knightmb wrote: ↑
Sun Apr 11, 2021 1:37 pm
I should have clarified, I was referring to the wire itself when it comes to heat being generated at dangerously high temperatures for the amount of power being sent through it,
In practice, the limitations of the breaker and of the connections at the ends of the wire always control over the wire's current handling ability itself, absent elevated temperatures or bundled conductors. For example, 2017 NEC table Table 310.15(B)(16) tells us that the ampacity of 90C insulated #10 copper is 40A. That means that when 3 conductors of that size are near each other (in the same cable or raceway) they can reliably carry 40A without overheating insulation rated for 90C use.
[Going back to your 6' vs 60' example, if the portion of the wire outside the terminations is operating all under the same condition (e.g. run through fiberglass backs and wood studs), I would expect the temperature for the wire to be the same along most of its length. There will be some end effect that extends beyond the terminations, and possibly the end effect is longer than 3', so a 6' run would differ from a 60' run, but maybe once you reach 10' and up, the worst case in the middle of the run will be the same independent of length.]
But you will almost never see #10 Cu wire on a 40A breaker, because for most applications it is limited to 30A by the small conductor rule. [Motor circuits, e.g. HVAC, is one exception, but there the 40A breaker would not be providing overload protection, it's just providing short circuit and ground fault protection.] And even when it's not it would be extremely rare to be able to use #10 at its full 90C ampacity, as terminations are generally limited to 75C at best, for which #10 Cu has an ampacity of 35A.