Shower cable rating

[JohnW2]

Wow this has generated quite the debate, thanks everyone for your comments. .... Are the regs applicable relevant to the circumstances of the usage of the circuits? For example, as noted above, the shower is only ever on for 5-10 mins at a time. The ring it runs alongside in the ~2m conduit run is supplying some sockets in the conservatory, which are almost never used.

Well, per what I have most recently written, the regs (or, rather, 'guidelines to regs) seem to be as clear as mud to me.

However, in the spirit of what I have also recently written, I think the 'electrical common sense' answer to your question is fairly clear (as you seem to realise) - although you probably can't expect anyone here to advise you to "ignore the regulations" (even if we can't really work out {or agree about}what the regulations actually 'require')!

Kind Regards, John.

 

[SUNRAY]

Wow this has generated quite the debate, thanks everyone for your comments.

Are the regs applicable relevant to the circumstances of the usage of the circuits? For example, as noted above, the shower is only ever on for 5-10 mins at a time. The ring it runs alongside in the ~2m conduit run is supplying some sockets in the conservatory, which are almost never used.

You put forward a very reasoned question and in many ways I sympathise.
Is it likely to be a problem? - probably not.
Will it catch fire? - probably not.
Do you need to panic? - from your comments probaly not.
Is it correct? - Not in my interpretation of the regs. Other interpet the regs in a different way (as is apparent from this thread).


Sometimes I do make decisions to install using incorrect methods as I do a fair amount of temporary power, however I'm experienced and able to keep total control of the installation to maintain its safe operation. Would I make a similar decision for a permanent installation in a customers property? No.

 

[JohnW2]

Is it likely to be a problem? - probably not.
Will it catch fire? - probably not.
Do you need to panic? - from your comments probaly not.

You know that we differ 'quantitatively' in our view of risks, which is fine. I certainly don't disagree with any of what you say above, but if I were writing it I would probably change your three "probably not"s to three "almost certainly nots"s **.

Is it correct? - Not in my interpretation of the regs. Other interpret the regs in a different way (as is apparent from this thread).

As you say, the poor clarity of the regs is such that there cannot be universal agreement as regards their interpretation - but that obviously doesn't mean that any one of the differing interpretations is necessarily 'what was intended'.

** As you have said, without any de-rating 6mm² cable in conduit has a tabulated current rating of 38A. That means that, by implication, the regs deem it to be able to carry at least 55.1A (38A x 1.45) for 1 hour without coming to harm, and appreciably higher currents that for shorter periods without coming to harm. So, even if you believed (which you have said you do) that you need to apply de-rating ('grouping') factor of 0.7 then, even per the figures implied by BS7671 itself, the cable would come to no harm during a shower of any credible duration.

However, in the present context, what you don't seem to have grasped is that one of the major problems with Table 4C1 is (as is effectively admitted in "Note1" to that Table) that the factors in it are only 'applicable' (or even remotely sensible) if all of the 'grouped' cables are similarly loaded (which will usually also mean 'of similar CSA'). For example, you surely don't believe that the CCC of, say 10mm² (or even 16mm²!) cable would have to be de-rated by 30% if it were 'grouped' with two 1.0mm² cables serving lighting circuits, do you?

Edit: ... and I forgot add. As for your belief that the two cables of a ring final, if 'grouped', should count as "2 cables", if the circuit is not overloaded then the total current in the two legs of a 32A ring will not exceed 32A - so no different total current (or total heating) than a single cable serving a 32A radial (which I would hope you would only count as "one cable" ).

Kind Regards, John

 

[SUNRAY]

If that were to be 2x 16A radials would consider it to be one or two cables.
I imagine you say two, what makes those two cables different to two cables on a 32A ring final?

 

[JohnW2]

If that were to be 2x 16A radials would consider it to be one or two cables.....I imagine you say two, what makes those two cables different to two cables on a 32A ring final?

Quite - and if there were cables relating to 5 x 6A circuits, we would presumably both say that there were "5 cables" (or, to get silly, if there were 32 cables each protected by a 1A MCB {they do exist },we would presumably say that there were "32 cables"!).

Do you not think that this illustrates that Table 4C1 greatly over-simplifies a potentially complicated situation and, as such makes little, if any, electrical common sense in many real-world situations - particularly if the 'grouped' cables vary considerably in CSA and/or the currents they are carrying?

Kind Regards, John

 

[SUNRAY]

Quite - and if there were cables relating to 5 x 6A circuits, we would presumably both say that there were "5 cables" (or, to get silly, if there were 32 cables each protected by a 1A MCB {they do exist },we would presumably say that there were "32 cables"!).

Do you not think that this illustrates that Table 4C1 greatly over-simplifies a potentially complicated situation and, as such makes little, if any, electrical common sense in many real-world situations - particularly if the 'grouped' cables vary considerably in CSA and/or the currents they are carrying?

Kind Regards, John

I really struggle with such questions. A cable is surely a cable and if bunched requires derating. I for one don't find that to be a difficult concept. I don't see any relevance how many cables are on a circuit etc.
AFAIC the cable has a CCC and the OCPD will be below that figure.

 

[JohnW2]

I really struggle with such questions. A cable is surely a cable and if bunched requires derating. I for one don't find that to be a difficult concept. I don't see any relevance how many cables are on a circuit etc.

