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Instrumentation · Signal Types · 4-20 mA Current Loop

4-20 mA Current Loop Explained: How It Works, Wiring, Troubleshooting and Common Mistakes

A complete beginner-to-intermediate guide to the 4 to 20 mA current loop: why it became the global standard, how Ohm's law makes it work, 2-wire vs 4-wire transmitters, loop-powered vs externally-powered, the mA-to-percentage formula, NAMUR fault levels, HART on a current loop and field troubleshooting.

Why 4 mA Not 0 mA 2-Wire vs 4-Wire mA Formula with Examples Troubleshooting Table

4 to 20 mA Current Loop

Ask any instrumentation engineer in the world what signal connects a field transmitter to a control system and the answer will almost always be the same: 4-20 milliamps. The 4-20 mA current loop has been the universal standard for transmitting process measurements in industrial plants since the 1960s. It connects pressure transmitters, temperature transmitters, flow meters, level instruments and analyser outputs to DCS, PLC and SCADA systems on every continent.

The 4-20 mA loop is so fundamental to process instrumentation that every other signal type (HART, Foundation Fieldbus, even wireless) is compared to it or built on top of it. Yet many engineers and technicians who use 4-20 mA loops every day have never fully understood why it works the way it does: why the signal starts at 4 mA instead of 0, why current is used instead of voltage, what loop power actually means, and why the loop still works accurately over 1,000 metres of cable.

This guide explains all of it from first principles, clearly and practically. By the end you will understand how to calculate the mA value for any measurement, how to wire both 2-wire and 4-wire transmitters, how to read NAMUR fault signals, and how to diagnose the most common field problems with a multimeter. For the broader context of analog signals in process control, see our article on analog vs digital signals in instrumentation.

What this guide covers

Where the 4-20 mA standard came from and why it replaced pneumatic signals · How Ohm's law explains the current loop · Why current is used instead of voltage · Why the signal starts at 4 mA not 0 mA · The five components of a current loop · 2-wire vs 4-wire transmitters · Loop-powered vs externally-powered transmitters · mA to percentage formula with worked examples · NAMUR NE43 fault signal levels · HART on the 4-20 mA loop · Ground loops · Troubleshooting common problems.

Where Did the 4-20 mA Standard Come From?

To understand why 4-20 mA exists, you need to know what came before it. Before electronic instrumentation, process plants used pneumatic control signals: compressed air at 3 to 15 psi (pounds per square inch) carried measurement information from field instruments to control rooms through copper tubing. At 3 psi the measurement was at 0%, at 15 psi it was at 100%.

The 3 psi live zero was intentional. It meant a working system always had at least 3 psi of air pressure present. If the air supply failed or a tube ruptured, the signal dropped to 0 psi, which was instantly distinguishable from the minimum measurement value. The live zero made fault detection simple and reliable.

In the 1950s and 1960s, as electronics became affordable, the process industry needed an electrical equivalent of the pneumatic signal. Engineers designed the 4-20 mA standard deliberately to mirror the logic of the pneumatic system: a live zero at 4 mA (equivalent to 3 psi), a maximum of 20 mA (equivalent to 15 psi) and the same fault-detection principle. A working loop always carries at least 4 mA. A broken wire or dead transmitter drops to 0 mA, immediately signalling a fault.

The parallel between pneumatic and electronic signals

Pneumatic: 3 psi = 0% measurement, 15 psi = 100% measurement, 0 psi = fault

Electronic: 4 mA = 0% measurement, 20 mA = 100% measurement, 0 mA = fault

The logic is identical. The 4-20 mA standard was consciously designed to replace pneumatic signals with the same live-zero fault detection built in from the start.

Why Current, Not Voltage? Ohm's Law Explains It

The most important question in understanding the 4-20 mA loop is: why send a current signal instead of a voltage signal? The answer comes directly from Ohm's Law: V = I × R.

When you send a signal as a voltage (for example, 1-5 V), that voltage must travel through the resistance of the cable. The cable has electrical resistance, and Ohm's law tells us that voltage drops across resistance (V = I × R). The longer the cable, the higher the resistance, and the larger the voltage drop. By the time the signal reaches the DCS, the voltage has fallen below its original value. The receiver sees a lower voltage than the transmitter sent, creating a measurement error. This error gets worse the longer the cable.

