4-20mA Current Loop Explained: How It Works and How to Wire It to a PLC
The 4-20mA current loop explained — why current beats voltage, loop components, 2-wire vs 4-wire, live-zero diagnostics, and how to wire and scale it in a PLC.
A 4-20mA current loop is the dominant analog signal standard in process instrumentation. A transmitter converts a physical measurement — pressure, temperature, flow, level — into a DC current that varies proportionally between 4 milliamps (0% of span) and 20 milliamps (100% of span). The current travels around a single pair of wires and is read by a PLC analog input card or a panel meter.
Current loops have wired process plants since the 1950s. Decades later they remain the go-to choice because they are reliable, tolerant of electrical noise, immune to cable resistance, and easy to diagnose — all at low cost.
Try it: convert mA ↔ engineering units ↔ raw counts with the free 4-20mA Scaling Calculator.
What the 4-20mA Loop Is
A current loop is exactly what the name implies: current flows out of a power supply, through a transmitter, through the cable, through a receiving device (a PLC input card or resistor), and back to the supply — completing the circuit. Because current is the same everywhere in a series circuit, the value read at the PLC is identical to the value the transmitter is producing, regardless of wire length or resistance (within limits).
The signal range is 4 mA to 20 mA:
| Current | Meaning |
|---|---|
| 4 mA | 0% of the measured range (live zero) |
| 20 mA | 100% of the measured range |
| < 3.6 mA | Typically indicates a broken wire or transmitter fault |
| > 21 mA | Typically indicates a transmitter fault or over-range condition |
The span of the signal is 16 mA (20 − 4). Any measurement maps linearly onto that 16 mA window.
Why Current, Not Voltage?
Voltage signals (0–10 V, 1–5 V) are simpler to generate but suffer two practical problems in industrial environments.
Noise Immunity
Electromagnetic interference induced by motor drives, contactors, and high-voltage cables adds noise voltage to the signal wires. A 100 mV noise spike on a 0–10 V signal causes a 1% reading error. On a current loop the same interference induces negligible current — the high impedance of the loop limits it. Current loops are therefore standard practice in electrically noisy plant environments.
Voltage Drop Is Irrelevant
Ohm's Law tells us that resistance in a wire creates a voltage drop (V = I × R). If a sensor produces 5 V and the cable to the PLC has 50 Ω of resistance, the voltage at the PLC input is lower than 5 V — a calibration error that changes with cable temperature.
In a current loop the transmitter regulates current, not voltage. The transmitter automatically adjusts its terminal voltage to maintain the commanded current regardless of wire resistance. A cable with 50 Ω extra resistance does not change the 12 mA the transmitter is sending — the PLC still reads 12 mA exactly.
This makes 4-20 mA loops practical over cable runs of hundreds of metres, common in large process plants, pipelines, and water treatment facilities.
Loop Components
Every 4-20 mA current loop contains four elements.
1. Power Supply
A DC power supply — most commonly 24 VDC — drives current around the loop. The supply must provide enough voltage headroom to overcome the voltage drops across the transmitter, the cable resistance, and the burden resistor (see below). Typical loop supplies are 12–36 VDC; 24 VDC is the industrial standard.
2. Transmitter
The transmitter (sensor + signal conditioning electronics) measures the process variable and converts it to a proportional current. Examples include pressure transmitters, temperature transmitters, and types of industrial sensors such as ultrasonic level sensors and flow meters.
A 2-wire transmitter draws its own operating power from the loop — it does not need a separate supply. A 4-wire transmitter has a separate power supply for its electronics and uses the signal wires only for the 4-20 mA output.
3. Receiver (PLC Analog Input Card)
The PLC analog input card measures the loop current. Most modern cards present a differential input and either measure voltage across an internal burden resistor or measure current directly. The card converts the analog value to a digital count (typically 0–4095 for a 12-bit card, 0–32767 for a 15-bit card) that the PLC program reads.
4. Wiring
A single shielded twisted-pair (STP) cable is standard. The shield is grounded at one end only (typically the panel end) to avoid ground loops. Cable resistance must be accounted for when calculating loop voltage headroom.
Loop voltage headroom formula:
V_supply ≥ V_transmitter_min + (I_max × R_cable) + (I_max × R_burden)
Where I_max = 0.020 A and R_cable is the total two-way cable resistance.
