A C-Wire Adapter is needed when installing a Honeywell Home Smart Thermostat and a common wire or C-Wire (24 volts) is not present. ?The C-Wire Adapter makes Honeywell Home smart thermostats compatible with most homes that don’t have a common wire.? All Honeywell Home Smart Thermostats require 24 volts power (C-Wire)?. Some homes may have an unused C-wire sometimes tugged behind the wall where thermostat is installed.