I will agree, sort of, almost, for most practical purposes anyway. But to be precise, the magnetic field is never completely ?cancelled,? not even in a perfectly balanced three phase run, nor in a perfectly balanced single phase run from a single phase transformer.
We generally treat such fields (i.e., as viewed outside the conduit) as being non-existent, although "negligible" might be a better term. Let?s consider your single phase run. Current flowing outwards on one wire will create a field around itself, and current returning on the other wire will create a field of opposite orientation. Since the currents are equal, the fields should be equal and opposite, and thus cancel each other. But the locations of the two wires are not identical; they are separated in space by a distance at least as much as the thickness of their insulation systems. A person viewing the wires from outside the conduit will be one distance from one wire, and a slightly, ever so slightly different distance from the other wire.
Who cares about such small differences, you ask? A recent client of mine, I reply. I just finished a project (it is being built at the moment) in which there will be lab instruments that are highly sensitive to stray magnetic fields. It was necessary to account for the small differential magnetic fields caused by currents flowing in the conduits that fed branch panels from distribution panels. It was a first time for me to have to deal with fields so small, but it was important for this particular type of lab.
