Boxing in of cables

[JohnW2]

I would have expected this to be a very common question, but a search of the forum archives reveals very little, whilst (again rather surprisingly) the regs also seem pretty silent on this question (as always, unless I'm missing something) ....

Clipped direct cables (e.g. in the vicinity of a CU, cable runs between different levels of a property etc.) in visible places are never pretty (particularly if one spaces cables to avoid grouping issues), and many people therefore want to (and do) 'box them in'. If the boxing is very generous in size (and, ideally, ventilated) I presume that it makes no material difference to current carrying capacity, so that Reference Method C continues to apply. However, the smaller the boxing gets, the closer it gets to being 'trunking' - hence Reference Method B, with current-carrying capacities around 15% lower than with Method C.

Am I missing something in the regs which gives some guidance as to when boxing (which can be 'ignored') becomes 'trunking' (withy resultant down-rating) - or, if the regs are indeed silent about this, are there any 'rules of thumb', or even just opinions, as to how 'small' boxing can be without cable downrating having to be considered?

Kind Regards, John.

 

[SgtTrojan]

I would say that the installation method would be considered to be B (Number 56) p.264 BS7671:2008

"Sheathed........multicore cables in an open or ventilated cable channel run horizontally or vertically." and the note goes onto say that restricted access is recommended so that the reduction in current carrying capacity, fire hazards, accumulation of debris can be prevented.

"C" aka clipped direct will ALWAYS be uncovered and ventilated on all sides apart from the surface it is clipped to.

 

[ban-all-sheds]

Don't forget 523.5 & Appendix 4 §2.3..

 

[davelx]

I would say that the installation method would be considered to be B (Number 56) p.264 BS7671:2008

"Sheathed........multicore cables in an open or ventilated cable channel run horizontally or vertically." and the note goes onto say that restricted access is recommended so that the reduction in current carrying capacity, fire hazards, accumulation of debris can be prevented.
.

Would you therefore consider cables clipped to floor joists in an uninsulated floor void, e.g. between ground and first floors, also to be method B, since that description fits very well?

 

[JohnW2]

I would say that the installation method would be considered to be B (Number 56) p.264 BS7671:2008
"Sheathed........multicore cables in an open or ventilated cable channel run horizontally or vertically." ...

I must say that I wouldn't have interpreted Installation Method 56 as applying to traditional boxing. Both in descriptions and drawing, 56 appears to refer to 'channels', which I take to to be (open or ventilated) 'recesses' created 'within' (below the surface of) walls, rather than 'boxing' which is generally 'surface mounted'. Trunking (Installation methods 8 & 9, also B) would seem much closer to the sort of boxing one generally sees - which is most commonly 'surface mounted', 'closed' and not ventilated (hence technically not Method 56).

However, the issue of scale remains, and I'm surprised that the regs don't address this. Without any reference to scale, it could be argued than an alcove (e.g. beside a chimney breast) technically qualified as an 'open channel' (hence 56), but that would be plain silly, and not, I imagine, what the authors of the regs intended!

...and the note goes onto say that restricted access is recommended so that the reduction in current carrying capacity, fire hazards, accumulation of debris can be prevented.

It does indeed - but that clearly doesn't happen in relation to domestic premises, even though boxing is very common. Maybe the saving grace is that the places where 'debris' is likely to accumulate (dare I mention 'meter cupboards'? ) are not ones where aesthetics are usually much of an issue, so most people probably wouldn't bother with boxing.

"C" aka clipped direct will ALWAYS be uncovered and ventilated on all sides apart from the surface it is clipped to.

Yes, but we're back to the issue of 'scale' again - in other words, how should we interpret 'uncovered and ventilated'? Unless the wiring is clipped to the outside wall of a building, it is always going to be contained within a finite bounded space - be that a room, a cupboard or whatever - so I guess one way to look at my question is to ask how small a cipboard has to be before it should be regarded as trunking.

In real-world terms, I wonder how often any of this really gets taken into account in domestic premises, particularly given that boxing is very often 'retrofitted' after the electrical installation has been designed and installed.

