With conventional light switch wiring using NM cable, a NM cable supplies line voltage from the electrical panel to a light switch outlet box.
Another NM cable connects from this switch box to the light fixture box. Both line and neutral travel from the switch box to the light fixture box.
Alternate (California Style) Switch Wiring
With alternate light switch wiring, an NM cable supplies line voltage from the electrical panel to a light fixture outlet box.
Another NM cable connects from the light fixture box to the switch box. In the diagram below right, a 2-wire NM cable that connects the light fixture to the switch carries 2 line wires (one line, and one switched line).
The white wire carries line, the black wire carries the switched line. Since the white wire is hot, a black piece of electrical tape is placed on each end.
This alternate type of wiring is no longer allowed by the NEC as of 2011 if a neutral wire is not present inside each switch box.
Diagrams shown on this page are simplified for clarity. Electrical outlet boxes can have numerous NM cables going in and out. See Actual Switch Box Wiring.
Typical NM Ground Wire Connections Diagram is shown below.