Final answer:
The statement regarding the thermostat control circuit being classified as a Class 1, extra-low voltage power circuit is true, as it uses a secondary voltage of 24V and a power rating typical for HVAC control circuits.
Step-by-step explanation:
The statement that the thermostat control circuit of a forced-air furnace fed from a 120/24 V 40VA transformer is usually classified as a Class 1, extra-low voltage power circuit is true. This classification is based on the fact that the secondary voltage supplied by the transformer is 24 volts (which is considered extra-low voltage) and is typically used for control circuits in HVAC systems, such as thermostats.
Furthermore, the power rating of 40VA (volt-amps) falls within the range commonly used in these applications. It's important to note that within electrical engineering, the classifications of circuits can have implications on safety regulations and the types of materials that can be used.