One would have to do experiments to be sure, but my intuition tells me that boxing 2 or 3 inches deep with some ventilation (maybe just top and bottom for vertical runs) would not have any appreciable effect on cable temperature - but I may be wrong.

Kind Regards, John.

 

[JohnW2]

Don't forget 523.5 & Appendix 4 §2.3..

Indeed, but that's all about grouping, which is the other issue I hinted at in my original post.

However, I have to say that I find it very hard to work out what one is meant to do about grouping in the most-common 'grouped' situation - where many cables of different sizes, some carrying low loads (maybe less than 30% of their grouped capacity) emerge from a CU. Tabulated grouping factors are not applicable in this situation and any attempt at calculation would be difficult. I actually read somewhere (can't remember where!) that grouping usually does not have to be taken into account in this specific situation, but the regs don't seem to say this. However, tight grouping of all cables emerging from a CU seems to continue to be a very common practice, almost the norm - unless I'm not seeing the exceptions!

Much more generally, I have to wonder to what extent all the regulations about current-carrying capacity are having any impact on domestic circuit design. Maybe it's just that I don't see enough, but I can't say that I've seen any installation in ordinary domestic properties (new builds or recent re-wires) that have had anything other than 2.5mm² RFCs and 1.5mm² lighting circuits, seemingly regardless of anything! Am I just not seeing the right places? Are many 'ordinary houses' actually having 4mm² RFCs and/or 2.5mm² lighting circuits?

Kind Regards, John

 

[JohnW2]

Would you therefore consider cables clipped to floor joists in an uninsulated floor void, e.g. between ground and first floors, also to be method B, since that description fits very well?

Such floor voids are 'closed' and not usually (deliberately) ventilated, so I guess would not technically qualify as Method 56 ('open or ventilated').

This is perhaps a good example of the sort of 'large volume boxing' I was talking about - which intuitiuon suggests to me is unlikely to have any appreciable impact on cable temperatures.

Kind Regards, John

 

[ban-all-sheds]

cable runs between different levels of a property etc.) in visible places are never pretty (particularly if one spaces cables to avoid grouping issues), and many people therefore want to (and do) 'box them in'.

Mine are in 2 runs of 50mm galvanised trunking which is¹ then boxed in.








¹ By that I mean, of course, "is in the process of being"

 

[JohnW2]

Mine are in 2 runs of 50mm galvanised trunking which is¹ then boxed in.

That's pretty tight 'grouping'. may I ask if this had any impact on cable sizing?

A further question about current-carrying capacity. It is very common (probably the norm) for a run of cable to have more than one 'Installation Method' along its length and I assume that one is meant to design on the basis of the 'worst' installation method that applies to a significant length of that cable. However, what counts as a 'significant length'? For example, if otherwise clipped direct and non-grouped cables spend 'an inch or four' going through some insulation, grouped through a hole in a joist or whatever, is one meant to design as if the entire length of the cable run was in that 'worse' situation? As always, I'm probably missing it, but I haven't yet found an answer to that in the regs.

¹ By that I mean, of course, "is in the process of being"

Ah, the list of 'jobs to be done/finished - I have a very long one of those, and some of the items have been on the list for more years than I care to remember [tip: if you leave things on the list for long enough, some of them cease to be necessary ] .

Kind Regards, John.

 

[holmslaw]

..

 

[JohnW2]

This is the calculation for groups of cables from page 257 of the big red bumper book of fun. View media item 30385

Indeed - but, as I said, actually applying that equation to the sort of situation we are talking about is 'difficult' (essentially impossible, on the basis of what is available from 'the book').

The equation requires knowledge of Cg, the "applicable rating factor for grouping given in Tables 4C1-4C5". Of those tables, only 4C1 and 4C4 are applicable to multicore cables above ground; 4C1 specifically relates to "uniform groups of cables, equally loaded" (Note 1), which is not the situation we normally have, whilst 4C4 only relates to Installation Methods 31-34, which are not the ones which interest us.

If you think that this equation (or, indeed, any other method of calculation) can be applied in the sort of situation we are discussing, perhaps you could explain how you would ascertain Cg for a typical mix of cables one might expect to see emerging from a CU. Thanks.

