I wonder if you have any thoughts about my original question?

Kind Regards, John.

 

[mikhailfaradayski]

Hello John, I hope you are well.

We are all familiar with the fact that, in the context of an RFC wired entirely with 2.5mm² cable, Appendix 15 ("Informative") of the regs indicates that an unfused spur should feed only one socket, whereas a spur connected via an FCU (fuse 13A) can feed an unlimited number of sockets.

I presume that this recommendation relates essentially to the current-carrying capacity of the cable - both within the spur circuit and also in the ring itself (since an extensive spur connected close to one end of the ring could result in most of the current going through one 'leg' of the ring - just as if many heavily-loaded sockets were installed close to one end of the ring).

If that is the case, then I think I could produce a fairly compelling argment for it to be acceptable to have an unfused spur with several sockets if it were wired in 4mm² cable, particularly if it were connected roughly at the mid-point of a ring. With such an arrangement, no cable would carry any more current than in the 'recommended' designs for RFCs or radial circuits. The wiring of the spur circuit itself would certainly be no problem since, with 4mm² cable, it would be identical to what is recommended for an unfused spur to a 4mm² radial final. As for the ring itself, the situation would be no worse than having several sockets close to each other on the ring at the point at which the spur was to be attached.

An advantage of such an arrangement would be that it could often result in an appreciable reduction in the total length of the ring (where most of the load would probably be), not to mention various possible practical installation advantages.

Any thoughts, or arguments against what I have suggested?

Sure, why bother producing a compelling argument for your concept?

What practical scenarios can you envisage which would make your idea the best solution when considering other options that may be available?

Kind Regards, John

 

[mikhailfaradayski]

oops, double post

 

[ban-all-sheds]

As I said, I really don't think think that 433.15 is anything more than a description of one arrangement of an RFC (the arrangement nearly always used) which is deemed to satisfy the requirements of 433.1.1.

It's the only one which satisfies it without deviant design which would require you to document it as an exception on an EIC and show that it was at least as safe as the one explicitly deemed to be safe.

I have to say that my interpretation is that the regulations do not pretend to be exhaustive and therefore permit any type of circuits, provided that the designer can demonstrate that the circuit satisfies the stated underlying requrements of the regs

Not necessarily a non-trivial thing to do.

However, I don't see that as in any way preventing a designer from undertaking his/her own analysis and calculations to demonstrate that a proposed circuit (ring final, or anything else) satisfies the general requirements of the regs - provided, of course, they have the knowledge and ability to do that.

Indeed not.

But why would you?

I wonder if you have any thoughts about my original question?

My thoughts are that there is little if any merit in gratuitous novelty, therefore unless you had a really pressing reason to do it the decision should be to not do it.

 

[holmslaw]

..

 

[JohnW2]

As I said, I really don't think think that 433.15 is anything more than a description of one arrangement of an RFC (the arrangement nearly always used) which is deemed to satisfy the requirements of 433.1.1.

It's the only one which satisfies it without deviant design which would require you to document it as an exception on an EIC and show that it was at least as safe as the one explicitly deemed to be safe.

I have to say that my interpretation is that the regulations do not pretend to be exhaustive and therefore permit any type of circuits, provided that the designer can demonstrate that the circuit satisfies the stated underlying requrements of the regs

Not necessarily a non-trivial thing to do.

However, I don't see that as in any way preventing a designer from undertaking his/her own analysis and calculations to demonstrate that a proposed circuit (ring final, or anything else) satisfies the general requirements of the regs - provided, of course, they have the knowledge and ability to do that.

Indeed not.

I'm pleased to hear that you agree with me. I was merely responding to your comment "The whole validity of a ring final rests on 433.1.5 - unless the circuit complies with it it fails in other ways as it no longer qualifies for the exemptions.". I never said that it would be a non-trivial thing to do - merely that a person with the necessary knowledge and ability could.

But why would you?

As I said, I wouldn't, and nor do I think hardly anyone would - and therefore think that EFLImpudences's suggestion is essentially academic. If you want an answer to "Why would you?", I guess it would have to come from him.

I wonder if you have any thoughts about my original question?

My thoughts are that there is little if any merit in gratuitous novelty, therefore unless you had a really pressing reason to do it the decision should be to not do it.