2-Wire vs 3-Wire vs 4-Wire Transmitters
Understanding wiring configurations is essential before connecting any transmitter to a PLC analog input card.
| Configuration | Power Source | Signal Wires | Typical Use |
|---|---|---|---|
| 2-wire (loop-powered) | Loop supply (24 VDC) | 2 wires carry both power and signal | Most common for field instruments |
| 3-wire | Separate 24 VDC supply | 1 common, 1 power, 1 signal | Sensors needing more power than the loop allows |
| 4-wire (self-powered) | Separate supply (AC or DC) | 2 power, 2 signal | Analyzers, high-power transmitters |
2-wire (loop-powered) transmitters are the most common in process plants. The transmitter electronics must complete their work while consuming no more current than the 4 mA live-zero value — typically 3.5 mA or less — leaving at least 0.5 mA of margin. This constraint limits what the transmitter can do (bright displays and high-speed processors require more current), but the simplicity of two-wire installation is compelling.
3-wire transmitters add a separate DC positive supply wire while sharing a common negative. They are common for sensors such as pressure transducers that need more power than a 2-wire loop allows but do not carry an isolated supply.
4-wire transmitters power their electronics from an entirely separate supply (often 115/230 VAC or a dedicated 24 VDC rail) and use the two signal wires purely for the 4-20 mA output. Analyzers, chromatographs, and high-precision instruments typically use this arrangement.
Live Zero and Diagnostics: Why 4 mA, Not 0 mA?
The choice to start the range at 4 mA rather than 0 mA is one of the most important design decisions in the standard — and the reason it is called a live-zero signal.
Broken-Wire Detection
If the signal started at 0 mA, a broken wire (which also produces 0 mA) would be indistinguishable from a genuine zero reading. Starting at 4 mA means that 0 mA on a live loop always indicates a wiring or transmitter fault.
Most PLC analog cards include a configurable under-range alarm that triggers when current drops below approximately 3.6 mA. This alarm can drive a fault indicator, suppress the PV display, and prevent control logic from acting on a bad signal — all automatically.
Fault Thresholds (NAMUR NE43)
The NAMUR NE43 recommendation defines standard fault indication levels:
| Signal Level | Status |
|---|---|
| 3.6 – 4.0 mA | Under-range warning |
| 4.0 – 20.0 mA | Normal operation |
| 20.0 – 21.0 mA | Over-range warning |
| < 3.6 mA | Transmitter failure / broken wire |
| > 21.0 mA | Transmitter failure / short circuit |
Configuring these thresholds in the PLC program allows the system to distinguish between "the process is at zero" and "the instrument has failed."
Calculating mA From a Measurement (Span)
Before writing scaling code, understand the linear relationship between current and measurement.
Forward calculation — measurement to mA:
I (mA) = 4 + [(Measurement − Range_Min) / (Range_Max − Range_Min)] × 16
Example: A pressure transmitter is ranged 0–100 bar. What current represents 65 bar?
I = 4 + [(65 − 0) / (100 − 0)] × 16 = 4 + (0.65 × 16) = 4 + 10.4 = 14.4 mA
Reverse calculation — mA to measurement (what the PLC does):
Measurement = Range_Min + [(I − 4) / 16] × (Range_Max − Range_Min)
How to Wire a 4-20mA Transmitter to a PLC Analog Input Card
This section covers the practical wiring of both loop-powered (2-wire) and separately powered (4-wire) transmitters to a typical PLC analog input card.
Understanding Sinking vs Sourcing Inputs
PLC analog input cards are classified by how they handle loop current:
- Sinking input (current sinking): The PLC card is the passive load. Current flows from an external supply, through the transmitter, into the card's positive terminal, and back to the supply via the card's negative terminal. The card measures the voltage across its internal burden resistor.
- Sourcing input (current sourcing): The PLC card provides the loop power internally. The card's positive terminal supplies 24 VDC directly; current flows out, through the transmitter, and back into the card's negative terminal.
Most modern PLC cards support both modes through jumper or software configuration. Check your card's manual for the default.
Wiring a 2-Wire (Loop-Powered) Transmitter — Sinking Card
24 VDC (+) ──────────────────── Transmitter (+)
Transmitter (-)
│
PLC AI (+) terminal
PLC AI (−) terminal
│
24 VDC (−) / COM ────────────────────┘
Step-by-step:
- Connect the positive 24 VDC supply to the + terminal of the transmitter.
