What is the meaning of 'compliance voltage' in a 4-20 mA loop?

• A. The maximum voltage the transmitter can tolerate
• B. The voltage required across the transmitter and wiring for the selected loop current; the power supply must exceed this to maintain operation
• C. The baseline voltage of the current loop
• D. The voltage drop across the measuring resistor only

Answer: (B)

In a 4-20 mA loop, compliance voltage is the minimum headroom that must be available across the transmitter and all wiring and devices in the loop to sustain the selected loop current. The transmitter controls the current through the loop, and the load (including the sensing resistor and any connected devices) drops a portion of the supply voltage. To maintain, say, 20 mA, the supply must be high enough so that the voltage drop across the transmitter plus the wiring and the load can be met without forcing the current lower than commanded.

So the correct idea is that compliance voltage is the voltage required across the transmitter and wiring for the chosen current; the power supply must provide at least this amount (and usually a bit more for margin) to keep the loop operating properly. If the supply isn’t high enough, the current cannot reach the desired level, and the loop will not behave as intended. This isn’t just the baseline voltage, nor the drop across the resistor alone, and it isn’t about the transmitter’s maximum tolerance in isolation.