A disconnecting switch is commonly used to provide a residential furnace with a _________ power supply.

Residential furnaces commonly operate on standard household electrical power, which is typically supplied at 120 volts in North America. This voltage is suitable for powering various components of the furnace, including the blower motor, control circuitry, and other electrical elements.
To provide a means of disconnecting power to the furnace for maintenance or safety purposes, a disconnecting switch is often installed. This switch allows the electrical supply to be easily and safely disconnected without the need to shut off power to the entire dwelling.
115V was a more common voltage in the past, but 120V is now the standard.