Can you wire a light switch with only one 14-2 wire?

Answer 1

If the power feed goes directly to a fixture, or if you are wishing to switch a receptacle that is not now switched, you can us a 14-2 NM cable to install a "switch loop", provided the circuit is no larger than 15 amps.

To do this, after running the 14-2 NM cable, connect the WHITE conductor of the 14-2 to the hot wire. At the switch, connect the black and white conductors to the switch terminals. At the fixture or receptacle, the black wire of the 14-2 NM cable now becomes your switch leg and is connected to the black conductor of the fixture or brass terminal of the receptacle.

The NEC (for the United States) requires that the switch be fed with the white wire and the black wire be used as your switch leg.

Answer 2

No, you need a minimum of two 14-2 wires to wire a light switch properly. One wire is needed for the power supply, and the other wire is needed to carry the switched power to the light fixture. Without two wires, you won't be able to complete the necessary connections for the light switch to work correctly.