Where compliance voltage looks at a loop from the transmitter's need for voltage, burden resistance looks at the same loop from the other side: how much total resistance the transmitter is being asked to push current through. Every device in series - the sense resistor, a barrier, an indicator, the cable - adds to that burden, and every transmitter has a maximum it can drive at a given supply voltage. Exceed it and the device runs out of headroom and cannot hold 20 mA. This guide explains total loop burden, the maximum-load spec, and the classic 250 ohm HART resistor that sits at the center of it.
Loop Burden Resistance in one line: Loop burden resistance, also called loop load, is the sum of every series resistance a 2-wire transmitter must drive current through - the sense or load resistor, intrinsic-safety barriers, indicators, and the cable itself. A transmitter's datasheet gives a maximum load, usually as a curve or formula against supply voltage, and if the total burden exceeds that maximum the transmitter cannot maintain full 20 mA and the reading clips. The classic component of loop burden is the 250 ohm HART sense resistor.
A 4-20 mA loop is a single series circuit, so the transmitter has to drive its current through everything in that circuit one after another. The total loop burden is simply the sum of all those series resistances. The sense or load resistor across which the receiving device reads the signal is usually the largest single contributor. On top of it come any intrinsic-safety barrier's internal resistance, a series panel indicator, a signal isolator's input resistance, and the resistance of the cable itself, out to the field and back. Add them all and you have the burden the transmitter faces.
Cable resistance is the term people most often forget. It is small per meter, but a long run adds up, and it counts twice because current flows out on one conductor and back on the other. On a short bench loop the cable is negligible; on a kilometers-long field run with thin conductors it can rival a load resistor. Because burden is a pure sum, the way to reduce it is to attack whichever term is largest - a smaller load resistor, a shorter or heavier cable, a lower-resistance barrier - rather than assuming the cure is always more supply voltage.
Burden and compliance voltage are two views of one constraint, linked by the loop current. Burden resistance times 20 mA gives the voltage those resistances consume at full scale, and the supply must cover that consumed voltage plus the transmitter's compliance voltage. So you can express the limit either as a maximum resistance the transmitter can drive at a given supply, or as a minimum supply needed to drive a given resistance. They are the same inequality rearranged, which is why loop-design tables often present a maximum-load curve: resistance on one axis, supply voltage on the other.
Every 2-wire transmitter datasheet states a maximum load, and it is almost always given relative to supply voltage rather than as a single number - often a straight-line graph or a simple formula of the form maximum resistance equals supply voltage minus the transmitter's minimum voltage, all divided by 20 mA. The higher your supply voltage, the more burden the transmitter can drive; the lower the supply, the less. This is why you cannot state a loop's maximum resistance without also stating the supply that feeds it - the two are inseparable.
When the actual burden exceeds the maximum for your supply, the transmitter runs out of voltage headroom before it reaches 20 mA. It can still drive the lower part of the range where less voltage is needed, but as it tries to push toward full scale it hits its limit and the current flattens - the reading clips below 20 mA and the top of the range becomes unusable or nonlinear. The failure is one-sided and predictable: the low end works, the high end saturates. That signature is the fingerprint of an over-burden loop and distinguishes it from a sensor fault, which would misbehave across the whole range.
Fixing an over-burden loop means either lowering the burden or raising the supply. Lower the burden by using a smaller load resistor, removing an unnecessary series indicator, choosing a barrier with less internal resistance, or upsizing the cable. Raise the supply, within the transmitter's and barrier's rated limits, to buy more headroom. The disciplined approach is to total the burden at design time against the datasheet's maximum-load curve for your chosen supply, leaving margin, so the loop is never sitting right at the edge where a later addition or a warm-day supply sag tips it into clipping.
The most famous single item on any burden list is the 250 ohm resistor. HART communication rides a small digital signal on top of the 4-20 mA analog current, and the receiving equipment reads that digital signal as a voltage developed across a series resistance - conventionally 250 ohms. That resistor is deliberately part of the loop burden: it is what turns the HART current signal into a readable voltage. At 20 mA a 250 ohm resistor drops 5 volts, so it alone consumes a large slice of the loop's voltage budget, which is exactly why so many loop calculations start from that 5 volt figure.
The practical upshot is that a plain analog loop and a HART-enabled loop have different burden budgets. Add the 250 ohm resistor to communicate with the transmitter or to connect a HART interface, and you have added 5 volts of drop at 20 mA that a purely analog loop would not carry. A loop designed with no margin for it can slip into compliance-voltage trouble the moment HART is enabled. Conversely, a loop that already has a 250 ohm load can usually support HART without change, because the resistance the protocol needs is already there.
For field and SCADA work this matters when instruments are commissioned or diagnosed digitally. A cloud SCADA platform such as Merobix reads the analog value at the RTU or edge device regardless of whether HART is present, but crews increasingly rely on HART to configure, range, and health-check transmitters in place. When a technician taps into a loop with a handheld or a HART modem and the reading starts clipping at the high end, the extra burden of the communication resistance is the likely culprit - the loop had just enough headroom for the analog signal and not enough for the added 250 ohms. Knowing that keeps the diagnosis on the loop budget rather than sending a crew after a phantom instrument fault.
Everything in series with the loop current: the sense or load resistor, any intrinsic-safety barrier's internal resistance, series panel indicators, a signal isolator's input resistance, and the resistance of the cable out and back. Loop burden is the simple sum of all of these. The load resistor is usually the biggest single term, and cable resistance is the one most often forgotten on long runs.
HART overlays a digital signal on the 4-20 mA current, and the receiving equipment reads that digital signal as a voltage developed across a series resistance, conventionally 250 ohms. That resistor is a deliberate part of the loop burden - it converts the HART current into a readable voltage. At 20 mA it drops 5 volts, so it consumes a significant portion of the loop's voltage budget and must be accounted for in the burden total.
The transmitter runs out of voltage headroom before reaching 20 mA. The lower part of the range still works because less voltage is needed there, but as the output tries to climb toward full scale the current flattens and the reading clips below 20 mA. The fix is to lower the burden - smaller load resistor, heavier or shorter cable, lower-resistance barrier - or to raise the supply voltage within rated limits.
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