Switches controlling line-to-neutral loads must have a neutral conductor installed at a switch serving bathrooms, hallways, stairways, or rooms suitable for human habitation or occupancy, as defined in the applicable building code [404.2(C)] (Fig. 1).
When wiring for 3-way and 4-way switches, only the ungrounded conductors are switched [404.2(A)]. The neutral conductor must not be switched. New with the 2017 NEC: Where 3-way and 4-way switches are visible in a room, o