... 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