What Is 4-20 mA Loop Power?
4-20 mA loop power is the elegant trick behind the two-wire transmitter: one pair of wires delivers the device's power and carries its measurement signal at the same time. It is why so much field instrumentation needs only a single twisted pair. This guide explains how loop power works, the difference between two-wire and four-wire devices, sink versus source wiring, and what loop burden means in oil and gas.
4-20 mA Loop Power in one line: 4-20 mA loop power is a scheme where a two-wire transmitter draws its operating power from the same current loop it uses to signal. A DC supply drives the loop, and the transmitter regulates the current between 4 and 20 mA to represent its reading while living on that current.
How Two-Wire Loop Power Works
A loop-powered transmitter has just two terminals. A DC power supply - commonly 24 V - sits in the loop and drives a current through the transmitter, its wiring, and a load resistor at the input. The transmitter does not have a separate power feed; instead it controls how much current flows, regulating the loop between 4 and 20 mA to represent its measurement. Its own electronics must run entirely on that current, which is why it must draw at least 4 mA even at the bottom of the range.
That live-zero of 4 mA is deliberate: it guarantees enough current to power the transmitter and, usefully, makes a broken wire (0 mA) distinguishable from a genuine minimum reading. The whole measurement and the device's power share one two-wire pair.
Two-Wire vs Four-Wire Devices
A two-wire (loop-powered) device is limited to the small power budget the 4 mA minimum allows, which suits simple transmitters. A four-wire device has separate terminals for power and signal - two wires for a dedicated supply and two for the 4-20 mA output. Four-wire is used when the instrument needs more power than a loop can provide, such as an analyzer, a display-heavy device, or one driving multiple outputs.
The trade is wiring and power: two-wire minimizes cabling and is the default for basic field transmitters, while four-wire adds a power run but removes the power-budget limit. Selecting the right type is part of instrument specification.
Sink, Source, and Loop Burden
When wiring a loop you must match how the transmitter and the input handle current direction - sinking versus sourcing. A sourcing output pushes current out; a sinking input pulls it. Mismatched wiring means the loop will not conduct, so the device documentation defines which is which. Getting this right is a common field wiring detail.
Loop burden is the total resistance the supply must drive - the input load resistor plus wiring resistance plus any HART resistor, isolators, or barriers in the loop. The supply voltage must be high enough to push 20 mA through all of it while leaving the transmitter its minimum operating voltage; otherwise the loop saturates below full scale. A cloud SCADA such as Merobix reads the value the controller digitizes from this loop - the loop power and burden are field electrical details handled below the SCADA layer.
Sizing a Loop: A Worked Example
The arithmetic behind loop sizing is Ohm's law and nothing more. The supply must satisfy one inequality at full scale: supply voltage minus the voltage dropped across everything in the loop at 20 mA must still exceed the transmitter's minimum operating voltage, a figure that comes from the manufacturer's datasheet. Each element drops voltage in proportion to its resistance. At 20 mA, a 250 ohm load resistor - the conventional value that turns 4-20 mA into a 1-5 V signal - drops 5 V by Ohm's law; the wiring drops current times its loop resistance; and any isolator or barrier adds the insertion drop its datasheet states.
So the check runs: total the load resistor drop at 20 mA, the wiring drop, and the stated drops of any series devices; subtract from the supply voltage; compare what remains against the transmitter's minimum lift-off voltage. If the margin is thin, the loop can read correctly at low currents yet saturate before reaching 20 mA - a fault that looks like a transmitter problem but is a power-budget problem. Long home-run cables and intrinsic safety barriers are the usual margin eaters, and the fixes are a higher supply voltage within the transmitter's rating, heavier conductors, or fewer series elements.
Fault Signaling Outside the 4-20 Band
The live zero enables more than broken-wire detection. NAMUR NE 43 standardizes how a smart transmitter signals its own failure: it drives the loop below the measuring range, at 3.6 mA or less, or above it, at 21 mA or more, to say the value cannot be trusted, with the direction selectable so the system fails toward the safe state for that measurement. For this to be useful, the receiving analog input must be configured to treat those bands as fault rather than clamping them - an RTU that silently clamps 3.6 mA to the bottom of range converts a declared sensor failure into a plausible-looking minimum reading. How the input card handles out-of-band current belongs on the commissioning checklist, alongside the fundamentals in the 4-20 mA current loop reference.
Loop Checks at Commissioning
- With the loop powered, verify polarity and measure the actual loop current with a meter in series or a loop clamp meter - not just the voltage at the input terminals.
- Use the transmitter's test mode or a loop calibrator to drive fixed currents, for example 4, 12, and 20 mA, and confirm the value the controller digitizes at each point.
- Confirm the engineering-unit scaling in the RTU or PLC matches the transmitter's configured range; a range mismatch passes a check at 4 mA and lies everywhere else.
- Force an out-of-band current and confirm the system flags it as a fault rather than a reading.
- Record as-left values: supply voltage, measured loop resistance, and the current at a known process condition.
The mid-scale point matters: a loop can be right at both endpoints and wrong in between if a scaling stage is misconfigured or a wrong range was entered consistently in two places. If the transmitter is HART-capable, also compare the analog reading against the digital value the device reports; a disagreement between the two points at loop electrical problems rather than the sensor. The digital side of that check is described in the HART protocol guide.
Grounding and Shared Supplies
A current loop is robust, but it is not immune to grounding mistakes. The rule is one ground reference per loop: land the cable shield at a single end, conventionally the panel end, and never let the loop conductors touch ground at two points, or a parallel path forms and part of the signal current bypasses the input. Multiple loops routinely share one supply, and that is fine when the input channels are isolated or share a common properly; but a multi-channel input with a shared common can couple loops together in ways that look like one transmitter disturbing another. When a reading shifts as some unrelated device switches, suspect a shared-common or double-ground path before suspecting the transmitter, and involve the site's electrical personnel before lifting grounds in a classified area.
Frequently Asked Questions
How does a two-wire transmitter get its power?
It draws power from the same 4-20 mA loop it signals on. A DC supply drives current through the loop, and the transmitter runs on that current while regulating it between 4 and 20 mA to represent its reading. That is why it must draw at least 4 mA even at zero.
What is the difference between a two-wire and four-wire transmitter?
A two-wire device draws power and signals over one pair, limited to a small power budget. A four-wire device has separate power and signal wiring, so it can use more power - suited to analyzers and complex instruments - at the cost of an extra cable run.
What is loop burden?
Loop burden is the total resistance the supply must drive: the input load resistor plus wiring, any HART resistor, isolators, and barriers. The supply voltage must push 20 mA through all of it while leaving the transmitter its minimum voltage, or the loop cannot reach full scale.
Why would a loop read correctly at 4 mA but never reach 20 mA?
That is the signature of insufficient loop voltage. At low current the drops across the load resistor and wiring are small, so the transmitter has voltage to spare; as current rises the drops grow, and at some point the transmitter falls below its minimum operating voltage and cannot force more current - the loop saturates. Recompute the budget: supply voltage minus all series drops at 20 mA must exceed the transmitter's datasheet minimum. The cure is more supply voltage within rating, less series resistance, or fewer series devices.
Can two inputs share one 4-20 mA loop?
Electrically yes - current is identical everywhere in a series loop, so a local indicator and an RTU input can both read it. Each added input increases the loop burden, so the power budget must be rechecked, and any device failing open kills the loop for everything in it. The devices also must not create a second ground path between them. It is workable for adding a display; for anything more, a signal isolator or splitter selected per the manufacturer's datasheet is the cleaner answer.
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