inaccessible Junction Boxes

[EFLImpudence]

Unless I'm missing something - limiting the current of the junction box does not limit the temperature of the conductors and so does not limit the temperature of the junction box.

The 804 could have eight 20A conductors at 70°; the 803 can only have six 32A conductors at 70°.

 

[JohnW2]

Unless I'm missing something - limiting the current of the junction box does not limit the temperature of the conductors and so does not limit the temperature of the junction box. The 804 could have eight 20A conductors at 70°; the 803 can only have six 32A conductors at 70°.

You could have a thousand conductors all at 70° and the temp within the box would not rise above a maximum of 70° - the issue is in limiting the conductor temperatures to 70°, not the number of conductors at that temperature.

The point is that the current required to raise the temperature of any of the conductors to 70° will depend upon the ambient temp within the box, which will rise if there are more conductors carry substantial currents within the box.

As I said, it's no different from 'grouping of cables' (without the enclosure) or, indeed, restricting heat loss from a cable by surrounding it with thermal insulation. In either case, the maximum current that can flow without conductor temp rising to above 70° (i.e. the CCC, after taking de-rating factors into account) is less than it would be if there were no grouping or thermal insulation.

Kind Regards, John

 

[EFLImpudence]

You could have a thousand conductors all at 70° and the temp within the box would not rise above a maximum of 70°

If so, restricting the current of the box has no effect, then.
A thousand 20A conductors can have the same temperature as a thousand 32A conductors.

- the issue is in limiting the conductor temperatures to 70°, not the number of conductors at that temperature.

But that is not achieved by limiting the current of the box which does not limit the temperature of the conductors.

The point is that the current required to raise the temperature of any of the conductors to 70° will depend upon the ambient temp within the box,

That will apply equally to 20A conductors carrying 20A as 32A conductors carrying 32A.

which will rise if there are more conductors carry substantial currents within the box.

Does that not contradict the first quote?

As I said, it's no different from 'grouping of cables' (without the enclosure) or, indeed, restricting heat loss from a cable by surrounding it with thermal insulation. In either case, the maximum current that can flow without conductor temp rising to above 70° (i.e. the CCC, after taking de-rating factors into account) is less than it would be if there were no grouping or thermal insulation.

But the lower current box allows more fully loaded conductors than the higher current box.

 

[JohnW2]

If so, restricting the current of the box has no effect, then. A thousand 20A conductors can have the same temperature as a thousand 32A conductors.

Maybe your difficulty arises because of your use of these terms "20A conductor" and "32A conductor". What do you actually mean by those terms?

If you had a cable which, alone, had a 'tabulated' CCC of 20A, we take that (rightly or wrongly) to mean that it will attain a conductor temp of 70° when 20A is flowing (when there are no 'de-rating factors'). If several such cables each carrying the same current were 'grouped' together, particularly if within an enclosure, then you would have to reduce the current through each of them to below 20A in order to restrict conductor temp to 70°. So, the effect of 'restricting current' is to restrict the temp to the permitted maximum.

Kind Regards, John

 

[EFLImpudence]

Maybe your difficulty arises because of your use of these terms "20A conductor" and "32A conductor". What do you actually mean by those terms?

I mean it in the same way as Hager?Ashley do.
Their boxes allow (apparently) six cables carrying 32A or eight cables carrying 20A.
Is that what the box ratings mean?
Bear in mind the boxes are exactly the same but for the extra connector block.

If you had a cable which, alone, had a 'tabulated' CCC of 20A, we take that (rightly or wrongly) to mean that it will attain a conductor temp of 70° when 20A is flowing (when there are no 'de-rating factors').

Yes.

If several such cables each carrying the same current were 'grouped' together, particularly if within an enclosure, then you would have to reduce the current through each of them to below 20A in order to restrict conductor temp to 70°.

OK. Does the 20A box (803) do that?
Are you assuming that the same csa cable is being used in both box ratings? whereas, in fact, a greater number of smaller cables may be used in the 20A one which could achieve the same temperature.

So, the effect of 'restricting current' is to restrict the temp to the permitted maximum.

That depends on the csa of the cables.

Six 4mm² cables (CCC 37A) in the 32A box carrying 32A each, or
Eight 1.5mm² cables (CCC 20A) in the 20A box carrying 20A each.

If you agree that the 20A box will be hotter, what is the point of the rating differenc?[/QUOTE][/quote]

 

[JohnW2]

deleted - finger hit wrong button!

