Double-pole thermostats are used to control line voltage -- electrical power to heating devices. Line voltage is the electricity that comes from the main circuit breaker or fuse panel. When wiring these devices, typically there are two values for line voltage, 120 VAC and 240 VAC. The lower value, 120VAC, is found at the common wall outlet. The higher voltage, 240 VAC, is generally used for providing power to electrical hot water heaters and baseboard heaters. Baseboard heaters will most often use a two-pole line voltage thermostat to control the on and off operation of the heater.
Observe the rear of the double-pole thermostat. There are four screws for the electrical connections. Two of the screws are identified as "Line" and the other two screws are labeled "Load." Typically, the line screw terminals are at the top of the thermostat and the load screws are at the bottom. Each physical side of the thermostat represents one pole of the double-pole internal switch mechanism.
Remove all electrical power from the wires that will provide power for the thermostat circuit. This will entail shutting off the controlling circuit breaker or pulling the fuses.
Use the wire strippers and remove ¾ inch of the wires' insulation to reveal the bare copper underneath. There should be two power wires coming from the main panel and two wires going to the electrical device the thermostat is controlling.
Bend a small U shape in the bare copper ends of the wires with the needle-nose pliers. The radius of the bend should be of a size that it will fit under the screw head of the thermostat connections.
Place the two power feed wires to one each of the line screw terminals. Tighten the screw with the screwdriver. Attach the two wires going to the device being controlled by the thermostat under the two load screws. Again, place one wire each to each screw terminal.
Turn the thermostat counterclockwise to its lowest setting. Re-energize the main power source. Turn the thermostat clockwise until an audible "click" is heard. The thermostat should now be conducting power to the device it is controlling.