In a standard thermostat wiring scheme, the R wire originates from the __________.

• A. secondary of the 24V control transformer
• B. primary of the 24V control transformer
• C. thermostat mounting screw
• D. outdoor unit contactor

Answer: (A)

R is the 24-volt hot supply that powers the thermostat. It comes from the secondary winding of the 24V control transformer inside the HVAC equipment. The transformer’s primary is connected to line voltage, which is stepped down to 24V on the secondary; the thermostat uses that 24V to operate and to energize control circuits by connecting R to various terminals (W, Y, G, etc.). The other side of the secondary is common (C), providing the return path. The primary side isn’t used by the thermostat, the mounting screw is just hardware, and the outdoor unit contactor is a load controlled by the thermostat, not the source of power.