Be aware: this is an extension to how can i upload a “c” wire to my thermostat which asks about jogging the wires via the wall. This question is set where to connect the wire at the hvac. After finding the transformer inside the diagram, you may must decide which wires are related to the primary coil, and that are connected to the secondary coil. Check the diagram for a key, which will give you a hint. On a few diagrams a thicker (formidable) line denotes line voltage wiring, at the same time as a thinner line represents low voltage wiring. Others may additionally listing the voltage and frequency on every facet of the transformer.