A 2-wire transmitter needs a certain minimum voltage across its own terminals just to keep its electronics running. That minimum is called the compliance voltage, and it is the number at the heart of every loop-design calculation. If the supply cannot deliver that minimum voltage to the transmitter after every resistor and barrier in the loop has taken its share, the transmitter starves and the reading falls apart - usually right at 20 mA, when the loop draws the most and the drops are largest. This guide defines compliance voltage and shows how it sets the floor on your supply voltage.
Compliance Voltage in one line: Compliance voltage, sometimes called minimum operating voltage or terminal voltage, is the lowest voltage a 2-wire transmitter needs across its own terminals to function correctly at full 20 mA output. Because the loop's series resistances all drop voltage, the supply must be high enough that, after those drops, at least the compliance voltage still appears at the transmitter. In short: supply voltage must exceed the transmitter's compliance voltage plus every series voltage drop in the loop at 20 mA.
A loop-powered transmitter runs on the loop it modulates, which means its own electronics have to be powered by whatever voltage appears across its two terminals. There is a floor below which those electronics cannot function - the internal regulator has no headroom, the sensor conditioning browns out, and the device can no longer hold an accurate current. That floor is the compliance voltage. A datasheet might state it directly as a minimum terminal voltage, often somewhere in the range of about 8 to 12 volts for a modern transmitter, though the exact figure varies by model and rises for devices with displays or extra features.
The reason compliance voltage bites hardest at 20 mA is that current and voltage drop go hand in hand. Every series resistance in the loop - the sense resistor, an intrinsic-safety barrier, an indicator, the cable itself - drops a voltage equal to its resistance times the loop current. At 4 mA those drops are small; at 20 mA they are five times larger. So the worst case for the transmitter is when it is trying to output 20 mA, because that is exactly when the loop is stealing the most voltage before it reaches the device. A loop can look perfectly healthy at low readings and then fail at high ones for this reason.
It helps to think of compliance voltage as the transmitter's rent: it must be paid out of the supply voltage first, before any resistor gets its share, or the device is evicted. Everything else in loop design is about making sure that after the loop's resistances take their cut, the transmitter still keeps at least its compliance voltage. Get this number wrong, or ignore it, and you build a loop that runs fine on the bench and then reads erratically or clips near full scale once the real barriers, resistors, and cable are in place.
The core loop-design inequality is short: the supply voltage must be greater than or equal to the transmitter's compliance voltage plus the sum of all series voltage drops in the loop at 20 mA. Those drops include the sense or load resistor - a 250 ohm HART resistor alone drops 5 volts at 20 mA - plus any barrier, isolator, panel meter, and the resistance of the cable out and back. Add them all up at the worst-case 20 mA, add the compliance voltage, and that total is the minimum supply your loop can tolerate.
This is why 24 VDC became the near-universal loop supply. Take a transmitter needing, say, 10 volts of compliance, add a 250 ohm resistor's 5 volts, allow a few volts for a barrier and cable on a real run, and you are already well past a 12 volt supply. A 24 volt supply leaves comfortable headroom for all of that plus margin for supply tolerance and future additions. The margin is not waste - it is what absorbs a longer cable run, a barrier added later, or a supply that sags a bit under load, all without pushing the transmitter below its compliance voltage.
The design discipline, then, is to budget from the transmitter outward. Start with its compliance voltage as the non-negotiable floor, add every drop the loop imposes at 20 mA, and confirm the supply beats the total with margin to spare. If it does not, you have three levers: raise the supply voltage, reduce a drop - a smaller load resistor, a shorter or heavier cable, a lower-burden barrier - or choose a transmitter with a lower compliance voltage. Compliance voltage is the constant the whole calculation orbits, and respecting it is the difference between a loop that works only near zero and one that works across its full range.
Out in the field, the loops that fail on compliance voltage are usually the long ones and the ones that grew over time. A gathering site commissioned with a short loop and a bare transmitter can slip below compliance when someone later inserts a barrier for a hazardous-area upgrade, adds a local indicator, or extends the cable to a relocated instrument. Each addition quietly raises the total voltage the loop consumes at 20 mA, and if nobody rechecks the budget, the transmitter begins to clip or read erratically at the top of its range - a fault that is maddening to chase because it only appears when the process is running high.
The symptom pattern is distinctive once you know it: readings that track correctly at the bottom and middle of the range but flatten, jump, or read low near full scale. That signature almost always points back to a starved loop rather than a bad sensor, because it is the 20 mA end where the drops are largest and the compliance margin is thinnest. Recognizing it as a voltage-budget problem, not an instrument problem, sends you to the loop sheet and the supply rather than to a truck roll for a replacement transmitter.
A cloud SCADA platform such as Merobix cannot add voltage headroom to a field loop, but by trending every channel continuously it makes a compliance-voltage failure legible. A tag that reads faithfully across most of its span and then behaves oddly only at the high end, especially one that started misbehaving after a documented site change, is telling you the loop ran out of voltage. Seeing that pattern on the historian - rather than catching it by luck at a panel meter - turns an intermittent, high-range-only glitch into a clear diagnosis, so the crew arrives knowing to check the supply and the loop budget, not to swap the instrument.
Compliance voltage is the minimum voltage the transmitter needs across its own terminals to work, typically stated on its datasheet. Supply voltage is what your DC power source provides to the whole loop. The supply must be larger than the compliance voltage plus all the series drops in the loop, so that after those drops the transmitter still sees at least its compliance voltage - especially at 20 mA, when the drops are greatest.
Because voltage drop is proportional to current. At 20 mA every series resistance in the loop drops five times the voltage it does at 4 mA, so far more of the supply is consumed before it reaches the transmitter. If the remaining voltage at the transmitter falls below its compliance voltage at full scale, the device starves and the reading clips or goes erratic - even though the same loop had plenty of voltage at the low end.
Enough to cover the transmitter's compliance voltage plus every series drop at 20 mA, with margin left over for supply tolerance and future additions. This is why 24 VDC is standard: after a transmitter's compliance voltage, a load resistor, a barrier, and cable resistance, a 24 volt supply still leaves comfortable headroom. Keeping several volts of margin protects against a longer cable, an added barrier, or a supply that sags under load.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.