How 4-20 mA Current Loops Work: The Complete Field Guide

Why 4-20 mA? The Live Zero Principle

The 4 mA lower range value is not arbitrary. It serves a critical diagnostic function called "live zero." If the signal wire breaks or the transmitter loses power, the loop current drops to 0 mA. Because 0 mA is outside the normal 4-20 mA range, the control system can immediately distinguish between a legitimate zero-percent reading (4 mA) and a fault condition (0 mA). If the range started at 0 mA, you could never tell whether a 0 mA reading meant the process variable was at zero or the transmitter was dead.

The 4 mA minimum also powers two-wire transmitters. A two-wire (loop-powered) transmitter uses the same two wires for both signal and power. It must operate on whatever current it is signaling. At 4 mA, the transmitter has 4 milliamps to power its sensor, signal conditioning, and output circuitry. Modern two-wire transmitters are designed to operate on as little as 3.5 mA, leaving margin for the 4 mA lower range value.

The 16 mA span (from 4 to 20) was chosen because it provides a clean ratio. Each milliamp represents 6.25% of span. Each 4 mA increment represents 25% of span: 4 mA = 0%, 8 mA = 25%, 12 mA = 50%, 16 mA = 75%, 20 mA = 100%. This makes mental math easy during calibration and troubleshooting.

The 20 mA upper limit was selected because it provides enough current to drive indicator circuits and maintain signal integrity over long runs, while remaining low enough to be intrinsically safe in many hazardous area classifications. Higher currents would generate more heat in the loop components, creating problems in both safety and reliability.

Signal Scaling: Converting mA to Engineering Units

Every 4-20 mA signal represents a process variable range. A pressure transmitter ranged 0-100 PSI outputs 4 mA at 0 PSI and 20 mA at 100 PSI. The scaling formula converts between milliamps and engineering units in both directions.

To convert mA to engineering units: PV = ((mA - 4) / 16) * span + LRV where PV is the process variable, span is the difference between upper and lower range values (URV - LRV), and LRV is the lower range value. For the 0-100 PSI example at 12 mA: PV = ((12 - 4) / 16) * 100 + 0 = 50 PSI.

To convert engineering units to mA: mA = ((PV - LRV) / span) * 16 + 4. For 75 PSI on that same transmitter: mA = ((75 - 0) / 100) * 16 + 4 = 16 mA.

These formulas work for any linear range, including ranges that do not start at zero. A temperature transmitter ranged 200-600 degrees F has LRV = 200, URV = 600, span = 400. At 12 mA: PV = ((12 - 4) / 16) * 400 + 200 = 400°F. The key is to always use the actual LRV and span, not assume zero-based ranges.

For reverse-acting (inverse) signals where 4 mA represents the high value and 20 mA represents the low value, simply swap LRV and URV in the formula. This is common in some level applications where a full tank produces minimum current.

Two-Wire vs Four-Wire Transmitters

Two-wire (loop-powered) transmitters use the same pair of wires for both power supply and signal output. The control system or DCS provides a DC power supply (typically 24 VDC) in series with a 250-ohm sense resistor. The transmitter modulates the loop current between 4 and 20 mA, and the DCS reads the voltage across the 250-ohm resistor (1-5 VDC corresponds to 4-20 mA).

Two-wire transmitters are simpler to install because they require only one pair of wires. They are also easier to make intrinsically safe because the energy in the loop is limited. The downside is that the transmitter must operate entirely on the loop current, which limits the power available for the sensor, display, and signal processing. At 4 mA with 24 VDC supply and loop resistance consuming some of the voltage, the transmitter might have only 60-80 milliwatts of power available.

Four-wire transmitters have separate power supply wires and signal output wires. They receive power from a dedicated supply (often 120 VAC or 24 VDC) and output 4-20 mA on a separate pair. This gives them essentially unlimited power for sensors, displays, and advanced diagnostics. Analyzers, magnetic flowmeters, and Coriolis meters are commonly four-wire because their sensor electronics require more power than a loop can provide.

The four-wire signal output can be either "sourcing" (the transmitter provides the current from its own power supply) or "sinking" (the transmitter controls the current from an external loop supply). Most four-wire transmitters are sourcing, which means they do not need a separate loop power supply from the DCS. This is a key wiring difference: connecting a sourcing four-wire transmitter to a DCS channel that also provides loop power will cause a conflict.

