Open the datasheet of almost any industrial pressure transducer and the first output listed is 4-20 mA. The standard is so universal that engineers rarely question it — but it exists for concrete reasons that still matter in an age of fieldbus and wireless: a 4-20 mA current loop is immune to voltage drop, shrugs off electrical noise that destroys voltage signals over long runs, powers the instrument over the same two wires that carry the signal, and fails loudly when a wire breaks. No other analog standard gives you all four at once.
This guide explains why 4-20 mA became the industrial default, how to wire and size a two-wire loop correctly, when to choose a different output, and how the loop behaves in the field — including the failure modes that make it safe.
Why 4-20 mA and Not 0-20 mA?
The "live zero" is the defining feature of the standard. The lower end of the range is 4 mA, not 0 mA, and that is deliberate:
- Wire-break detection — if a loop cable snaps, the current falls to 0 mA, which is below the 4 mA live zero. The PLC sees a reading that is physically impossible for the transducer to produce and raises an alarm. With a 0-20 mA system, a broken wire is indistinguishable from a legitimate zero-pressure reading.
- Self-powered electronics — the 4 mA baseline is enough to power the transducer's internal electronics. The instrument is fully alive at zero pressure, so it can report faults, hold calibration data, and respond to commands.
- Headroom for diagnostics — HART digital communication rides on top of the analog loop, and the live zero guarantees there is always current flowing to carry it. Modern transmitters also use out-of-range currents, such as 3.6 mA or 21 mA, to signal specific faults — NE 43 diagnostics — which is only possible because the normal range is a narrow 4-20 band.
In short, 4-20 mA turns the signal itself into a self-monitoring system. That is why the standard has survived since the 1950s through fieldbus, Profibus and wireless eras.
Noise Immunity and Long Cable Runs
Current signals defeat voltage drop and noise in a way voltage signals cannot. In a voltage-output system, the reading at the PLC is the sensor voltage minus the voltage lost across the cable resistance — a 0-10 V transducer on a 300 m run of thin cable can deliver 9.2 V at the PLC even when the process is at full scale, silently biasing the measurement. And an inductive load switching on anywhere near the cable injects spikes that a high-impedance voltage input happily receives.
A current loop is a series circuit: the same current flows through the transducer, the cable and the receiver, so cable resistance has zero effect on the reading, only on the voltage headroom. And because the receiving end is a low-impedance current sense (typically a 250 Ω resistor), induced noise currents are far smaller relative to the 4-20 mA signal than they are relative to a 0-10 V signal. Practical result: reliable runs of 1,000 m and more with ordinary twisted-pair cable, in the same plant where a voltage output would need a local amplifier or a transmitter every few hundred metres.
How a Two-Wire Loop Works
The name says it: two wires carry everything — power in, signal out. The loop is a single series circuit:
→ transducer − terminal → receiver resistor (250 Ω, inside PLC input) → supply (−)
At 0 bar: loop current = 4 mA → 4 mA × 250 Ω = 1.0 V at the ADC
At 10 bar: loop current = 20 mA → 20 mA × 250 Ω = 5.0 V at the ADC
Inside the transducer, the sensing element and conditioning electronics run on the residual voltage — the supply voltage minus the drop across the receiver and the cable. The output stage is a current regulator: it continuously adjusts its internal resistance so that exactly the right current flows, whatever the cable resistance does. That regulation is what makes the loop immune to wiring variations; it is also why a 4-20 mA transducer is sometimes called a "two-wire" instrument — power and signal share the same pair.
Wiring and Loop Sizing
Sizing a loop correctly is a three-number check. The supply voltage must be high enough to power the transducer after every series element takes its share:
- Transducer minimum operating voltage — typically 8-12 V for 4-20 mA transducers (check the datasheet).
- Receiver resistor drop — 20 mA × 250 Ω = 5 V for a standard analog input; some cards use 500 Ω, which needs 10 V.
- Cable resistance drop — 20 mA × round-trip cable resistance; a 1,000 m run of 1.5 mm² wire adds roughly 23 Ω, or under 0.5 V. Thin cable over long runs is where people run out of headroom.