- Connect the transmitter − terminal to the PLC analog input + terminal (this is the signal return carrying the 4-20 mA).
- Connect the PLC analog input − terminal (or COM) back to the 24 VDC supply negative.
- Ground the cable shield at the panel end only.
The PLC card measures the current flowing through its internal burden resistor (typically 250 Ω). Across 250 Ω, 4 mA produces 1 V and 20 mA produces 5 V — resulting in a 1-5 V signal at the card's differential inputs. Some cards expose the burden resistor terminals so you can add an external 250 Ω resistor and read voltage with a voltmeter for commissioning checks.
Wiring a 4-Wire (Self-Powered) Transmitter — Sinking Card
Transmitter AC/DC supply ──── Transmitter PWR+
Transmitter PWR−
Transmitter output (+) ──────── PLC AI (+) terminal
Transmitter output (−) ──────── PLC AI (−) terminal / COM
The transmitter's output terminals carry only the 4-20 mA signal; its internal electronics are powered separately. Connect signal (+) and signal (−) directly to the PLC analog input terminals. Polarity matters — reversing these wires drives current backward and the card reads nothing or a fault.
The 250 Ohm Burden Resistor
If your PLC card has a 0-10 V analog input (not a dedicated current input), you need an external 250 Ω precision resistor (0.1% tolerance, 0.5 W minimum) across the input terminals to convert current to voltage:
V = I × R → 4 mA × 250 Ω = 1 V; 20 mA × 250 Ω = 5 V
This converts the 4-20 mA loop to a 1-5 V signal that a standard voltage input reads correctly.
Scaling Raw Counts to Engineering Units in the PLC
Once the current is wired into the card, the PLC firmware converts the analog current to a raw digital count. You must then scale that count to a meaningful engineering unit (bar, °C, m³/h, etc.).
Understanding Raw Counts
A 12-bit card produces counts from 0 to 4095. A 15-bit card produces 0 to 32767. Most cards map the full 4-20 mA range across the full count range, but some map 0-20 mA or use offset representations. Always check the card's documentation.
Assuming a 12-bit card mapped 4 mA = 0 counts, 20 mA = 4095 counts:
Linear Scaling Formula
EU_Value = EU_Min + (Raw_Count / Count_Max) × (EU_Max − EU_Min)
Structured Text Example (IEC 61131-3)
(* Scale a 12-bit PLC analog input (0–4095 counts) to engineering units *)
(* Transmitter range: 0.0 to 100.0 bar *)
VAR
rawCount : INT; (* Raw value from AI card, e.g. AI_Channel[0] *)
pressure : REAL; (* Scaled engineering-unit result *)
euMin : REAL := 0.0;
euMax : REAL := 100.0;
countMin : REAL := 0.0;
countMax : REAL := 4095.0;
END_VAR
(* Linear scaling *)
pressure := euMin + ((REAL(rawCount) - countMin) / (countMax - countMin))
* (euMax - euMin);
(* Clamp to valid range to handle minor over/under-range *)
IF pressure < euMin THEN pressure := euMin; END_IF;
IF pressure > euMax THEN pressure := euMax; END_IF;
Ladder Logic Approach
Most PLC vendors provide a built-in SCALE or SCL instruction that performs the same linear mapping. In Allen-Bradley ControlLogix the Scale instruction or a CPT (Compute) block implements the same formula. In Siemens TIA Portal, the SCALE_X function block maps a raw integer to a real-number range. Consult your PLC vendor's instruction reference for the specific function name and parameter order.
Handling the Live-Zero Offset
If the card returns 0–4095 across the full 0-20 mA range (rather than the 4-20 mA portion only), the count representing 4 mA is not 0. For a 12-bit card over 0-20 mA:
4 mA corresponds to count = (4/20) × 4095 = 819
In that case set countMin := 819.0 and countMax := 4095.0 in the scaling formula above.
Broken-Wire Detection in the PLC Program
Add a low-signal alarm to every analog channel:
(* NAMUR NE43 under-range alarm: < 3.6 mA = broken wire / transmitter fault *)
(* For 0-20mA card: 3.6 mA = count 738 *)
VAR
brokenWireAlarm : BOOL;
faultCountThreshold : INT := 738; (* adjust per card range *)
END_VAR
brokenWireAlarm := (rawCount < faultCountThreshold);
(* Suppress PV and inhibit control output when alarm is active *)
IF brokenWireAlarm THEN
pressure := euMin; (* hold safe value *)
(* Energize alarm output, set fault bit in HMI *)
END_IF;
This prevents a broken sensor from driving a control loop to an unsafe state — a critical safety practice for any automated process.