Loop Components and Voltage Budget

A current loop is a series circuit. Every component in the loop consumes voltage. The power supply must provide enough voltage to drive the rated current through all the loop components. If the total voltage drop exceeds the supply voltage, the loop saturates and the signal becomes inaccurate.

The voltage budget calculation is: V_supply >= V_transmitter + (I_max * R_total) where V_transmitter is the minimum operating voltage of the transmitter (typically 10-12 VDC for modern units), I_max is 20 mA (0.020 A), and R_total is the total loop resistance including the sense resistor, wire resistance, and any other series devices like intrinsic safety barriers.

Common loop components and their resistance contributions: DCS analog input sense resistor (250 ohms), intrinsic safety barrier (50-300 ohms depending on type), cable resistance (varies with gauge and length, approximately 5.2 ohms per 1000 feet for 18 AWG), and any indicators or trip amplifiers wired in series. A typical loop with a 250-ohm sense resistor, a 300-ohm IS barrier, and 500 feet of 18 AWG cable has about 555 ohms total resistance.

At 20 mA through 555 ohms: V_drop = 0.020 * 555 = 11.1V. Add 12V minimum for the transmitter: total required supply voltage is 23.1 VDC. A standard 24 VDC supply barely covers this. If the supply dips to 22 VDC under load, the loop will saturate below 20 mA. This is a common cause of signals that read correctly at low values but clip or become non-linear near 100%.

Loop Fault Detection: NAMUR NE 43

The NAMUR NE 43 standard defines signal levels for fault indication. Under this standard, the normal operating range is 3.8 to 20.5 mA. Signals below 3.6 mA indicate a downscale fault (broken wire, transmitter failure, loss of power). Signals above 21.0 mA indicate an upscale fault (sensor short, saturation). The bands between 3.6-3.8 mA and 20.5-21.0 mA are uncertainty zones.

Most modern transmitters and DCS systems support NAMUR NE 43. When a transmitter detects a sensor failure (open thermocouple, shorted RTD), it drives its output to either 3.6 mA (downscale burnout) or 21.0 mA (upscale burnout), depending on configuration. Upscale burnout is more common because it drives the control system to its safe state (closing a fuel valve, shutting down a burner).

The DCS or PLC analog input module compares the raw signal against these thresholds. Below 3.6 mA triggers a wire-fault alarm. Above 21.0 mA triggers an overrange alarm. Between 3.8 and 20.5 mA is normal operation. This layered approach catches both wiring problems and sensor failures automatically.

During troubleshooting, the first measurement should be loop current. If it reads 0 mA, you have an open circuit — check wiring, fuses, terminal connections. If it reads above 21 mA, the transmitter is likely failed or saturated. If it reads a stable value between 4 and 20 mA but the process reading is wrong, the problem is in the transmitter calibration or sensor, not the loop wiring.

Field Troubleshooting: Measuring Loop Current

There are two ways to measure loop current without breaking the circuit. The preferred method uses a milliamp clamp meter, which clips around one conductor and reads the current magnetically. The second method measures the voltage across the 250-ohm sense resistor at the DCS and divides by 250 to get current. A reading of 3.00 VDC across a 250-ohm resistor means 12.0 mA.

If you must break the loop to insert a milliamp meter in series, use the mA function on your multimeter, not the amp function. The mA input has a low-resistance shunt (typically 10-50 ohms) that does not significantly affect the loop. The amp input has a much lower resistance shunt but some meters have high-resistance internal paths on the wrong setting that can disrupt the signal.

When simulating a transmitter for calibration checks, a milliamp source replaces the transmitter in the loop. Set it to 4.000, 8.000, 12.000, 16.000, and 20.000 mA and verify the DCS reads the correct corresponding process values. This is a five-point calibration that checks linearity across the full range.

For two-wire transmitters, the milliamp source must be set to "source" mode — it provides both the current and the voltage. For four-wire transmitters, the milliamp source simulates only the signal output while the transmitter's power supply connections are disconnected. Getting this wrong is a common mistake that can damage the calibrator or the DCS input.