The Complete Guide to 4–20 mA Current Loops

9 min read · updated 2026-07-12

The 4–20 mA current loop has carried process measurements for over half a century, outliving several generations of "replacements." This guide explains why it works the way it does — and links to the calculators for every computation along the way.

Why current, not voltage?

Kirchhoff's current law is the secret: in a series loop, the same current flows through every element, no matter how long the cable. A voltage signal sags with cable resistance — every metre of wire steals a little of it — but a current signal arrives at the control room exactly as it left the transmitter, whether the cable is 10 metres or 1,000. Add the fact that a slowly changing current is highly immune to induced electrical noise, and you have a signal format perfectly suited to industrial plants.

The live zero: why 4 and not 0

If 0 mA meant "0% of range," a broken wire and an empty tank would look identical. Starting the scale at 4 mA creates a live zero: a healthy loop never carries less than 4 mA, so 0 mA is unambiguously a fault. The offset serves a second purpose — that ever-present 4 mA powers the electronics of two-wire transmitters over the same pair of conductors that carries the signal. One cable pair: power in, measurement out.

The scaling math

Between 4 and 20 mA lie 16 mA of signal, mapped linearly onto the instrument range from LRV (lower range value) to URV (upper):

The fixed anchor points are worth memorising: 4 mA = 0%, 8 = 25%, 12 = 50%, 16 = 75%, 20 = 100%. For everything in between, use the 4–20 mA to PV calculator (range-aware, both directions) or the quick mA ↔ percentage converter.

Powering the loop

Every device in the loop — the DCS input resistor (typically 250 Ω), indicators, IS barriers, and the cable itself — drops voltage in proportion to current. What remains at the transmitter terminals must exceed its minimum operating voltage (often 9–12 V DC) at maximum current, or the loop saturates early or dies entirely. It's one of the most common commissioning faults and takes thirty seconds to check with the loop voltage drop calculator.

Fault signalling: NAMUR NE 43

Smart transmitters use the margins just outside 4–20 mA to talk about their own health. Per NAMUR NE 43, currents of roughly 3.8–4 and 20–20.5 mA indicate a process just beyond the calibrated range (saturation), while ≤ 3.6 mA or ≥ 21 mA are deliberate failure signals. Configure your DCS alarm limits to catch them — a loop that parks at 3.6 mA is telling you something.

The square root exception

One signal family breaks the linear rule: flow from a differential-pressure element. DP grows with the square of flow, so recovering flow requires a square root — done in exactly one place, transmitter or DCS, never both and never neither. On a linear DP signal, 12 mA is 50% DP but 70.7% flow. The square root extraction calculator converts all four quantities (DP %, DP mA, flow %, flow mA) and the classic "8/12 test" in its notes tells you what your transmitter is actually doing.

Calibrating a loop

A routine check runs the transmitter through 0-25-50-75-100% of its range and compares the output current against ideal values within a tolerance (commonly ±0.25% of span = ±0.04 mA). The calibration table generator produces the full test table — ideal mA, acceptance limits, as-found and as-left columns — ready to print for the bench. For what those tolerances mean in different units, see the error converter, and to confirm your calibrator is good enough for the job, the TUR calculator.

Where HART fits

HART superimposes a digital signal on the same 4–20 mA wires, giving configuration and diagnostics access without disturbing the analog value. Two practical implications: the loop needs at least ~250 Ω of resistance for HART to communicate, and the analog current — not the digital value — usually remains the control signal, so analog calibration still matters.

The bottom line

4–20 mA endures because it solves four problems with one pair of wires: distance-independent measurement, noise immunity, device power, and built-in fault detection. Master the scaling math, check the loop power budget, know the NE 43 limits, and respect the square root — that's 90% of current-loop competence.

Calculators used in this guide: 4–20 mA ↔ PV · mA ↔ % · loop voltage drop · square root extraction · calibration table