Furnace Wiring Colors

White

• White wires, also called “returns,” indicate heat. They are most often associated with a gas burner, oil burner, electric heat and auxiliary heat on a heat pump. They are associated with the W, W1 and W2 terminals and must be able to handle 24 volts. Any white wire most likely holds voltage that is returning to the breaker from the appliance.

Black and Red

• Red wires attach to the R terminal and are hot wires, those that carry current from the breaker to the appliance. On a furnace, red is run to the hot side of the transformer, while black leads to the common side of the transformer. Depending on the manufacturer, black is sometimes replaced with blue or brown. This is one situation where testing with a volt-ohm meter is a good idea.

Ground Wires

• A ground wire is a bare wire attached to the appliance frame. These wires carry no current unless there is a short circuit. The ground prevents shocks and electrocution by tripping the circuit breaker and stopping all electrical current. The minimum size for this type of wire is 14-2, though 12-2 is a better choice in modern homes with lots of electrical needs. The thicker wire greatly reduces the risk of shortages.

Additional Colors

• Yellow wires control compressor activity, usually for cooling or heating. Green runs to the furnace blower and is needed for air conditioning, heat pumps and in some models of electric furnace. Orange and blue are only occasionally used by certain manufacturers and connect to heating components. Emergency heat relays are wired with blue, pink, gray or tan depending on the manufacturer. There is no universal code for this kind of project, and it is essential to seek the aid of a professional if you are uncertain about any part of the installation.