Line voltage thermostats control electrical power to heating and cooling devices. The two basic types of line voltage thermostats are single-pole and double-pole units. Single-pole thermostats control 120-volt electrical power. A double-pole thermostat can control either 120 volts or 240 volts. The "pole" acts like a basic switch that will either turn on or turn off the electrical power, depending on the temperature setting of the thermostat. Installing a line voltage thermostat takes approximately one hour.
Shut off all electrical power from the circuit supplying electricity to the heating or cooling device being controlled. Turn off the circuit breaker or pull the fuses that send power to the device.
Remove ¾ inch of the insulation covering the electrical wires, using the wire strippers. Expose the bare copper underneath. The bare copper end of the wire is connected to the screw terminals of the thermostat.
Attach the electrical wire(s) coming from the circuit breaker or fuse to the screw terminal(s) marked as "line." In most cases, this is the black wire coming from the power supply. Wrap the bare copper end of the wire around the screw terminal. Tighten the screw around the wire with the screwdriver, turning counterclockwise.
Connect the wire(s) going to the device being controlled to the screw terminal labeled "load." Turn the screw clockwise to secure the bare copper end of the wire to the screw.
Twist the two white wires together, using the wire pliers if applicable. Cover the twisted bare copper end with a plastic wire nut. The 120-volt power supply has a white neutral wire that comes from the main electrical power panel. A 240-volt power supply does not use a white neutral wire. Instead, both wires on a 240-volt supply power circuit are connected to the thermostat terminal screws on a double-pole thermostat.
Install the thermostat into the electrical box if applicable. Some line voltage thermostats may fit on the device itself. Use the vendor-provided screws to secure the thermostat to either the electrical box or the device.