A control circuit in HVAC equipment is often powered through an isolation transformer to limit the voltage to a maximum of ______ volts.

• A. 100
• B. 150
• C. 230
• D. 480

Answer: (B)

Isolation transformers used with HVAC control circuits keep the control-side voltage within a safe, manageable level. By limiting the maximum voltage that can appear on the control side, they protect people and equipment from high line voltages while still providing enough voltage for relays, sensors, and thermostats to operate reliably. The 150-volt ceiling is a practical standard for this purpose, balancing safety with the needs of typical control devices.

A voltage limit of 100 volts would be unusually low for standard control components, while 230 and 480 volts are high line voltages that would defeat the purpose of isolating and protecting the control circuitry.