By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 31, 2026
A loop-powered device draws its operating power from the 4-20 mA signal loop itself, with no separate power connection. This works because the current is identical at every point in a series loop. The voltage drop a device takes for itself therefore does not change the signal. In practice the term means the same thing as a 2-wire device.
The idea is simple; the engineering is in the budgets. Every loop-powered device must run its electronics on less than 3.5 mA and must leave enough voltage for everything else in the loop. This guide works through both budgets with real numbers, compares 2-wire, 3-wire and 4-wire instruments, and covers loop-powered indicators, hazardous areas and wiring. For how the loop itself carries the signal, start with our 4-20 mA current loop guide.
Contents
- What loop powered means
- The 3.5 mA budget
- Voltage budget
- Worked example
- 2-wire vs 4-wire
- Loop-powered indicators
- Hazardous areas
- Wiring the loop
- FAQ
What loop powered means
A 4-20 mA loop is a series circuit: one power supply, one transmitter regulating the current, and one or more receivers. Because the circuit is a series loop, the same current flows through every device. A device can therefore sit in the loop, take a voltage drop across its terminals, and run its own electronics from that drop. The signal passes through untouched.
That is all “loop powered” means. The transmitter measures the process and regulates the loop current. A loop-powered indicator displays that current as engineering units. Both live entirely off the two signal wires: no power terminals, no third conductor, no local supply.
The 3.5 mA budget
The first constraint is current. A loop-powered device cannot consume more current than the loop guarantees, and the loop guarantees very little. The signal floor is 4 mA, and fault signaling per NAMUR NE43 uses 3.6 mA or less to report a failure.
If the device’s own electronics drew 4 mA, the loop could never fall to a fault level. Designers therefore hold the quiescent draw below about 3.5 mA.
That budget caps what the electronics can do. At 4 mA and a 12 V terminal drop, a transmitter has 48 mW to work with. A display taking a 3 V drop has 12 mW.
It is enough for an LCD, a piezoresistive cell or an open-collector output. It is not enough for an LED display, a mechanical relay or a heated sensor. Those features push you to externally powered instruments.
The same logic rules out zero-based ranges. A 0-20 mA device would have no current at all to live on at the bottom of its range. A true loop-powered device is therefore always a live-zero, 4-20 mA device.
Voltage budget
The second constraint is voltage. Every device and every meter of wire in the loop takes a voltage drop. At 20 mA, the sum of those drops must stay below the supply voltage. The transmitter states this as a minimum terminal voltage, often called the lift-off or compliance voltage; a common figure is 10.5 to 12 V.
Rmax = (Vsupply − Vmin) / 0.020 A
The formula turns the budget into a maximum loop resistance. With a 24 V supply and a 12 V transmitter minimum, the loop can carry up to 600 ohms of load and wire. The table assumes that 12 V minimum; substitute the figure from your transmitter datasheet.
| Supply voltage | Max loop resistance | Room for |
|---|---|---|
| 24 V DC | 600 ohms | One 250 ohm input plus an indicator and wire |
| 28 V DC | 800 ohms | Two 250 ohm inputs plus an indicator |
| 36 V DC | 1200 ohms | Long cable runs and several series devices |
Computed as (supply − 12 V) / 20 mA. Keep the working total below about 80 percent of the limit to ride through supply variation. HART communication also needs at least 230 ohms in the loop.
Worked example
A 24 V supply feeds a 2-wire pressure transmitter with a 12 V minimum terminal voltage. The loop also carries a loop-powered field indicator with a 3.0 V maximum drop, a 250 ohm analog input at the control system, and 10 ohms of cable resistance out and back.
Drops at 20 mA = 5.0 V (250 ohm) + 3.0 V (indicator) + 0.2 V (cable) = 8.2 V
Left for the transmitter = 24 − 8.2 = 15.8 V, above the 12 V minimum
The loop works, with 3.8 V of margin. Now add a chart recorder with another 250 ohm input. Drops rise to 13.2 V, the transmitter is left 10.8 V, and the loop fails at the top of its range, where readings matter most.
The fixes, in order: feed the recorder from the control system, raise the supply toward 28 V within device ratings, or add an externally powered repeater. Our 4-20 mA loop calculator runs this arithmetic for any combination.
