What voltage typically powers a 2-wire transmitter loop?

• A. Usually 12-30 VDC
• B. 120 VAC
• C. 5 VDC
• D. 48-75 VDC

Answer: (A)

Two-wire transmitter loops are powered by a single low-voltage DC source that both powers the transmitter and provides the return path for the 4-20 mA signal. The supply must cover the voltage drops across the transmitter and the loop load at the highest current (20 mA). In most industrial setups, a common loop voltage is around 24 V DC, with an acceptable range of roughly 12 to 30 V DC. This range gives enough headroom to power the transmitter electronics and the load while maintaining proper current regulation.

Voltage outside this range isn’t typical for standard 2-wire loops. 120 VAC isn’t used because the loop runs on DC. 5 V DC is usually too low to power typical transmitter circuits and the load. Higher ranges like 48-75 V DC do occur in some specialized or long-distance applications, but they are not the standard for a typical 2-wire loop.