Learn about the common or C wire

In most cases, Nest thermostats can get enough power from your system’s heating and cooling wires. But in a small number of situations you may need to connect a common or C wire to deliver enough consistent power to your thermostat.

During installation, the app will give you a custom wiring diagram and tell you if you need to connect a C wire. It may also test to check if the C wire delivers enough power to your Nest thermostat.

Unlike other wires connected to your thermostat, a common wire doesn’t control heating or cooling functions. It only provides constant power to your thermostat.

Nest thermostats don’t typically need a C wire

Nest thermostats are engineered to use very little power, less than 1 kWh/month. For comparison, a typical 40W light bulb that’s only used 4 hours per day consumes about 5 kWh/month. So even though Nest thermostats have Wi-Fi, activity sensors, color displays and more, they typically don’t need a C wire to stay charged up.

The app will tell you to connect a C wire

During installation, always use the app to get a wiring diagram so you can correctly connect your thermostat.
  • If your current thermostat doesn't have a wire in the
    C terminal
    : the Nest app won't tell you to connect one. Do not connect a C wire to your Nest thermostat unless the Nest app tells you to.

  • If your current thermostat has a wire in the C terminal: the Nest app will tell you to connect the C wire to your Nest thermostat.

  • The app may also test the connected C wire to make sure it can deliver enough power to your thermostat.

Nest thermostat wiring diagram


Why your Nest thermostat may need a C wire

During setup

During initial setup, the app will tell you if you should connect a C wire to your thermostat. 

The app may also test to check if the C wire can deliver enough consistent power to your Nest thermostat. If it can’t, you have a few choices: 

  • Check if there’s an existing thermostat wire you can use as a C wire.
  • Contact a local HVAC professional to install a C wire, or an accessory that can act as a C wire.

After installation and setup

In some cases, you may experience problems after installing your Nest thermostat that can be fixed by connecting a C wire, or by removing the wire from the C connector.

If your system can’t deliver consistent power to your thermostat to keep its battery charged or correctly control heating and cooling, you may experience one or more of these symptoms:

  • Reduced thermostat battery life
  • Thermostat motion sensing is disabled
  • Your thermostat occasionally disconnects from Wi-Fi
  • Your system unexpectedly turns on or off
  • Your system is making strange noises: chattering, stuttering, clicking or thumping
  • Heating or cooling is always on, and won’t turn off
  • Heating or cooling repeatedly turns on and off in a short period
  • There is a Delayed or Starts in message on your Nest thermostat’s screen (for example: “Heating is delayed for 2:30 mins”)
  • The system fan is always running or won’t turn on
  • The system fan turns on and off repeatedly in a short period

Go to the section below for help fixing these power related issues.

Troubleshoot or connect a C wire

The best solution for power issues is to contact a Nest Pro in your area. They can diagnose and help fix any possible problems with your system and thermostat installation. If needed, they can also connect a C wire or a C wire accessory, such as a Fast-Stat Common Maker, to your Nest thermostat.

If you’re comfortable troubleshooting your system and take necessary precautions, you might be able to fix C wire issues or connect a C wire yourself. Go to the following article for detailed steps:

Fix or connect a C wire

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