I was taught to always count the neutral as current-carrying ... shared or not.
George, you also bring up an interesting point ... I was given to understand that current flowing through neighboring wires created an impedance in a conductor, hence the de-rating. Less resistance countering the higher impedance, as it were.
If this is so, does it matter what circuit the current is from? Does it matter to our conductor if the wire next to it is from the same circuit? The same phase? Is it an ideal situation when the two wires are of differing phases - or one a neutral, with the current flowing in the opposite direction?
I always imagined this issue - de-rating - applied to pipe carrying multiple circuits. This thread has me thinking ... what about switch legs? Imagine your typical office, with four lighting "zones," and three-ways to boot. You have one 'power leg' shared between the four switches, and 8 travelers. That's 9 wires in a single pipe. De-rate? I never thought about that situation.
What say you all?
