Often called induction, even though some say that this is not strictly correct.
Running only the hot conductor in a steel conduit will cause excesive voltage drop, with the "lost" energy appearing as heat.
Adding a ground wont help since the ground/EGC is purely a safety feature and normaly carries virtualy no current.
What is required is to add one or more other current carrying conductors, such that the currents "cancel out" and dont induce currents in the conduit.
On a 2 wire 120 volt circuit, hot and neutral should be run together.
On a 2 wire 240 volt circuit, the 2 hot conductors should be run together, no neutral is needed to avoid induction effects. If however the load requires a neutral for proper operation, then this must be run in the same conduit as the hots.
The two wires to a light switch should run in the same conduit, both are hot, not neutral, but are in oposite directions and therefore cancel out.
If plastic conduit is used, then no heating will be caused, but the same rules should still be followed, since electromagnetic fields will be radiated by unbalanced currents, and some believe this to be dangerous to health.
