4–20 mA Current Loop Wiring

Page review information

Edition covered: ISA analog signal practice; NFPA 79 / IEC 60204-1 (wiring-practice chapters) — NFPA 79:2024, IEC 60204-1:2016+AMD1:2021
Content status: Reviewed
Coverage: Transmitter types, active/passive matching, loop-resistance budget, grounding/shield policy and loop calibration at chapter/article level; vendor terminal designations, torque and HART configuration deliberately excluded.
Last reviewed: August 2026
Primary reference: The official published standard — required for final decisions

Confirm the governing edition, jurisdiction, and project-specific requirements before applying anything on this page.

Safety. This guide is educational reference material, not a work instruction. Electrical work is performed de-energized and verified by qualified personnel under your site’s LOTO procedures, following the device manufacturer’s manual and the authority having jurisdiction. A live loop shares a panel with hazardous voltages — treat the enclosure as energized until proven otherwise.

Overview

A 4–20 mA loop transmits an analog value as a current, not a voltage, and that single choice explains almost everything about how it is wired. Because the loop is a series circuit, the same current flows at every point — so the resistance of the wire, the terminals, and any barrier in the loop does not change the reading. A voltage signal depends on a shared reference and degrades over distance; a current signal does not.

Two more properties fall out of the same idea. Live zero: 4 mA is 0% of span and 20 mA is 100%, so a reading below 4 mA cannot be a legitimate process value — it means a broken wire, a dead transmitter, or lost loop power, and that built-in broken-wire detection is why the floor is 4 mA and not 0 mA. Noise immunity: the low-impedance loop shunts capacitively coupled noise that would corrupt a high-impedance voltage node.

Every loop, whatever the transmitter, has three functional roles spread across one to three terminal groups: the transmitter (modulates the loop current; needs a minimum terminal voltage), the loop power source (the DC supply, commonly nominal 24 V DC), and the receiver / input (reads the current as a voltage across a sense resistor, commonly 250 Ω, card-dependent).

flowchart LR
    PS[Loop supply<br/>24 V DC] --> TX[Transmitter]
    TX --> RS[Sense resistor<br/>read by analog input]
    RS --> PS

This guide covers wiring one 4–20 mA loop and matching its ends. Its companion, 0–10 V analog signal wiring, covers the voltage alternative and where the two differ — read both when choosing a signal type. Terminal designations, minimum operating voltage, active/passive designation, and torque are vendor-specific and come from the device manual, never a guide.

Before You Start

Have on hand before wiring:

Transmitter type — the defining decision. Confirm whether each device is 2-wire (loop-powered) — draws power from the same pair that carries the signal, simplest wiring but tightest voltage budget; 3-wire — separate power feed and common with the signal on a third conductor; or 4-wire — fully separate power input and an isolated output pair. The detail is in Control / Signal Wiring below.
Who powers the loop — active vs passive. Every device is either active (it sources loop power) or passive (it needs the loop energized by the other end). The loop must contain exactly one power source. An active input card paired with a self-powered (active) transmitter double-powers the loop — the classic magic-smoke mistake. Confirm the designation of both card and transmitter against their manuals.
Input card burden and impedance — note the card’s sense resistance (burden); it is a term in the loop-resistance budget below.
Drawings and environment — loop diagrams, supply location, cable route relative to power and VFD cabling, and any hazardous-area or barrier requirement decided upstream.

Sizing & Protection

There are no conductor-ampacity tables here — a 4–20 mA loop carries milliamps. The sizing that matters is the loop-resistance budget: at 20 mA the supply must overcome every series drop and still leave the transmitter its minimum operating voltage.

The budget inequality. Verify, at 20 mA worst case:

V_supply ≥ V_transmitter(min) + 0.020 A × (R_sense + R_wire + R_barrier + …)

where V_transmitter(min) is a vendor value (consult the manual), R_sense is the input burden (e.g., 250 Ω drops 5 V at 20 mA), and R_wire is twice the one-way conductor resistance — the term that grows with distance and shrinks with a heavier gauge.
Wire gauge for long runs. On a long loop, undersized conductors eat the voltage budget. The resistance arithmetic is the same concept behind cst voltage-drop — use it to keep the run inside the budget; never invent a resistance-per- length value, take it from the wire tables.
Compliance voltage. The transmitter’s usable voltage span from its supply is its compliance budget. Exceed the loop-resistance limit and the loop simply cannot reach 20 mA — it reads correctly low and clips near 100%. That symptom is a resistance-budget failure, not a transmitter fault.
Supply protection. Fuse or current-limit the loop supply per device and panel practice — generally accepted practice, verify for your installation.