A current signal solves this problem completely. In a series circuit (which is exactly what a 4-20 mA loop is), the current is the same at every point in the loop regardless of cable resistance. It does not matter whether the cable is 10 metres or 1,000 metres long: the current at the DCS input is identical to the current the transmitter set. The resistance of the cable affects the voltage distribution around the loop, but it does not change the current magnitude that the transmitter controls.

Figure 1: Why Current Beats Voltage Over Long Cable Runs

Figure 1: A voltage signal degrades with cable resistance. The DCS receives a lower voltage than the transmitter sent. A current signal stays identical throughout the loop regardless of cable resistance. This is why current is used for long-distance signal transmission.

The water pipe analogy

Think of a 4-20 mA loop like water flowing through a pipe. The pump pressure is the voltage (24V DC supply). The water flow rate is the current (4-20 mA). Even if the pipe has bends, restrictions and rough sections that reduce the pressure at various points, the flow rate remains the same throughout the pipe. The transmitter is a valve that controls how much water flows. The DCS measures the flow rate, always accurately, regardless of what happens to the pressure (voltage) around the loop.

Watch: The Fundamentals of 4-20 mA Current Loops (Video)

This recorded webinar from Precision Digital is one of the clearest introductory explanations of the 4-20 mA current loop available. It covers Ohm's law, loop components and practical applications:

The Fundamentals of 4-20 mA Current Loops: Precision Digital Corporation

The Five Components of a 4-20 mA Current Loop

Every 4-20 mA loop contains the same five elements. Understanding each one makes wiring, commissioning and troubleshooting much simpler.

1. Sensor (Measuring Element)

Detects the physical process variable: pressure, temperature, level, flow, etc.
Converts the physical quantity into an electrical signal (resistance change, millivolt, capacitance, etc.)
Examples: RTD, thermocouple, diaphragm capsule, piezoelectric element, float
The sensor is the input to the transmitter. The two are often integrated into a single instrument.

2. Transmitter (Signal Converter)

Converts the sensor signal into a 4-20 mA output proportional to the measurement
In a 2-wire loop-powered transmitter, the transmitter both controls the current AND is powered by that same current
Configured with the Lower Range Value (LRV) and Upper Range Value (URV) that define the 4 mA and 20 mA points
Modern smart transmitters also carry HART digital data on top of the 4-20 mA signal

3. Power Supply (Loop Supply)

Provides the DC voltage to drive current through the loop. Standard voltage: 24V DC.
The power supply must have enough headroom to power all loop components at the maximum current (20 mA) plus a safety margin
Common voltage range for transmitters: 12V DC minimum to 42V DC maximum (check datasheet)
In most DCS systems, the AI card provides 24V DC loop power built in. No external supply is needed.

4. Cable (the "Loop")

The two-wire twisted pair cable connecting transmitter to DCS. Called the loop wire.
Standard size: 0.5 mm² to 1.5 mm² twisted pair, overall shielded
Cable resistance is a real but manageable factor: a 1,000 m run of 0.5 mm² cable has about 70 ohms of resistance
The shield must be grounded at ONE end only to prevent ground loops
Polarity matters: always connect positive (+) to positive and negative (-) to negative at every termination

5. Receiver (DCS / PLC Input Card)

The device that reads the 4-20 mA signal and converts it to a digital value for the control system
Contains a precision 250 ohm input resistor. Ohm's law converts the current to a voltage: V = I × R. At 4 mA: V = 0.004 × 250 = 1.000 V. At 20 mA: V = 0.020 × 250 = 5.000 V.
An analogue-to-digital converter (ADC) inside the card digitises the voltage and scales it to engineering units in the DCS database
Most modern DCS AI cards provide loop power (24V DC) and the 250 ohm sense resistor in one card
HART communication also uses this 250 ohm resistor as the minimum impedance for HART signal detection. See our full guide on how HART protocol works.

