The voltage from terminal G to the load side of the transformer (thermostat closed, fan auto) is:

• A. 6 V
• B. 12 V
• C. 24 V
• D. 48 V

Answer: (C)

The control circuit for HVAC thermostats runs on 24-volt AC. The G terminal switches the fan by energizing a relay coil that is connected across the transformer's secondary (one side to G, the other to the common/load side). When the thermostat is closed and the fan is set to auto, current flows from the transformer's hot side through the thermostat to G and returns through the load side, so the coil sees the full transformer's voltage. That voltage is 24 V AC, which is why the correct answer is 24 V. Other voltages like 6 V, 12 V, or 48 V aren’t standard for these low-voltage thermostat circuits.