As I've said, Table 4C1 over-simplifies a complicated situation, such that, in the real world, merely 'counting cables' can lead to conclusions that do not correspond with electrical common sense. At the very least, one has to consider what the cables are "doing"

Consider what you may regard as an 'extreme'; situation - a 10mm² cable in conduit (CCC=52A) together with three other cables(of whatever size), each of which is a switch drop from an LED light - so perhaps a maximum of about 0.1A for each of those three cables. You will count "4 cables" and thereby presumably decide (per 4C1) that each cable has to be subjected to a 'grouping factor' of 0.65 - correct?

If so, do you think that it is (electrically) sensible to 'de-rate' the 10mm² cable from a CCC of 52A to 33.8A (a reduction of over 18A) because of the (maximum of) 0.3A that may be flowing in the other cables?

In my opinion, it's one of those situations in which, given that regulations cannot be 'fully comprehensive' (e.g. have no explicit provision for the situation I have just described), there needs to be an option for flexibility, based on discretion/judgement, in individual real-world situations. You will undoubtedly disagree

Kind Regards, John

 

[SUNRAY]

As I've said, Table 4C1 over-simplifies a complicated situation, such that, in the real world, merely 'counting cables' can lead to conclusions that do not correspond with electrical common sense. At the very least, one has to consider what the cables are "doing"

Consider what you may regard as an 'extreme'; situation - a 10mm² cable in conduit (CCC=52A) together with three other cables(of whatever size), each of which is a switch drop from an LED light - so perhaps a maximum of about 0.1A for each of those three cables. You will count "4 cables" and thereby presumably decide (per 4C1) that each cable has to be subjected to a 'grouping factor' of 0.65 - correct?

If so, do you think that it is (electrically) sensible to 'de-rate' the 10mm² cable from a CCC of 52A to 33.8A (a reduction of over 18A) because of the (maximum of) 0.3A that may be flowing in the other cables?

In my opinion, it's one of those situations in which, given that regulations cannot be 'fully comprehensive' (e.g. have no explicit provision for the situation I have just described), there needs to be an option for flexibility, based on discretion/judgement, in individual real-world situations. You will undoubtedly disagree

Kind Regards, John

So please explain what alternative calculations you advocate or practice to arrive at a safer set up

 

[EFLImpudence]

To be fair, 4C1 Note 9 does state that cables loaded to <30% of their grouped ratings it may be ignored.



As we are discussing conductors in conduit, what if the wiring were in singles?

Taking the OP's example of one ring and one radial in the conduit, then presumably, as there are no multicore cables, for 4C1 one would have to decide that there were indeed only two circuits.

However, what difference would this make to the 0.7 or 0.8 derating apart from the CCCs would be slightly different to begin with?

 

[EFLImpudence]

Also, if there are two apples and one orange, what is the answer to the question:

how many apples OR oranges are there?

 

[JohnW2]

So please explain what alternative calculations you advocate or practice to arrive at a safer set up

In relation to the hypothetical example I desribed, if I were able to exercise it, my judgement would probably be that no 'calculation' was necessary (i.e. I would "ignore" the 'grouping') - since I do not believe that the presence of other cables which, between them, might carry a total of 0.3A would have any noticeable effect on the CCC of a cable which otherwise would have a CCC of 52A.

In fact, in comparison with the situation in which the 10mm² cable were 'floating about on its own' within the conduit, the presence of three other cables (carrying negligible current) might well improve the thermal contact between the 10mm² cable and the conduit, thereby possibly slightly increasing its theoretical CCC.

Others (other than just yourself!) may disagree with me!

Kind Regards, John

 

[Colin Brenton]

To answer the question, I'd personally want to get either an RCD or RCBO in place to feed the shower if it were mine tbh.

I'll leave cable derating discussions to the other lads

 

[JohnW2]

If that were to be 2x 16A radials would consider it to be one or two cables. ... I imagine you say two, what makes those two cables different to two cables on a 32A ring final?

Does this mean that you do not believe that it is ever compliant to have a single 2.5mm² cable of a ring final in conduit if there are any other cables in the conduit?

Kind Regards, John

 

[JohnW2]

To be fair, 4C1 Note 9 does state that cables loaded to <30% of their grouped ratings it may be ignored.

True. In my desire to find an 'extreme' example to cite, I overlooked that.

However, I think the whole idea of basing everything on a 'count of cables' is very suspect. 4C1 does not seem to say anything about "how 'multi' " a multicore cable can be, so if (ignoring CPCs) one used an N-core cable to carry what would otherwise be N/2 separate cables, one would have magically reduced the cable count from N/2 to one - but I don't see any electrically logical reason why one should then be able to apply a much less onerous 'de-rating factor'!

Kind Regards, John

 

[JohnW2]

To answer the question, I'd personally want to get either an RCD or RCBO in place to feed the shower if it were mine tbh.

That's certainly the 'standard advice' which any of us would probably give, and seems "intuitively obvious" as a good (maybe even 'essential') thing if one does not think too deeply.

However, in reality, it's very difficult to think of any situation in which an electric shower could present a hazard (against which an RCD might provide some protection) unless its case were smashed to pieces (in which case one would image/hope that no-one would touch it!).

Kind Regards, John