A c-wire (also referred to as a common wire) is a low voltage power wire that many modern thermostats require for their features to work.

The Nest Thermostat E does not require a c-wire to work. Nest has a built-in battery that can be charged using the heating and cooling wires.

If you already have a c-wire, however, Nest recommends that you connect it. To determine if you have a c-wire, remove your old thermostat's cover plate. You should see several wire terminals labeled with letters. If a wire is connected to the “C” terminal, you have a c-wire.