I need to find out the science behind the reason we cut a channel in a metal J-box when we pass wires through it. An example would be wires from a generator to cam-locks in a wiring box.
If your example means that a single conductor is passing through a single hole in a ferrous metal enclosure, then 300.20(B) is the NEC passage that is invoked.
The alternating current in the conductor creates an expanding and contracting magnetic field around the conductor. As the current increases, the magnetic field strengthens and expands, and, as the current decreases, the magnetic field weakens and collapses.
Ferrous metal is, generally, a conductor. It's not as good a conductor as copper or aluminum because the resistance in ferrous metal is higher. Given conductors of iron and copper, that are the same gage, the same amount of current in each conductor will cause more heat in the iron conductor.
Now, the ferrous metal around the hole that the single conductor passes through, experiences the "generator effect". The ferrous metal is in a moving magnetic field. And, being a conductor, the ferrous metal has current induced in it. When one works out the
Right Hand Rule, one finds that the
Eddy Current, that is, the induced current, travels in the ferrous metal around the hole.
Now, cutting a slot in the ferrous metal between two holes turns them into a single "bar bell shaped" hole. The Eddy Current is forced to go around the bar bell shaped hole.
When the conductors of a circuit are together in a single slot-linked hole, the Eddy Current created by each conductor, when added to the other conductor's Eddy Currents, cancel out.
None of my kids were around to do it for me!!!:grin: