4-20 mA Current Loops Explained: Wiring, Powering, and Troubleshooting

The 4-20 mA current loop is the most stubbornly persistent analog signal in industrial control. It predates the microprocessor, it’s been declared obsolete every five years for forty years, and it’s still the default way pressure transmitters, level sensors, flow meters, and temperature transmitters talk to PLCs and DCS systems. The reason it won’t die is that it works: noise immune, intrinsically self-checking (4 mA = “I’m alive at zero”), and powered over the same two wires that carry the signal.

If you’re new to instrumentation or you’ve been bridging this for years and never quite picked up the wiring rules, here’s the practical version.

Why Current, Not Voltage

Voltage signals (0-10 V, 1-5 V) drop along long wires. The longer the cable run and the smaller the gauge, the more error you accumulate from IR drop. A 100 Ω round-trip in the wire on a 5 V signal is a 1% error before you even get to the noise.

Current signals don’t drop with distance. A series loop carries the same current at every point in the loop, so the wire resistance changes nothing about the value the receiver sees — it only eats into the available voltage budget. You can run a 4-20 mA loop a thousand feet through a noisy plant and the receiver gets the same 12.000 mA the transmitter sent, plus or minus a little EMI that the loop itself rejects.

The mapping is simple:

4 mA = lower range value (0% of scale)
20 mA = upper range value (100% of scale)
<3.6 mA or >21 mA = fault, by NAMUR NE 43 convention

The 4 mA “live zero” is the killer feature. A truly dead loop reads 0 mA, so the receiver knows the difference between “the tank is empty” and “the wire is broken.”

The Three Transmitter Types

This is where most confusion starts. Not all transmitters are powered the same way, and miswiring a 3-wire as a 2-wire is one of the most common installation mistakes.

2-Wire (Loop-Powered)

The transmitter has only two terminals. The same two wires deliver power and carry the signal. The transmitter modulates how much current it draws from the supply — between 4 and 20 mA — and that current is the signal.

Wiring: +24 V → transmitter (+) → transmitter (−) → receiver (+) → receiver (−) → 24 V common.

The receiver is anywhere in the loop where you can put a known resistor. The PLC analog input typically has a 250 Ω sense resistor built in (or you add one externally) to convert current to a 1-5 V signal it can read.

Loop-powered transmitters are the most common and the easiest to wire. The catch is that the available voltage at the transmitter is the supply voltage minus the drops across every device in the loop, which limits how much instrumentation you can string in series.

3-Wire

The transmitter has separate power and signal terminals: V+, signal, common. Power and signal share a common, but the signal is a current source driven by the transmitter’s internal electronics rather than its draw from the supply.

3-wire devices are usually low-power transmitters (RTD heads, simple temperature transmitters) where loop-powered operation isn’t feasible because the transmitter itself draws more than 4 mA quiescent.

4-Wire

The transmitter has its own AC or DC power supply on one pair, and the 4-20 mA signal is on a separate isolated pair. These are big transmitters — flow computers, gas chromatographs, magnetic flow meters — that need substantially more power than a loop can provide.

4-wire devices are the most flexible and the most expensive to wire. Two cables, two terminations.

Loop Powering and Voltage Budget

The 24 V supply has to push current through everything in series with the loop. Every device adds some “burden” — a voltage drop at 20 mA — and the transmitter needs a minimum compliance voltage to operate (typically 8–12 V).

Voltage budget = Supply − (Sum of burdens at 20 mA) − Transmitter minimum

Typical burdens:

250 Ω sense resistor: 5.0 V at 20 mA
HART communicator pickup: ~1.5 V
Indicator/totalizer in the loop: 3.0 V
Barriers / isolators: 5–7 V each
Long cable run with #18 wire: a few tenths of a volt per 1000 ft

If your supply is 24 V and you’ve got a 250 Ω resistor, an indicator, and an intrinsic safety barrier, you’ve burned 13 V before the transmitter sees anything. If the transmitter needs 11 V minimum, you’re already at the edge. Bump the supply to 28 V or eliminate a series device.

This is why some loops mysteriously stop working at 20 mA but read fine at 4 mA — the transmitter starves at high signal because the burden across the loop scales with current.

Sourcing vs Sinking

A “sourcing” output drives current out of the device. A “sinking” input pulls current down to common. A loop-powered transmitter sources its own loop current; the PLC analog input sinks. Match these up:

PLC sourcing input + loop-powered transmitter: doesn’t work. Both want to push current.
PLC sinking input + loop-powered transmitter: standard configuration.
PLC sourcing input + 4-wire active output transmitter: doesn’t work. Same problem.
PLC sinking input + 4-wire active output transmitter: works.

If two devices in a loop both want to be the current source, the loop won’t function. Read the data sheet. The phrase “active” output means the device sources current; “passive” means it sinks. Most modern Allen-Bradley 1769-IF analog inputs are passive (sinking) — good. Some old Siemens cards are active — check carefully.

Troubleshooting a Dead Loop

A loop reading 0 mA at the PLC is usually one of:

Broken wire
Wrong polarity
Dead transmitter
Insufficient supply voltage
Missing sense resistor

Here’s the procedure that catches all five with a multimeter.

Step 1 — Verify Supply Voltage at the Transmitter

Disconnect the loop wires at the transmitter and measure the voltage on the field side with the multimeter. Should read close to 24 V (open circuit). If you see 0 V, the supply or a fuse upstream is the problem. If you see 12 V, something in the loop is shorted or there’s a barrier eating voltage.

Step 2 — Measure Loop Current

Reconnect the wires. Set the multimeter to mA DC. Open the loop in series — break one wire at the transmitter, place the meter inline — and read the current.

0 mA: open circuit. Wire is broken or polarity is reversed (a reverse-polarity-protected transmitter will show 0).
3.5 mA or less: NAMUR low fault. Transmitter detects a sensor fault internally.
4 mA: process is at low scale. Real reading.
12 mA: midscale. Real reading.
22 mA or more: NAMUR high fault.

Most decent multimeters handle 20 mA easily. Watch the fuse rating — many meters have a 200 mA fuse on the mA jack that blows fast if you hook it across a 24 V supply by accident.

Step 3 — Substitute a Loop Calibrator

If the transmitter reads OK but the PLC sees nothing, the transmitter isn’t the problem. Disconnect at the transmitter and substitute a loop calibrator (or a 4-20 mA simulator). Drive a known 12 mA into the loop. If the PLC now reads midscale, the wiring and PLC are fine and the transmitter is the suspect. If the PLC still reads zero, the problem is downstream of the transmitter.

A loop calibrator is the single most useful tool for any instrumentation tech. They’re under $200 and pay for themselves the first time you need to commission a new transmitter.

Practical Tips

Use shielded twisted pair for the loop. Ground the shield at one end only — usually the control panel side. Grounding both ends creates ground loops that inject 60 Hz noise.
Keep loop wiring out of the same conduit as VFD output cables. Drive harmonics will couple into the loop and cause a low-frequency wobble in the reading.
Label every wire at both ends. The day you have to trace a loop in a panel with 200 transmitters is the day you’ll wish you had.
Don’t reuse the spare conductors in a loop cable for thermocouple or other low-level signals. Capacitive coupling between the loop and the spare pair causes mysterious noise.

The 4-20 mA loop is older than most of the people maintaining it, but the diagnostics are straightforward when you understand the four numbers — 4, 12, 20, and 24 V — and where each one should appear.