The Complete 4-20 mA Loop: How It All Connects

Figure 2: Complete 4-20 mA Two-Wire Loop: All Five Components

Figure 2: The complete 4-20 mA two-wire loop. The DCS AI card provides 24V DC and contains a 250 ohm resistor. The transmitter controls how much current flows, proportional to the measurement. Current is identical throughout the loop. The junction box is where field cables meet panel cables.

2-Wire vs 4-Wire Transmitters: What Is the Difference?

One of the most common questions when commissioning an instrument is whether the transmitter is 2-wire or 4-wire. These are fundamentally different wiring arrangements with different power requirements.

Feature	2-Wire (Loop-Powered)	4-Wire (Externally-Powered)
Number of wires	Two wires only. Both power and signal on the same pair.	Four wires. Two for power supply, two for the 4-20 mA signal output.
How it is powered	The transmitter draws its operating power directly from the 4-20 mA loop current. It must operate on the minimum loop current (as low as 3.5 mA at zero output).	The transmitter has its own separate AC or DC power supply input. The 4-20 mA output is an independent active source.
Typical power consumption	Very low. 3.5 to 4 mA is enough to power the electronics.	Higher. Can draw 100-500 mA from the separate supply.
Wiring cost	Lower. Only one cable pair needed between transmitter and DCS.	Higher. Requires an additional power supply cable to the field.
Typical instruments	Most modern pressure, level and temperature transmitters. Compact and field-mounted.	Gas analysers, chromatographs, complex multi-sensor devices, older instruments. Any instrument requiring significant processing power.
DCS input card compatibility	Works directly with standard DCS AI card that provides loop power.	Requires a DCS AI card in "passive" or "current sensing" mode (no loop power). The transmitter drives the current actively.
Output type	Passive current source. The DCS provides the voltage to drive the current.	Active current source. The transmitter itself drives the current into the loop.

Critical wiring mistake: connecting a 4-wire transmitter to a loop-powered input

If you connect a 4-wire (externally-powered, active) transmitter to a DCS input card that is also providing loop power, you will have two voltage sources in the same loop. Both will try to drive the current simultaneously. This creates a conflict that can damage the transmitter, the DCS card or both. Always check the transmitter datasheet: if it says "active output" or "externally powered," the DCS input card must be set to passive mode (no loop power supplied by the card).

The 4-20 mA Formula: Converting mA to Engineering Units

Every instrumentation engineer must be able to quickly convert a mA reading to a percentage of range or an engineering unit value, and vice versa. There is one formula that does everything.

The universal 4-20 mA formula

% of range = (mA reading - 4) / 16 × 100

Engineering value = LRV + [(mA reading - 4) / 16 × Span]

Where: LRV = Lower Range Value (value at 4 mA) | Span = URV minus LRV | URV = Upper Range Value (value at 20 mA)

Figure 3: The 4-20 mA Scale with Key Calculation Points

Figure 3: The 4-20 mA scale with percentage values and NAMUR NE43 fault zones. Below 3.6 mA and above 21.0 mA indicate faults. The saturated zone (20.5-21.0 mA) indicates the measurement is above the upper range.

Worked examples

mA reading	% calculation	Example: 0 to 10 bar range	Example: 0 to 200°C range
4.000 mA	(4-4)/16 × 100 = 0%	0.0 bar (tank empty)	0.0°C
8.000 mA	(8-4)/16 × 100 = 25%	2.5 bar	50.0°C
12.000 mA	(12-4)/16 × 100 = 50%	5.0 bar	100.0°C
16.000 mA	(16-4)/16 × 100 = 75%	7.5 bar	150.0°C
20.000 mA	(20-4)/16 × 100 = 100%	10.0 bar (full range)	200.0°C
Below 3.6 mA	NAMUR NE43 fault zone	Broken wire / transmitter failure	Broken wire / transmitter failure
Above 21.0 mA	NAMUR NE43 fault zone	Overrange / transmitter fault	Overrange / transmitter fault

NAMUR NE43: The Standard for Fault Signalling

The 4 mA live zero gives a simple yes/no fault indication: if the loop reads below 4 mA, something is wrong. But NAMUR NE43 (NAMUR Recommendation NE43) goes further, defining specific signal levels for four different alarm conditions and leaving the full 4-20 mA range available for process measurement.

