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Article 200 provides general rules for grounded conductors. Which of these apply to the calculations you must do to correctly size conductors and overcurrent protection devices?
Actually, there’s only one. You can’t use a neutral conductor for more than one branch circuit (multiwire or not) or for more than one set of ungrounded feeder conductors except where specifically permitted in the NEC [200.4].
The NEC distinguishes between the neutral and the grounded conductor, giving them separate definitions in Article 100. A conductor may do double duty as both, but the grounded conductor isn’t automatically the neutral.
In a single-phase, three-wire system, there really isn’t a neutral, per se, even though we commonly call it that. The white wire is the grounded conductor (the green isn’t an energized conductor; it’s the ground). The “neutral” isn’t a return path for electrons, as is often suggested. What flows in an alternating current circuit is the alternation, not electrons.
In a DC circuit, electrons do flow, though the way we name the direction is backwards—a fact which gave rise to hole flow theory. The holes between the electrons move from positive to negative. The electrons themselves move in the other direction. For semiconductor theory, this is important.
In a three-phase system, the neutral is the return path for undesirable current (the unbalanced load) to return to the source. It’s not needed in a purely three-phase system, but tap that transformer for a single phase and you get imbalance.