I have no idea how sensitive they are, but would doubt that they (at least, of the type that an electrician might have) are necessarily sensitive enough to detect very small differences in temperature,I thought thermal imagining cameras were designed to be sensitive, so you can see for example where the heat is leaving your house?
Yes, but you're now talking about something very different, and essentially irrelevant to the question being discussed. If, by virtue of the current flowing through them, all of the conductors entering the JB were at 70°C, then, if there were not a lot of heat losses, then all of the terminals in the JB would eventually have a temperature of 70°C. However, that's true whether there are 3, 4 or even 40 such cables (all at 70°C) going into the JB - so can't be the reason why a 4-'terminal' J803 is said to have a much lower maximum current capability that a 3-terminal' J804. Only if substantial amounts of heat were being generated at/by the terminals (which just shouldn't happen) would the number of terminals be relevant to the 'current rating' of the JB.Also there must be heat generated under high load in the cables let alone the connections as copper has a good resistance and we only limit it to 70c in PVC. If it didn't heat up how would screw terminals on cooker circuits loosen themselves.
Kind Regards, John