2-wire vs 4-wire
The wire count tells you where the power comes from. The table below sorts the terms that get mixed up in the field.
| Term | What it means |
|---|---|
| 2-wire, loop powered | Same thing. The signal pair carries both power and signal; lowest cost to wire |
| 3-wire | Separate DC supply lead with a shared common; watch for ground loops on the common |
| 4-wire | Fully separate power, AC or DC; the output loop floats, which avoids ground loops |
| Active vs passive | Which side sources the loop voltage; exactly one device in the loop is active |
| Sink vs source | A 2-wire transmitter sinks current; a 4-wire output sources it |
Whether an instrument can be loop powered comes down to its power demand. A pressure, temperature or hydrostatic level transmitter measures with milliwatts, so 2-wire is the norm. Browse our pressure transmitters and you will find 2-wire 4-20 mA as the standard output.
A magnetic flow meter has to drive its coils, so its converter is externally powered, and ultrasonic and Coriolis converters are the same. When someone asks for a loop-powered magnetic flow meter, the practical answers are a battery-powered version or an external supply.
Loop-powered indicators
The most common loop-powered device after the transmitter is the indicator: a local or panel display wired in series that shows the loop current in engineering units. An analog movement takes about 1 V from the loop; a typical digital LCD indicator takes up to about 3 V. Datasheets state this burden in several formats, so normalize before you budget. A maximum voltage drop, a voltage at 20 mA and an equivalent input resistance all describe the same thing.
The limits from the 3.5 mA budget apply in full. A loop-powered indicator can give you an LCD readout, a bargraph and open-collector alarm outputs. It cannot give you a bright LED display, a mechanical relay to start a pump, or a retransmitted signal; the loop cannot feed those loads. The moment a display has to switch something, plan on local power.
Application example
Agricultural irrigation, United States. A farm operator asked for two deep-well water level sensors with digital displays, and wanted the displays to trigger pump relays directly.
We explained the split: the 2-wire hydrostatic level transmitter runs loop powered down the well on its signal pair. A relay, though, is a switched load the loop cannot feed. We proposed externally powered indicators with relay outputs at the well head, keeping the sensor side 2-wire and the switching side on local power.
Hazardous areas
Low power is a safety feature. Because a loop-powered device handles milliwatts, it is a natural candidate for intrinsically safe and nonincendive approvals. That is why 2-wire transmitters are the default in classified areas.
One budget note: an intrinsic safety barrier sits in the loop and takes its own voltage drop, often several volts across its end-to-end resistance. Add the barrier to the voltage budget like any other series device, or the loop that worked on the bench will starve in the field. For hazardous-area service, see our HART pressure transmitter line for smart 2-wire options.
Wiring the loop
Wiring a loop-powered device is simple by design: supply positive to the transmitter positive, transmitter negative onward through each series device, and back to supply negative. Run shielded twisted pair with the shield grounded at one end only, and observe polarity. Most 2-wire devices are diode-protected against reversal but will simply not run backward.
Series order does not matter electrically, since the same current flows everywhere. Put the indicator where a technician can see it. For terminal-by-terminal diagrams of 2-wire, 3-wire and 4-wire hookups, see the pressure transducer wiring diagram guide.
FAQ
What are loop-powered devices?
Loop-powered devices are 2-wire instruments that draw their operating power from the 4-20 mA signal loop instead of a separate supply. Common examples are process transmitters, field and panel indicators, and signal isolators. They work because a series loop carries the same current everywhere, so a device’s voltage drop does not alter the signal.
What is a 4/20mA loop powered?
A loop-powered 4-20 mA arrangement means the field device and the signal share one pair of wires. The control room supply drives the loop, the transmitter regulates the current between 4 and 20 mA to represent the measurement, and the transmitter’s electronics live on less than 3.5 mA of that current.
What power supply is needed for 4-20mA loop?
24 V DC is the standard choice. The supply must exceed the transmitter’s minimum terminal voltage plus every series drop at 20 mA. With a 12 V transmitter minimum, 24 V supports up to 600 ohms of load and wire. Use 28 to 36 V within device ratings when the loop carries more resistance.
What is a 4-20mA loop connection?
It is a series circuit: power supply, transmitter, and receivers connected in one ring with a twisted pair. Current leaves the supply, is regulated by the transmitter, passes through each receiver in turn and returns to the supply. Every device sees the same current, which is what makes the signal immune to voltage drops.
Request a quote
Send us the measurement, the loop supply voltage and what else sits in the loop, and we will confirm the terminal voltage math along with the quote. Tell us the application and we configure one unit, not a shelf part. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.