To be frank, it would be totally impractical for the regs to attempt to tabulate values of Cg for all possible permutations, arrangements and loadings of the sort of cables we are talking about, and any attempt to work it out for ourselves from first principles would get us very deeply into thermodynamic theory, and without knowledge of the values of many of the parameters we would need. I therefore wonder (a) what the regs think one should do and (b) what people actually do (I suspect I know the common answer to the latter!).

Kind Regards, John.

 

[holmslaw]

..

 

[JohnW2]

Its not an exact science there will always too many variables.....

Indeed - but a British Standard (or a set of 'regulations') really ought to address such very common practical issues (even if only to indicate what 'rules of thumb' can be applied, or what practices are 'deemed to satisfy'), rather than leaving people to work on the basis of guesses and opinions. Is that not what a 'Standard' is meant to be?

Thats why all of the methods in the grouping tables are littered with qualifying notes.

Quite, except the first of the notes to 4C1 (which you go on to mention) is the very last thing one would expect, since, far from acknowledging the great variability of real-world situations, it appears to be trying to exclude most of the practical situations we are talking about!

As a general rule I would still use the most relevant table, including 4C1 for enequally loaded cables.

Fair enough - it's probably the closest one can get, but I still don't think it's very satisfactory for a Standard to effectively make it necessary for most people to 'misuse' it! If you did use the grouping factors from 4C1, I suspect that 'standard' cable sizes could well be theoretically inadequate in relation to what goes on around many a (old, or recently installed) CU/DB.

Also, what does 4C1 mean by equally loaded?
Does it means the same size cables carrying the same current?
Does it means the different size cables carrying the same current?
Or does it mean different sizes cables but carrying the same percentage of current?

Well, it actually says "uniform groups of cables, equally loaded" and my interpretation (perhaps wrong - but, again, a Standard really should be clear about meaning!) of 'uniform groups' is that it refers to cables of equal size - in which case it would have to be talking about the first of the interpretations you mention.

But if you want to be pedantic, for cables clipped to a surface I would use 30 or 31 from table 4A2.

Hmmm - 30 and 32 relate to trays. I don't think it could be called 'pedantic' ('incorrect' would probably be nearer the mark ) to use part of a Standard which related to a situation different from the one which actually applied.

But probably the best method is too never install tightly bunched groups. As an example, modern sparks have a tendency to tightly bunch groups of cables in db's and secure the bunch with ty-wraps. It all looks very neat but is very bad practice, I always get the ty-wraps removed.

Yes, I agree - although 'neat grouping of' cables around a CU/DB is not a new practice by any means. Most of the (I guess late 70s) installation I originally inherited in my present house had beautifully neat sets of tightly side-by-side cables held in place with galvanised table ties/clips (and, in most cases, 'sheathed' with a good few coats of Dulux ).

However, I keep coming back to the question I asked of BAS. Despite all these sophisticated and well-intentioned science/engineering-based regulations concerning current-carrying capacity, have you ever installed, or seen installed, a 'normal' domestic instalation in which RFCs were wired in cable larger than 2.5mm² or lighting circuits in cable larger than 1.5mm²?

Kind Regards, John.

 

[flameport]

In real-world terms, I wonder how often any of this really gets taken into account in domestic premises, particularly given that boxing is very often 'retrofitted' after the electrical installation has been designed and installed

Simple answer - never.
This isn't usually a problem, since most circuits in a house will never be used at anything like their maximum capacity, and for many circuits the cables are substantially oversized.

Cables for electric showers can be a problem, however easily solved by chasing the cable into the wall and covering with plaster, which is also method C.

 

[ban-all-sheds]

That's pretty tight 'grouping'. may I ask if this had any impact on cable sizing?

No - there's actually quite a lot of room for cables in 5000mm²....

The only one which carries significant current is the shower cable, and that of course is of short duration - 98% of the time it's carrying no current. The rest are lighting and non-kitchen sockets. We have no immersion heater, and the only electric heating is about 1kW worth of thermostatically controlled towel rail & panel heaters in bathroom/WC.
Cooker and kitchen circuits don't run through them.

I did do this:

View media item 30385
of course

but I can't quite lay my hands on the results. But I do definitely recall that it was OK.