How to Install a Line Voltage Thermostat for a Baseboard Heater

1. Attach a Wire Nut to Ground and White Wires

   There are two ways to approach the attachment of the ground wire to the thermostat:

   If there is no ground for the thermostat: Devices such as a Cadet Mechanical Thermostat are plastic, and thus, do not require grounding. You will wire-nut the ground wire coming in with the ground wire leaving the box, bypassing the device entirely.

   If you have a grounded thermostat: Does your thermostat have a green ground screw on it? Or similarly, does it have a green or bare wire coming out of it? If so, this is the wire that connects your device to the ground, and this is crucial for safety. Wire-nut the two ground wires, adding a third ground wire. That third ground will attach to the device (or you will use the wire already on the device). If the thermostat's electrical box is metal, it also must be grounded, with a pigtail wire or with a grounded conduit path. 

   If you have a white neutral wire, connect it so that wires coming in and out continue onward, bypassing the device. Alternatively, since you do not need the neutral for your 240V device, you can cap it off entirely in the box.

2. Wire-Nut One Hot Wire to Bypass the Thermostat

   Two hot wires, each carrying power, enter your box. Typically, one wire is coded black and the other wire is coded red (this is a hot wire, not a neutral wire) that it required to be identified via black or red tape.

   The important thing to remember is that only one hot wire enters the thermostat; the other is wire-nutted to bypass the device and continue onward.

   If you try to connect both hot wires to the thermostat, the circuit breaker will shut off, or you may experience a little electrical flash.

   You can choose either wire to continue onward.

3. Attach the Line and Load Black Wire to Thermostat

   Attach the other hot wire to your device. One end of your hot wire is a line wire, which means that it comes off of the power lines and carries power. The other black wire is a load wire, meaning that it flows to your baseboard heater.

4. Push the Green, White, and Red Wires Into the Box

   The wiring connection part of the project is now finished. Carefully push the green, white, and red wires back into the box. The farther back you can shove the wires, the better. This will give you more room to fit the body of the thermostat.

5. Line Up the Screws on the Box

   By turning the thermostat to the side, you can see where the screws line up with the electrical box. If you have to force the thermostat even to get the screws to touch the box, the wires are not correctly folded in. Try pulling out the green, white, and red wires and folding them back into the box.

6. Screw the Thermostat Into the Box

   If you are having a hard time screwing the thermostat into the box, it could be either that the wires inside the box are preventing the device from moving inward or something much simpler.

   The simple solution, at least with metal boxes, is to jiggle the screw side-to-side to get it to seat properly. If the screw is seated properly, it should easily turn in.

   Tip

   Use a manual screwdriver as this gives you better control.

7. Replace the Thermostat Faceplate

   Before you test your thermostat, install the faceplate. Even though a properly installed thermostat minus a faceplate should not have any powered-up exposed sections, you need to be extremely careful with high voltages. Even an errant screwdriver can touch hot areas in the box. Flip on the circuit breaker and turn up the thermostat until it clicks. The baseboard heater should start heating up.

When to Call a Professional

Consider calling in a licensed electrician when installing a line voltage thermostat at any point during the installation process, whether for the entire project or just for part of it. If you feel uncomfortable at all with wiring, even if you're halfway through the process and begin to have second thoughts, it's better to be safe and call a professional to help you.