Thermostat line voltage is a term used to describe the power that is supplied by an electrical circuit for the operation of a thermostat. It is defined as a single-phase AC voltage between two conductors, usually rated between 24-30VAC. This voltage is necessary for the proper operation of most thermostats used in HVAC systems. The voltage should be properly grounded and the wiring should meet all applicable codes. Failure to follow these guidelines can result in property damage or physical harm.