Signal level	NAMUR NE43 condition	What it means	DCS alarm
Below 3.6 mA	Sensor/transmitter failure or broken wire	The transmitter cannot produce a valid signal. The loop current has dropped below the minimum possible operating current. Broken wire, failed electronics or lost sensor.	Hardware fault alarm
3.6 to 3.8 mA	NAMUR Low Alarm	Measurement is below the lower range value. Process has gone below the minimum measurable value OR the transmitter has detected a sensor failure and is driving the output downscale (burnout down).	Low low alarm
4.0 to 20.0 mA	Normal measurement range	The transmitter is operating normally and the signal represents a valid process measurement between 0% and 100% of the configured range.	No alarm
20.5 to 21.0 mA	NAMUR Saturation / above range	Measurement is above the upper range value. The process has exceeded the maximum measurable value OR the transmitter has detected a sensor failure and is driving upscale (burnout up).	High high alarm
Above 21.0 mA	Transmitter fault	The transmitter has detected an internal fault and is driving the output above the saturation level to signal a hardware error. This is outside the saturation zone and indicates a definite fault.	Hardware fault alarm

The burnout function on a temperature transmitter uses the NAMUR signal levels to indicate sensor failure. When a thermocouple opens, the transmitter drives its output either up (above 20.5 mA) or down (below 3.8 mA) to signal the fault. See our article on the burnout function in temperature transmitters for more detail.

Ground Loops: A Common Problem on 4-20 mA Installations

A ground loop is an unwanted electrical current that flows through signal cable shields or through the earth path of a loop when two pieces of equipment in the same loop are grounded to earth at different points. The stray current adds to or subtracts from the legitimate 4-20 mA signal, creating a measurement offset that can be impossible to calibrate out.

Ground loops are one of the most common causes of unexplained measurement errors and signal noise in process plant instrumentation. They are particularly common when:

Cable shield is grounded at both ends (always ground the shield at ONE end only, typically the control room end)
The transmitter housing is grounded to local earth AND the DCS card is also grounded to panel earth at a different earth potential
Long cable runs pass through areas with different electrical earthing systems
Multiple instruments share a common earth return path

How to prevent ground loops

Ground the cable shield at ONE end only. Always connect it at the control room or marshalling cabinet end, never at the field instrument end. If the transmitter housing is grounded locally for lightning protection, use a galvanic isolator in the loop to break the DC current path between the field earth and the panel earth. Galvanic isolators are also called loop isolators or signal isolators, and they are the standard solution for ground loop problems in process instrumentation.

Troubleshooting 4-20 mA Loop Problems with a Multimeter

A calibrated multimeter set to DC milliamps (or DC millivolts measured across the 250 ohm resistor) is all you need to diagnose almost any 4-20 mA loop problem in the field.

Symptom at DCS	Likely cause	What to check with multimeter	Fix
Loop reads exactly 0 mA or below 3.6 mA	Broken wire, blown loop fuse, dead transmitter, reversed polarity, no loop power	Measure DC voltage at transmitter terminals. Should be 12V DC minimum. If zero, no loop power. Check voltage at DCS card output terminals.	Check fuse on DCS card or power supply. Check cable continuity. Verify polarity at all termination points. Replace transmitter if power is present but output is still zero.
Loop reads exactly 20 mA at all times (pegged high)	Transmitter in fixed-output mode (HART simulation). Transmitter electronics failed high. Short circuit in field wiring that is bypassing the transmitter control.	Connect HART communicator and check for fixed current mode or simulation mode active. Check for short to 24V on field cable.	Exit simulation or fixed-current mode via HART communicator. Check field cable for short circuit to supply wire. Replace transmitter if electronics failed.
Loop reading is constant but wrong (offset from expected value)	Wrong LRV or URV configured in transmitter or DCS. Transmitter zero or span needs calibration trim. Ground loop adding stray current.	Use HART communicator to read the transmitter's own digital PV value and compare to DCS reading. If they match but both are wrong, the issue is configuration. If they differ, the issue is wiring or ground loop.	Correct LRV and URV in transmitter configuration. Perform sensor trim if needed. Add galvanic isolator if ground loop is suspected.
Noisy, fluctuating signal despite stable process	Electrical interference from nearby VFDs, motors or power cables. Cable shield grounded at both ends causing ground loop. Loose terminal connections.	Check that cable shield is only grounded at one end. Inspect all terminal connections for looseness or corrosion. Route cable away from power cables.	Ground shield at control room end only. Tighten all terminal connections. Re-route cable in separate conduit away from power wiring. Add ferrite filters on cable near noise sources.
Loop works on the bench but not in the field	Total loop resistance too high for the transmitter's voltage compliance. Long cable plus additional devices in loop pushing total resistance above transmitter's compliance voltage limit.	Add up all resistance in the loop: cable resistance + DCS input resistor (250Ω) + any barriers or isolators. Total resistance × 0.020 A (max current) must be less than the transmitter's compliance voltage (typically 12-18V available at transmitter terminals).	Calculate total loop resistance. Increase power supply voltage. Reduce cable resistance by using thicker cable or shorter run. Remove unnecessary barriers from the loop.