A 24 VDC supply covers all three with margin in almost every plant configuration. If you are feeding multiple series instruments, such as a HART communicator or an indicator in line with the PLC input, add their drops too — and remember the rule of thumb: keep the total series resistance at 20 mA below (supply voltage − transducer minimum) ÷ 20 mA. For a 24 V supply and a 10 V minimum transducer, that allows up to 700 Ω of series load.
Wiring practice matters as much as math: use twisted-pair cable (shields reduce noise pickup), ground the shield at one end only to avoid ground loops, and keep the loop away from power cables and VFD drives. Polarity is marked on the transducer terminals; reversing a two-wire loop simply means no current flows, which the live-zero logic will flag as a fault — one more reason the standard is self-diagnosing.
Wire-Break and Fault Detection in the Field
The live zero pays off the day a cable gets cut. The PLC input reads 0 mA — impossible for a healthy loop — and trips an alarm, so a broken wire is found in seconds instead of being discovered as an unexplained process deviation hours later. The same logic catches a dead supply, a blown fuse, or a transducer that has lost its electronics. Many control systems are configured so that any reading below 3.6 mA or above 20.5 mA triggers a diagnostic, per NAMUR NE 43, and modern HART transmitters can be set to drive the loop to 3.6 mA or 21 mA on specific internal faults, giving maintenance a second channel of information over the same pair of wires.
When Not to Use 4-20 mA
4-20 mA is the default, not the only answer. Consider the alternatives in these cases:
- Short runs inside one cabinet or machine — 0-10 V is simpler (three wires, no loop math) and cheaper at distances under 100 m; HVAC and OEM controls are full of voltage-output transducers for exactly this reason.
- Many sensors on one bus — RS485/Modbus daisy-chains up to 32 devices on two wires plus shield, cutting cable and PLC input cards dramatically in digital plants.
- Smart configuration and diagnostics at distance — HART keeps the 4-20 mA loop but overlays digital communication for remote zero/span and calibration; our PT200 series offers HART and RS485 on a ±0.1% FS platform.
- Wireless or intrinsically safe extremes — battery-powered wireless transmitters and Ex ia loops have their own power budgets; current loops still dominate Ex because a 4-20 mA loop can be certified intrinsically safe with simple barriers.
Output Options Compared
| Output | Wiring | Max distance | Noise immunity | Best for |
|---|---|---|---|---|
| 4-20 mA | 2 wires, loop-powered | 1,000 m+ | Excellent — current is drop-immune | Industrial, PLC/SCADA, long runs, Ex areas |
| 0-10 V | 3 wires | < 100 m | Moderate — voltage drops and picks up noise | HVAC, OEM, local cabinets |
| RS485/Modbus | 2 wires + shield, multi-drop | 1,200 m, 32 devices | Good — differential | Digital plants, sensor networks |
| HART | 2 wires + digital overlay | 1,000 m+ | Excellent — analog loop plus FSK | Process plants needing remote config |
When in doubt, choose 4-20 mA. Every industrial PLC, DCS and SCADA system supports it, it works over the longest distances, and the live zero gives you fault detection for free. The pressure transducer range ships 4-20 mA as standard across the PT100 (0-0.1 to 40 MPa) and PT200 (±0.1% FS) series, with 0-10 V and RS485 available on request.
FAQ
Why is 4 mA used instead of 0 mA?
The 4 mA live zero serves three purposes: it powers the transducer's electronics, it allows wire-break detection (a broken loop reads 0 mA, which is physically impossible for a healthy transducer), and it provides headroom for HART communication and NE 43 fault currents below or above the 4-20 band.
How far can a 4-20 mA signal travel?
In practice 1,000 m and more with ordinary twisted-pair cable, because current is unaffected by cable resistance — the only limit is whether the supply voltage can overcome the cable and receiver drops while keeping the transducer above its minimum operating voltage. The same run with a 0-10 V output would need amplification or a repeater.
Can a 4-20 mA transducer be converted to 0-10 V?
Yes, two ways: install a precision 500 Ω resistor across the PLC input so 20 mA develops 10 V, or specify a voltage-output version of the same transducer. For long runs the loop remains the better choice; convert at the receiving end, not in the field.