How to Wire a 220 Thermostat

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A 220-volt thermostat, also known as a line-voltage thermostat, disconnects the power supply to a heater once the temperature reaches a set level. A 220-volt thermostat wires directly to the heating unit and is commonly used to control baseboard heaters. This differs from an air conditioning and heating system that uses a separate low-voltage circuit to power the thermostat and the other parts in its control circuit.

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Two kinds of 220-volt thermostats exist; single- and double-pole thermostats. A circuit with 220 volts contains two 110-volt legs of electricity. A single-pole thermostat interrupts one leg, while a double-pole thermostat interrupts both legs.

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How to Wire a 220-Volt Thermostat

Step 1: Turn Off the Power

Turn off the circuit breaker in the main panel that controls the heater and thermostat. Before touching any thermostat wires, test them with a voltage tester to make sure they aren't live.

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Step 2: Expose the Wires

Pull the ends of both wire sets from the thermostat's mounting box. The mounting box will hold the thermostat on the wall. One wire set leads to the fuse box or circuit breaker and the other wire set leads to the heating unit. Each wire set contains two coated wires and an uncoated wire. Remove 1/2 inch from each wire's coating with wire strippers. Separate each wire. The wires must not touch anything.

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Step 3: Find the Line and Load Wires

Turn on the heater's circuit breaker or fuse, then pick a wire set and place a multimeter set to measure AC volts probe on each coated wire: one probe on the white-colored wire and one probe on the black-colored wire. Read the voltmeter. If the meter reads 220 volts, then that wire set leads to the fuse box or circuit breaker. Consider the wire set that leads to the fuse box or circuit breaker "Line" and the wire set that leads to the heater "Load." When the test is complete, turn the power back off.

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Step 4: Check the Thermostat for Line and Load Terminals

Inspect the 220-volt thermostat. Labels on the back of the thermostat identify the "Line" and the "Load" wire(s). Single throw thermostats use one Line and one Load wire. Double throw thermostats use two Line and two Load wires.

Step 5: Connect the Ground Wires

Twist both uncoated wires together and wrap them around the thermostat's green screw. Tighten the screw with a Philips-head screwdriver. The green screw, the thermostat's ground connection, attaches to the thermostat's housing near its top mounting bracket. Mounting brackets, one on top and one on bottom, will hold the thermostat in the mounting box.

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Step 6: Connect the Line Wires

Twist the "Line" wire set's black-colored wire to the thermostat's "Line" wire and cover it with a wire cap. If the thermostat uses two "Line" or L1 wires, then twist the black-colored "Line" wire to either thermostat "Line" wire.

Step 7: Connect the Load Wires

Twist the "Load" wire set's black-colored wire to the thermostat's "Load" wire and cover it with a wire cap. If the thermostat uses two "Load" wires, then twist the black-colored "Load" wire to either thermostat "Load" wire.

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Step 8: Connect the White Wires

Connect the white-colored wires if the thermostat uses one "Line" and one "Load" wire, then twist both white-colored wires together and cover them with a wire cap. If the thermostat uses two "Line" and two "Load" wires, then twist the white-colored "Line" wire to the thermostat's remaining "Line" wire and twist them together. Then the remaining white-colored "Load" wire to the thermostat's remaining "Load" wire and twist them together. Cover each pair of "Load" wire connections with a wire cap.

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