Trane Furnace Wiring Diagram | Free Wiring Diagram Assortment of trane furnace wiring diagram. A wiring diagram is a streamlined standard photographic depiction of an electric circuit. It reveals the components of the circuit as simplified forms, and the power and also signal links in between the gadgets. Instructions for Wiring a Trane Thermostat | Hunker The green wire is connected to the fan relay and connected to the G terminal on the Trane thermostat. If it's a programmable thermostat and requires a common wire, this is usually the black wire and is connected to the common side of the secondary winding of the transformer--and, on the thermostat, it is connected to the C terminal.