The fastest field test: measure mA directly at the transmitter

Break the loop at the transmitter terminals and insert a calibrated milliammeter in series. Put the process at a known condition (for example, a temperature bath at 25°C) and read the mA directly. This tells you immediately whether the problem is in the transmitter itself (wrong mA output) or in the wiring between the transmitter and the DCS (correct mA out but wrong value at DCS). In most cases this single test identifies the fault location within 60 seconds.

Frequently Asked Questions: 4-20 mA Current Loop

Why does the 4-20 mA signal start at 4 mA instead of 0 mA?

The 4 mA live zero serves two purposes. First, it allows fault detection: a working loop always carries at least 4 mA, so a reading below 3.6 mA immediately signals a broken wire or dead transmitter. Zero mA is unambiguously a fault, not a valid measurement. Second, in a two-wire loop-powered transmitter, the transmitter itself draws its operating power from the loop current and needs a minimum of 3.5 to 4 mA to keep its electronics running even when the measurement is at zero.

Why is current used instead of voltage for long-distance signal transmission?

In a series circuit, current is the same at every point regardless of cable resistance. A voltage signal degrades as it travels through cable resistance (V = I x R), arriving at the receiver lower than it was sent. A current signal is controlled by the transmitter and stays constant throughout the loop, making it immune to cable length effects and small variations in cable resistance.

What is the difference between a 2-wire and 4-wire transmitter?

A 2-wire (loop-powered) transmitter uses the same two wires for both power and the 4-20 mA signal. The transmitter draws its operating power from the loop current. A 4-wire transmitter has a separate power supply input (two wires) and a separate 4-20 mA signal output (two wires). Connecting a 4-wire transmitter to a loop-powered DCS input can damage both devices.

What does 12 mA mean on a 0-100 bar transmitter?

12 mA is the midpoint (50%) of the 4-20 mA range. Using the formula: % = (12-4)/16 × 100 = 50%. For a 0-100 bar range with LRV = 0 and URV = 100, the pressure reading is 0 + (50% × 100) = 50 bar. The midpoint of the mA range always corresponds to the midpoint of the measurement range.

What is a ground loop and how do I fix it?

A ground loop occurs when a signal cable shield or the signal current return path is connected to earth at more than one point, creating a path for stray earth currents to flow through the signal wiring. These stray currents add measurement error or noise to the 4-20 mA signal. Fix it by grounding the cable shield at one end only (the control room end) and using a galvanic isolator to break the DC path between field earth and panel earth.

What does below 3.6 mA mean on a 4-20 mA loop?

According to NAMUR NE43, a signal below 3.6 mA indicates a hardware fault condition. It means the transmitter cannot produce a valid signal. This is typically caused by a broken wire, dead transmitter electronics, blown fuse, no loop power, or a sensor failure that has driven the output downscale. It is never a valid process measurement.

4 to 20 mA Current Loop Explained: How It Works, Wiring and Troubleshooting