How to Wire a Line-Voltage Thermostat for a Baseboard Heater

How to Wire a 240-Volt Double-Pole Thermostat

Generally, 240-volt baseboard heater thermostats are wired with 2-wire (with ground) cable, where both the black and white wires are hot. The thermostat has a total of four wire leads: two marked "Line" that connect to the circuit feed wires entering the box from the circuit breaker panel, and two marked "Load" that connect to the circuit wires that run onward to the heater. The thermostat's wire leads are often red and black, a color-coding that indicates that all wires carry hot current.

1. Make Sure the Power Is Off

   The cable delivering power to the thermostat box must be shut off before you connect the thermostat. Make sure the circuit breaker controlling the wires is shut off, and use a non-contact circuit tester to test for current at the thermostat box.

2. Read the Thermostat Schematic

   Unpackage the baseboard heater thermostat. Carefully read the wiring diagram and ensure you identify the wire leads marked "Line" and "Load" before beginning. "Line" will be attached to the circuit wires that feed current from the power source, while "Load" which will carry power onward to the baseboard heater.

3. Strip Wires

   Using wire strippers, strip about 3/4-inch of insulation from each insulated circuit wire entering the thermostat's electrical box, if necessary. This usually involves two supply wires from the power source and two wires passing onward to the heater. If the thermostat is controlling two baseboard heaters from the same location, there will be additional outgoing wires leading to the other heater.

4. Make Ground Wire Connections

   The bare copper wires from each cable entering the box need to be joined together with a wire connector, and the metal box also needs to be grounded with a "pigtail" wire that links to the circuit grounding wires. Typically, this is done with a short green or bare copper wire attached to the metal box with a green grounding screw, with the other end of the pigtail joined to the circuit grounding wires with a wire connector. The thermostat itself usually does not have any grounding connection. If it does have a green grounding lead, join this to the other circuit grounding wires.

5. Make 'Line' Connections

   Use wire connectors to join the two feed wires entering the box from the power source to the wire leads on the thermostat marked "Line."

   In most 240-volt wiring situations, the circuit wires will be black and white; it doesn't make any difference which color of wire lead gets attached to which feed wire, as both are hot.

6. Make 'Load' Connections

   Use wire connectors to join the outgoing circuit wires leading to the baseboard heater(s) to the wire leads marked "Load" on the thermostat. If the thermostat is controlling a single heater, each lead will be attached to a single wire. If the thermostat is controlling two or more thermostats from the same location, then each lead will be attached to multiple outgoing wires.

7. Complete the Installation

   Tuck the wires into the electrical box and secure the thermostat with the mounting screws. Complete the wire connections on the baseboard heater, if this has not already been done. Turn on the circuit breaker and test the heater and thermostat for proper operation.

How to Wire a 240-Volt Single-Pole Thermostat

Wiring for a 240-volt single-pole thermostat is similar to that for a double-pole, but in this case, one of the hot wires will simply bypass the thermostat and pass onward to the baseboard heater.

1. Make Sure Power Is Off

   Make sure the circuit breaker controlling the wires to the circuit you're working on is shut off, and use a non-contact circuit tester to test for current at the thermostat box.

2. Read the Thermostat Schematic

   Unpackage the thermostat and carefully read the instructions and the wiring diagram. In particular, make sure you identify the wire lead marked "Line," which will be attached to a circuit wire that feeds current from the power source, and the one marked "Load," which will carry power onward to the baseboard heater.

3. Make the Ground Connections

   The bare copper wires from each cable entering the box need to be joined together with a wire connector, and the metal box also needs to be grounded with a "pigtail" wire that links to the circuit grounding wires. Typically this is done with a short green or bare copper wire attached to the metal box with a green grounding screw, with the other end of the pigtail joined to the circuit grounding wires with a wire connector. The thermostat itself usually does not have any grounding connection. If it does have a green grounding lead, join this to the other circuit grounding wires.

4. Join the 'Line' Connection

   Connect the black circuit feed wire to the wire lead on the thermostat marked "Line," using a wire connector.

5. Join the 'Load' Connection

   Connect the wire lead on the thermostat marked "Load" to the black outdoing wire(s) leading to the heater(s), using a wire connector.

6. Complete the Bypass Connection

   Use a wire connector to join the white feed wire to the white outgoing wire(s); this connection is between the wires in the wall, not on the thermostat. This is also a hot wire, but in this case, it bypasses the thermostat and runs directly to the heater(s).

7. Complete the Installation

   Tuck the wires into the electrical box and secure the thermostat with the mounting screws. Complete the wire connections on the baseboard heater, if this has not already been done. Turn on the circuit breaker and test the heater and thermostat for proper operation.

When to Call a Professional

These preliminaries will be already be handled if you are replacing an existing thermostat, but a new heater installation will involve running a new cable, installing a new electrical box, and mounting a baseboard heater on the wall. Running new circuits for electric baseboard heaters is normally done by a professional electrician, because it requires specialized skills and involves working at the main circuit breaker panel, where there is a distinct danger of shock. Such work should be done by DIYers only if they have substantial knowledge and experience. However, once that work is done, connecting a line-voltage thermostat is a fairly simple job that you can easily do.