What is a C-Wire or Common Wire and How do I Know if I Have One?

Many conventional, non-connected thermostats don’t need a “c-wire” to function. But today, most Wi-Fi thermostats have new features such as Wi-Fi radios and LED color screens that require the unit be powered at all times. Thermostats can get this constant power through a common wire or “c-wire.”

 

It is easy to see if you already have a c-wire connected to your system. Simply remove your current thermostat face from its baseplate and look for the terminal labeled with the letter “c.” If this terminal has a wire attached to it, you have an active “c-wire.”

 

If you don’t see a wire connected to your “c” terminal, you have a few options. See our c-wire assistance page for more information and helpful videos.

 

I don’t have a common or “c-wire" and need one. What are my options? 

Sensi thermostats work with most heating and cooling systems, however, the need for a common wire or “c-wire” is different based on the Sensi thermostat model. Unsure if your thermostat needs a c-wire? Reference the master compatibility chart in this article.

 

If you need a c-wire, there are a few main options:

• Using an extra, unused wire as a c-wire
• Using your thermostat’s g-wire (fan wire) as a c-wire
• Installing an Add-a-Wire Kit (These kits can be purchased from another manufacturer and used with Sensi.)

If you would like to understand what a c-wire is and learn more about these three options, watch our helpful videos on our c-wire page.

If you have a 2-wire, heat-only system, you could also install a 24VAC power transformer to supply constant power to your Sensi thermostat.

 

I have extra wires coming from my wall that are not connected to my old thermostat. Can I use any of them for a c-wire?

 

Sometimes there will be extra, unused wires in the wall. If they are not used in wiring your Sensi, simply tape them off and push them into the wall and out of the way. If you need to install a c-wire, learn more about how you can use your extra wire as a c-wire.