I know the NEC requires all current carrying conductors of the same circuit to be included in the same cable or conduit [300.3(B)], but why? I think it is because the magnetic field generated by current within a single conductor would cause inductive heating in nearby metals, and if the other circuit conductors were included their magnetic fields would cancel each other out. But I don't understand how that works.
I watched several YouTube videos showing the magnetic fields around magnets and single conductors, but no one really explained how two sides of the same circuit, with one current traveling in one direction and the other equal current returning in the other direction, cancel each other's magnetic field. In one video, two magnets faced one another with their north poles forced together; it effected a reduction in the magnetic field. If one magnet was flipped, they attracted and this increased the magnetic field.
So, in a cable with the directions of current opposing one another, it seems to me this would also cause these to be opposite forces, thus attracting one another and making the magnetic field stronger. I guess I just don't understand the rules very well. Can someone explain?