Which statement about common control circuit voltages in LV systems is true?

• A. 24 V DC is never used
• B. 120 V AC is the only control voltage used
• C. 240 V AC is typical for control circuits
• D. 24 V DC is common for control circuits; some installations may also use 120 V AC

Answer: (D)

Low-voltage control circuits are designed to minimize shock risk and simplify wiring. A 24 V DC control supply is widely used because DC coils in relays and contactors respond reliably and quickly, and many control devices (PLC inputs, timers, door controls) are standardized around 24 V. While this voltage is common, some installations may also use 120 V AC for control, especially in older equipment or specific applications. 240 V AC is not typical for control circuits due to higher voltage and safety concerns. So, 24 V DC is common for control circuits, with some installations also using 120 V AC.