4-20mA and HART: Getting More From the Same Wires
The Highway Addressable Remote Transducer protocol superimposes a digital signal on top of the 4-20 mA current loop using frequency-shift keying (FSK). The digital signal carries device identification, diagnostic data, secondary variables, and configuration parameters — without interrupting the analog 4-20 mA signal.
HART allows a single instrument to report multiple variables (for example, a Coriolis flow meter reporting mass flow, density, and temperature on one pair of wires) while remaining compatible with any standard 4-20 mA receiver. See the full HART protocol tutorial for implementation details.
4-20mA in Context: Networks and Sensors
4-20 mA loops are point-to-point: one transmitter per wire pair. When a process requires dozens or hundreds of measurement points, a fieldbus or industrial Ethernet network such as those covered in the PLC communication protocols guide can reduce wiring cost substantially. However, loops remain dominant for safety-critical measurements and in environments where the simplicity and reliability of a dedicated wire pair outweigh the wiring cost.
For an introduction to the PLC hardware and software fundamentals that put analog signals in context, see the PLC programming basics guide.
For instrument-specific wiring examples, the same loop principles apply to a pressure transmitter and to temperature sensors — see RTD vs thermocouple for how RTDs are typically connected via 4-20 mA transmitters.
Frequently Asked Questions
What is a 4-20mA current loop?
A 4-20mA current loop is an analog signal standard used in industrial instrumentation. A transmitter converts a physical measurement into a DC current proportional to the measured value: 4 mA represents 0% of the measurement range and 20 mA represents 100%. The current travels through a single pair of wires in a series circuit, making the reading immune to cable resistance and electrical noise.
Why 4-20mA and not 0-20mA?
Starting the range at 4 mA rather than 0 mA creates a live zero: when the wire is intact and the instrument is powered, there is always at least 4 mA flowing. A reading of 0 mA can only mean a broken wire, an unpowered transmitter, or a short circuit — never a genuine measurement. This makes fault detection immediate and automatic. The 4 mA live zero is defined by NAMUR NE43 and is supported by every major PLC vendor's analog input card.
How do you wire a 4-20mA transmitter to a PLC?
For a 2-wire (loop-powered) transmitter with a sinking analog input card: connect the 24 VDC positive supply to the transmitter positive terminal; connect the transmitter negative terminal to the PLC analog input positive terminal; connect the PLC analog input negative terminal (COM) back to the 24 VDC supply negative. The loop is complete and current flows through the card's internal 250 Ω burden resistor. Ground the cable shield at the panel end only.
What is a 2-wire transmitter?
A 2-wire (loop-powered) transmitter powers its internal electronics from the same two wires that carry the 4-20 mA signal. The transmitter draws its operating current from the loop supply, so it must complete all its measurement and signal-conditioning functions while consuming no more than the 4 mA live-zero value (typically ≤ 3.5 mA). Two-wire transmitters are the most common type in process plants because they require only a single twisted pair from field to panel.
What does the 250 ohm resistor do in a 4-20mA loop?
The 250 Ω burden resistor converts current to a measurable voltage: 4 mA × 250 Ω = 1 V and 20 mA × 250 Ω = 5 V, producing a 1-5 V signal. PLC cards with dedicated current inputs have this resistor built in. Cards with only voltage inputs require an external 250 Ω precision resistor wired across the input terminals. It also sets the loop's electrical load, so the supply voltage must be high enough to drive 20 mA through the transmitter impedance, the cable resistance, and the burden resistor combined.
Summary
The 4-20mA current loop is the backbone of process instrumentation wiring. Its advantages — noise immunity, immunity to cable resistance, live-zero fault detection, and two-wire simplicity — explain why it has outlasted decades of competing signal standards. Understanding the four loop components, the sinking/sourcing distinction on PLC cards, and the linear scaling math from raw counts to engineering units is essential knowledge for anyone commissioning or programming a PLC-controlled process system. Pair that wiring knowledge with types of industrial sensors for a complete picture of how field measurements reach your PLC.