 

[John D v2.0]

I don't see that the number of cables has an effect, it's the overall length of cable. Admittedly there's a correlation, but if I loop the cable round and round inside the box it'll get hotter than straight in and out.

 

[EFLImpudence]

I don't see that the number of cables has an effect,

OK.

it's the overall length of cable. Admittedly there's a correlation, but if I loop the cable round and round inside the box it'll get hotter than straight in and out.

Well exactly, but the rating of the box is a flat amperage rating.

In other words, as long as your cable is carrying a maximum of 20A or 32A (depending on which box) then nothing else is cosidered.

 

[JohnW2]

I mean it in the same way as Hager?Ashley do. Their boxes allow (apparently) six cables carrying 32A or eight cables carrying 20A. Is that what the box ratings mean?

I'm a little confused by your numbers. Are you talking about conductors or 'cables' (e.g. T+E)? Assuming one uses them as intended/labelled, one block of connections is for earth (hence no current). A J803 then has provision for four N conductors and four L conductors - that's 8 'live' conductors or 4 T+E cables. A J804 has provision for 4 N conductors, four 'L' conductors and four 'loop' conductors - that's 12 'live' conductors or, say, 4 3C+E cables.

This then gets quite a bit more complicated, and I don't think it makes sense to 'add up' all the currents in all the conductors/cables in the way you seem to be doing....

For a start, if you consider, say, a case in which there are just two T+E cables being joined in the JB (one 'in' and the other 'out') then, from the point of view of things like conductor temperature, I don't think that it makes sense to think of that as 'two cables' or ' live conductors' (each carrying, say, 32A), since it is effectively one cable (or two live conductors) (carrying, say, 32A) passing through the box which happen to have a join in the middle.

When one moves to a 'full house' of cables/conductors, if one assumes that the 32A 'rating' of a J803 means that not more than 32A may pass through any connector then, assuming that one does not have 'feeds in parallel' (which would be very unusual), I think the 'worst case' would be when there was a 32A 'in' ('feed') cable (2 live conductors) carrying 32A, with the other 3 ('out') cables (6 live conductors) having a total of 32A shared between them. I certainly can't think of any way in which one could get anywhere near the scenario of 32A flowing in all live conductors - and, furthermore, as with the '2 cable scenario' above, at least one 'pair of cables' (pair of pairs conductors) could probably be considered as a single cable (pair of live conductors) 'passing through' the box which happened to have a join in the middle.

With a J804 it gets a fair bit more complicated, because it depends upon how it is being used, so I would need to think about that!

Kind Regards, John

 

[JohnW2]

I don't see that the number of cables has an effect, it's the overall length of cable. Admittedly there's a correlation, but if I loop the cable round and round inside the box it'll get hotter than straight in and out.

In general, the conductor temperature achieved will obviously not depend upon the length of the cable - with a given current flowing, the conductor temperature should be roughly the same throughout the length of the conductor, regardless of the length of the conductor. If that were not the case, CCC would be length-dependent, which would be a major headache!

I agree, however, that if the cable is contained within a confined space (e.g. 'an enclosure), then the ambient temp within that enclosure will rise, by an amount dependent upon the length of cable within the enclosure, which in turn will cause conductor temp to rise.

Kind Regards, John

 

[EFLImpudence]

JohnW2

Ok. I may have got carried away with the exaggerated number of conductors but my fundamental point remains.

E.g. in an 803, a 4mm² cable (t&e) merely joined with a load of 32A will not get as hot as a 1.5mm² cable with a 20A load.
Both will presumably be fine.



You have said you will have to think about the use of the 804, so for what do you think it may be used and to what does the amp rating refer?

If it is heating of the connectors themselves, then, as you have said, it may not be fit for purpose.
If it is the general heating of the box with three live conductors (and CPC) then my point about the heating of these conductors because of their csa and ccc is valid.

 

[EFLImpudence]

Another point.

You mentioned it was similar to grouping of cables causing a derating of their ccc.

Another way to overcome this is to increase the csa of the cable.
804s do not afford this option and merely state a 20A limit - even if the 20A were on 4mm² conductors.

 

[ban-all-sheds]

You're talking about 'certification' again, and I'd like to hear more about this from someone who actually understands. As I said before, it's surely possible for a product to conform to relevant Standards without any 'formalities', so I wonder what 'certification' you have in mind.

The question is can a manufacturer self-certify, or does it have to be done by an authorised testing house?

