Why is 24 V DC commonly used as loop supply in HART installations?

• A. It provides sufficient headroom to power the transmitter and to accommodate the drop across the loop components while maintaining the 4-20 mA range.
• B. It minimizes power losses and eliminates noise entirely.
• C. It is required to support USB communication with field devices.
• D. It is mandated by all standards to ensure safety.

Answer: (A)

Headroom in the loop supply is the key idea. The 4-20 mA current loop must power the transmitter and any associated electronics while also covering the voltage drops from the field wiring, the transmitter’s own electronics, and any load (like a sensing resistor or other series elements). When the current is at its maximum (20 mA), those drops add up, and you still need enough voltage left for the transmitter to operate correctly and for the HART signaling to ride on top of the loop current. 24 V DC provides enough headroom for typical transmitters and wiring plus the necessary margin for longer cables or extra loop components, while keeping within common industrial power practices and safety limits. That balance of sufficient voltage at the highest current and broad device compatibility is why 24 V is the standard loop supply in many HART installations.