What is the open-circuit voltage between the two terminals of a single-pole switch on a lighting circuit when the switch is off?

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

Answer: (A)

Open-circuit voltage is the voltage you would read across the switch’s two terminals when the circuit isn’t completed. In a typical US lighting circuit, the supply between hot and neutral is 120 volts. When the switch is off, current can’t flow, so there’s no voltage drop across the lamp. The lamp is tied to neutral, so its side of the switch sits at neutral potential (0 V). The hot side of the switch remains at the line potential, about 120 V above neutral. Therefore the voltage across the two terminals is the full 120 volts. It isn’t 0 volts because one terminal is connected to the live supply, it isn’t 240 volts because standard residential systems here use 120 V, and it isn’t 12 volts because that’s a different, low-voltage lighting setup.