I totally agree that nothing should be done in the name of gratuitous novelty. What counts as a 'pressing' reason is obviously open to debate, but I can certainly envisage situations in which such a design would offer benefits, particularly practical.

More generally, I'm interested to learn about the preparedness of 'in the field' designers of circuits to use their knowledge and ability to design circuits to comply with BS7671, rather than relying on what others have designed for them.

Kind Regards, John

 

[mikhailfaradayski]

As I said, I really don't think think that 433.15 is anything more than a description of one arrangement of an RFC (the arrangement nearly always used) which is deemed to satisfy the requirements of 433.1.1.

It's the only one which satisfies it without deviant design which would require you to document it as an exception on an EIC and show that it was at least as safe as the one explicitly deemed to be safe.

I have to say that my interpretation is that the regulations do not pretend to be exhaustive and therefore permit any type of circuits, provided that the designer can demonstrate that the circuit satisfies the stated underlying requrements of the regs

Not necessarily a non-trivial thing to do.

However, I don't see that as in any way preventing a designer from undertaking his/her own analysis and calculations to demonstrate that a proposed circuit (ring final, or anything else) satisfies the general requirements of the regs - provided, of course, they have the knowledge and ability to do that.

Indeed not.

I'm pleased to hear that you agree with me. I was merely responding to your comment "The whole validity of a ring final rests on 433.1.5 - unless the circuit complies with it it fails in other ways as it no longer qualifies for the exemptions.". I never said that it would be a non-trivial thing to do - merely that a person with the necessary knowledge and ability could.

But why would you?

As I said, I wouldn't, and nor do I think hardly anyone would - and therefore think that EFLImpudences's suggestion is essentially academic. If you want an answer to "Why would you?", I guess it would have to come from him.

I wonder if you have any thoughts about my original question?

My thoughts are that there is little if any merit in gratuitous novelty, therefore unless you had a really pressing reason to do it the decision should be to not do it.

I totally agree that nothing should be done in the name of gratuitous novelty. What counts as a 'pressing' reason is obviously open to debate, but I can certainly envisage situations in which such a design would offer benefits, particularly practical.

would you care to expand on that?

More generally, I'm interested to learn about the preparedness of 'in the field' designers of circuits to use their knowledge and ability to design circuits to comply with BS7671, rather than relying on what others have designed for them.

Kind Regards, John

 

[EFLImpudence]

As I said, I wouldn't, and nor do I think hardly anyone would - and therefore think that EFLImpudences's suggestion is essentially academic. If you want an answer to "Why would you?", I guess it would have to come from him.

Firstly, assuming all relevant regulations are met.

I'm not sure to which of my suggestions you are referring but if it is to my agreement with John's original suggestion of a 4mm² spur on a 2.5mm² ring, I cannot see anything wrong with it as it saves the need for a FCU and it enables each socket to be used to ITS full rating rather than two or three (or however many) being limited to a total of 13A.

Also, if this spur should be, more or less, in a line from the ring. Rather than incorporating it into the ring, from one socket, with a cable joint and a second leg running back in the same channel as the first, a single run in 4mm² seems eminently preferable. I said I probably wouldn't do it because of other people's possibly unfounded views but that doesn't mean it is wrong.

As to the question "Why would you?" See above.

Another suggestion was that a 1.5mm² cable from the ring to the FCU was a bodge. If it meets all requirements, how can it be a bodge?
433.1.5 refers to a RFC with or without 'unfused' spurs so I would suggest
that if the spur were a fused one then the requirement for 2.5mm² cable need not apply to the spur. Otherwise, when would 433.2.2 apply?

I was contradicted recently for suggesting the opposite i.e. using larger cables than necessary.

The main intention of my original post was to point out that the contents of Appendix 15 is not all there is.

 

[ban-all-sheds]

Another suggestion was that a 1.5mm² cable from the ring to the FCU was a bodge. If it meets all requirements, how can it be a bodge?

It does not meet the requirements for a 433.1.5 ring final.

433.1.5 refers to a RFC with or without 'unfused' spurs so I would suggest
that if the spur were a fused one then the requirement for 2.5mm² cable need not apply to the spur.

On the supply side of the FCU it would, because the circuit has to be wired in a minimum of 2.5mm².

Otherwise, when would 433.2.2 apply?

With radials.

 

[JohnW2]

would you care to expand on that?

Expand on what? - you quoted my entire message!

Kind Regards, John.