A 220-volt 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. 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. 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.
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.
Turn on the heater's circuit breaker or fuse.
Find the wire set that leads to the fuse box or circuit breaker with a voltmeter. Pick a wire set and place a voltmeter 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 voltmeter 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."
Disconnect the fuse or circuit breaker.
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.
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.
Twist the "Line" wire set's black-colored wire to the thermostat's "Line" wire and cover it with a wire nut. If the thermostat uses two "Line" wires, then twist the black-colored "Line" wire to either thermostat "Line" wire.
Twist the "Load" wire set's black-colored wire to the thermostat"s "Load" wire and cover it with a wire nut. If the thermostat uses two "Load" wires, then twist the black-colored "Load" wire to either thermostat "Load" wire.
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 nut. 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 the white-colored "Load" wire to the thermostat's remaining "Load" wire. Cover each pair of "Load" wire connections with a wire nut.