 

[ban-all-sheds]

Indeed. For what it's worth, the sort of process you describe (presumably in the belief that it would not be acceptable) is, to the best of my knowledge, essentially the way that 'CE marking' works.
Thanks, but are not the answers 'inevitable'? ... as discussed, I don't think that an enclosure alone can conform with BS 5733 or bear a 'MF' mark, because of it's nature - so, if the answers to (1) and (2) are 'no', the (3) also has to be 'no', since BS7671 requires an 'MF' mark.
As above, if what I've said about (1) and (2) above is correct then it will always, by definition, be impossible for an enclosure alone to conform with BS 5733 or bear an 'MF' mark - so the issue probably can never be 'resolved' by the manufacturers of enclosures. It might have to be for BS7671 to revise its requirements.

Kind Regards, John

I have had a reply.

1. Yes. Wagobox with Wago 773 and 222 connectors complies with the requirements of BS 5733 where they are applicable…
   You need to bear in mind that BS 5733 isn’t a product standard. BS 5733 is more like a generic set of rules for products that aren’t covered by their own standard. The standard for Junction boxes is BS EN 60670 and the Wagobox with Wago 773 and 222 connectors is fully compliant with this standard up to and including the performance standard BS EN 60670-22.
   
   
2. Yes. The MF mark is on the underside of the Lid. BS 5733 requires the MF mark to be visible during installation.
   The MF mark was introduced in BS 5733. For an accessory to have the MF mark the connectors and enclosure together must pass a number of tests specified in BS 5733.
   We have successfully verified the Wagobox with Wago 773 and 222 connectors pass those tests.
   
   
3. Yes. Providing the Wagobox is used and assembled in accordance with our MF instructions
   Note: The Wagobox is a component part of a system for assembling a maintenance free accessory see:
   BS 7671 Part 2 Definitions: accessory
   BS 5733 Terms and Definitions: 3.31 Maintenance Free Accessory

 

[JohnW2]

... but my fundamental point remains. E.g. in an 803, a 4mm² cable (t&e) merely joined with a load of 32A will not get as hot as a 1.5mm² cable with a 20A load. Both will presumably be fine.

Sure, but that's entirely because the 1.5mm² cable is running at 100% of its CCC (Table 4D5, not 4D2A!), but the 4mm² cable only at about 86% of its CCC. If you could find a cable whose CCC was exactly 32A, it would presumably reach the same temp as the 1.5mm² one (assumed 70 in both cases). However, I'm not sure of your point here.

You have said you will have to think about the use of the 804, so for what do you think it may be used ...

It could clearly be used for lots of things, most commonly applications which needed a S/L as well as L (e.g. lighting etc.), in which case at least one of the cables would usually be to/from a switch. In that situation, it would effectively be used in the same way as a rose, with cables for L/N-in, L/N-out, switch and load. Used like that, the currents in the latter two cables would usually be very small, whilst what flowed in the first two would depend entirely upon what loads were downstream (but, if a lighting circuit, probably very modest loads/currents.

... and to what does the amp rating refer? If it is heating of the connectors themselves, then, as you have said, it may not be fit for purpose.

You tell me. We've often had similar discussions about the 'ratings' of terminals in various accessories, and I don't really understand how they are arrived at. As you say, I previously commented that if the connectors had such high a contact resistance that they got significantly hot (at rated current), then they would probably not be fit for purpose. However, I suppose it's possible that the manufacturers believe that, whilst OK up to 'rated' current, they might get hot if one exceeded the current rating they specify. However, as you have observed, that could not explain the difference in ratings between J803s and J804s, assuming that we are right in assuming that the connectors in both are identical (they certainly look identical).

If it is the general heating of the box with three live conductors (and CPC) then my point about the heating of these conductors because of their csa and ccc is valid.

Maybe. I'm having difficulty in focussing my mind on this issue, since I struggle to think of practical situations in which one would want a '4-terminal' JB other than on a lighting or similar circuit, in which all currents would usually be low, much lower than even 20A. I just wonder whether the manufacturer's might have thought similarly, and specified an arbitrary 20A rating since they did not envisage anyone wanting to use it for higher currents than that - about the only advantage to them being that they would then only have to test it to 20A or thereabouts.

I must say that I do find it hard to see how the heat generated by an inch or three of each of two, three or four cables is going to make any difference to anything, even if one could contrive for them all to be simultaneously carrying 100% of their CCC. That again makes me wonder whether there actually is any particularly logical reason for the difference in current ratings of the J803 and J804 (assuming the connectors really are identical).

Kind Regards, John