In 24 VAC irrigation controller wiring, which color conductor is commonly used for the common (C) line?

• A. Green
• B. Blue
• C. Yellow
• D. Red

Answer: (B)

The key idea is identifying the return path in a 24V irrigation control circuit. In these systems, the transformer provides two conductors: the hot lead (R) that supplies 24V and the common (C) return path. Blue is commonly used for that C line, giving a consistent way to recognize the return side of the circuit. The hot side is typically red. So blue is the standard choice for the common conductor because it clearly distinguishes the return path used to energize the valve solenoids. Green and yellow are not the standard for C in this context, and always check a specific system’s wiring diagram if colors differ.