Ashley J803/J804 MF JBs

[JohnW2]

In a recent thread about the Ashley J803 (3 'terminal') JB, I wrote:

There is a corresponding one [J804] with 4 sets of terminals, designed primarily for lighting circuits (lower current rating, if I recall, although I don't understand why - terminals look identical!).

This prompted me to ask Hager/Ashley Tech Support why the J804 was only rated at 20A, whilst the J803 is rated at 32A, given that the terminals looked identical. The (very prompt) reply I got was:

Although the terminals are the same, with 4 core cabling within them,this affects the heat rise & temperature rating of the joint box enclosure.

Any thoughts/comments? Does this sound like a credible explanation for the difference between a 32A and 20A rating? I have to admit that it's roughly right in relation to the ratio between 8 (4*2) and 12 (4*3) current-carrying conductors, but would one really expect a significant temperature rise within the enclosure, given that there would only be a couple of inches of each conductor within the enclosure? Another consideration (which they clearly are not taking into account) is that, in practice (although I would admit not guaranteed), it's in the nature of most uses of '4-core' (3C+E) cables that only two of the conductors will be current-carrying at any point in time.

The question is, of course, fairly moot becuse I don't think one can get 3C+E with a greater CSA than 1.5mm² - so one would never want a rating >20A when using 3C+E. However, I can think of a few (albeit unusual) situations in which one might want a '4-terminal' JB for T+E - in which cases, Hager's argument would obviously become 'inapplicable'.

Kind Regards, John

 

[seasickstevie]

. . . it's in the nature of most uses of '4-core' (3C+E) cables that only two of the conductors will be current-carrying at any point in time.

Why do you say that?
The neutral will carry current, the loop-in line will carry current, and the switch wire will also carry current.
In most cases the switch wire is likely to carry the least, but could be relatively high feeding a large chandelier, bank of downlights or whatever.

I agree though, in most cases, if well terminated you wouldn't expect a build up of heat.

 

[JohnW2]

. . . it's in the nature of most uses of '4-core' (3C+E) cables that only two of the conductors will be current-carrying at any point in time.

Why do you say that? The neutral will carry current, the loop-in line will carry current, and the switch wire will also carry current.

I'm really not sure what you're talking about here - in what common situation would you expect to have neutral, 'loopin line' and 'switch wire' as the three live conductors in a 3C+E cable? Cables to things like PIR detectors, thermostats and other control devices are the only that come immediately to mind - and even in those cases, the neutral current is generally negligable (just used to power the detector/device itself, not to carry any 'load' current). Am I missing something? Surely by far the most common use for 3C+E cable is for 2-way light switching - in which case, as I said, there would never be more than two conductors carrying current.

I agree though, in most cases, if well terminated you wouldn't expect a build up of heat.

With proper terminations, the actual connections/terminals obviously should not generate significant heat at all. I presume they are thinking of the heat generated by the couple of inches of each conductor within the enclosure - which, as I said, I would also expect to be minimal.

Kind Regards, John

 

[Iggifer]

I'm really not sure what you're talking about here - in what common situation would you expect to have neutral, 'loopin line' and 'switch wire' as the three live conductors in a 3C+E cable?

I have L,N & SL in my 3C+E as my switch then feeds a 5amp socket for a desk lamp, would have ideally gone for 2 x T+E but couldn't get it in the chase without rechasing

 

[seasickstevie]

. . . three live conductors in a 3C+E cable?

I understand what you mean regarding 3C+E cable. I misunderstood the assumed use of the JB in this case. I was thinking more with the typical lighting situation using a JB to loop-in where the loop-in is carrying the rest of the circuit (both L & N), while the S/L is also carrying the more local light/s.
But it's obvious that as you're talking 20A/32A JBs, then lighting is not really relevant.

I think Ashley are just 'playing (extremely) safe'

 

[JohnW2]

I'm really not sure what you're talking about here - in what common situation would you expect to have neutral, 'loopin line' and 'switch wire' as the three live conductors in a 3C+E cable?

I have L,N & SL in my 3C+E as my switch then feeds a 5amp socket for a desk lamp, would have ideally gone for 2 x T+E but couldn't get it in the chase without rechasing

Sure, I dare say that most of us have used 3C+E in such ways occasionally (and also using the third conductor as a, very low current, control or sensor line), but I presume you would agree that, in terms of the big picture, it's an extremely rare way to find 3C+E being used?

Kind Regards, John

 

[Iggifer]

I'm really not sure what you're talking about here - in what common situation would you expect to have neutral, 'loopin line' and 'switch wire' as the three live conductors in a 3C+E cable?

I have L,N & SL in my 3C+E as my switch then feeds a 5amp socket for a desk lamp, would have ideally gone for 2 x T+E but couldn't get it in the chase without rechasing

Sure, I dare say that most of us have used 3C+E in such ways occasionally (and also using the third conductor as a, very low current, control or sensor line), but I presume you would agree that, in terms of the big picture, it's an extremely rare way to find 3C+E being used?

Kind Regards, John

Not neccessarily 'extremely rare', but I agree that obviously the most common use is 2-way switching

 

[JohnW2]

. . . three live conductors in a 3C+E cable?

I understand what you mean regarding 3C+E cable. I misunderstood the assumed use of the JB in this case. I was thinking more with the typical lighting situation using a JB to loop-in where the loop-in is carrying the rest of the circuit (both L & N), while the S/L is also carrying the more local light/s.

Fair enough - but, as you probably now realise, what Ashley appeared to be concerned about was the potential number of current-carrying conductors in the cabes (i.e. 4*3=12 for 3C+E, as compared with 4*2=8 for T+E).

I think Ashley are just 'playing (extremely) safe'

Yes, I presume that's the case, and I personally would 'make my own judgement'. The people who might be a little annoyed are those who are, or who feel, constrained (by employer, scheme operator, insurer or even just 'professionalism') to work to the letter of BS7671 as regards compliance with MIs and 'ratings'. Because, presumably in the name of extreme caution, they have 'rated' the 4-terminal J804 at 20A, it means that someone who wanted to use one to connect just two or three cables which might carry more than 20A would not be 'allowed to', even though Ashely's argument collapses when there are less than 4 cables.

Kind Regards, John

 

[JohnW2]

Not neccessarily 'extremely rare', but I agree that obviously the most common use is 2-way switching

Fair enough, but that's really just a question of words.

Kind Regards, John