The open-circuit voltage observed at a switch in the off position is typically equal to which value in a standard residential lighting circuit?

• A. 120 volts
• B. 0 volts
• C. 240 volts
• D. 12 volts

Answer: (A)

In a standard US residential lighting circuit, the supply is 120 volts between hot and neutral. A switch sits on the hot conductor to the fixture. When the switch is off, no current flows, but one terminal of the switch remains connected to the hot line. The other terminal leads to the light fixture, which is connected to neutral at the fixture. If you measure across the switch with a voltmeter, you’ll see the full line voltage, about 120 V, because the hot side is energized while the other side is referenced to neutral through the fixture path. The reading doesn’t require current to flow; it’s about the potential difference that’s present.

The other voltages don’t apply here: 0 volts would mean both sides are at neutral potential, 240 volts would be from a different, higher-voltage or multi-phase system, and 12 volts corresponds to low-voltage lighting—not the standard residential lighting circuit.