Power Wiring

One source, sourced deliberately. Establish which end supplies the loop (field supply or input card) and wire so there is exactly one. This is the active/passive match from Before You Start, made physical.
Series the sense element. The sense resistor sits in series with the loop, carrying the full loop current — not a tap across the signal. On a 2-wire device the supply, transmitter, and sense resistor form one series ring.
HART coexists on the same pair. Where the transmitter is HART-capable, the digital signal rides the same two wires as a small AC component on the 4–20 mA DC and does not disturb the analog value, provided the loop has enough resistance (commonly the 250 Ω burden) for the modem. HART setup is out of scope — consult the device manual.
Torque discipline. Terminal torque and wire ranges are vendor values; record what you used.

Control / Signal Wiring

The core of loop wiring is getting the series circuit and the active/passive match right. The three transmitter wiring patterns (verify terminal names against the device manual — the roles are universal, the labels are not):

Type	Power	Signal path	Sense resistor	Typical use
2-wire (loop-powered)	Drawn from the signal pair; loop supply external (panel end)	Current on the same two wires that power the transmitter	External, at the input card	Most field transmitters; simplest wiring, tightest voltage budget
3-wire	Separate + and common	Signal returned on a third conductor referenced to that common	At the input, or internal	Higher drive than 2-wire; local power available
4-wire	Fully separate power input (line or 24 V)	Isolated 4–20 mA output pair, independent of power	At the receiver	Analyzers, high-drive or isolated outputs

Where the sense resistor sits. At the receiving end, in series, so the input reads the loop current as a voltage. One sense resistor per loop unless a second (e.g., a HART tap) is deliberately budgeted.
Match active to passive. A passive transmitter needs an active (loop-powering) input or external supply; an active transmitter needs a passive input. Pairing two active ends double-powers the loop — the single most common way to release the magic smoke. Both manuals state the designation.
Ground the loop at one point only. A single ground reference (commonly supply-negative at the panel end) prevents circulating ground-loop current; a second, unintended ground turns the return or shield into a ground-loop conductor.

Grounding, Shielding & EMC

Device-specifics here; the deep treatment is owned by the noise & EMC mitigation guide and the shield-landing theory by panel grounding & bonding.

Shield landed at ONE end only for low-frequency analog. The shield drains capacitively coupled noise without becoming a conductor between two grounds. This is deliberately the opposite of the both-ends 360-degree rule for high-frequency VFD/motor cable — a current loop’s frequencies are low, so a both-ends screen across any ground-potential difference just carries 50/60 Hz hum into the signal. The frequency reasoning behind the two rules is owned by grounding & bonding.
Isolators break ground loops. Where the loop spans a ground-potential difference — long field runs, separate buildings, older plants, or a device that insists on grounding its own end — a loop isolator or isolated input card breaks the DC path instead of fighting it. That is where an isolator earns its cost; on a short in-panel loop it usually is not needed. Generally accepted practice — verify for your installation.
Separate from power and VFD cabling. Analog is a Class 1 (quietest, most sensitive) victim; VFD output is Class 4. Keep them apart, cross at right angles, never share a duct — separation classes are in the EMC guide and the network-side view on the copper Ethernet page.

Common Mistakes

Double-powering a passive loop. Two active ends (an active card and a self-powered transmitter) push current from both sides — best case nothing reads, worst case the input stage or transmitter output is damaged. Confirm active/passive on both manuals before energizing.
Loop resistance exceeding compliance. Undersized wire on a long run, or an extra barrier, drives total loop resistance past the budget. The loop reads correctly low but clips near 100% — the giveaway symptom. Check the resistance budget, not the transmitter.
Shield grounded at both ends. The screen becomes a ground-loop conductor across any ground-potential difference and injects a steady 50/60 Hz hum or ripple that tracks plant load. Land the analog shield at one end only.
Sharing the analog common with digital or high-current returns. The other circuit’s return current develops a voltage on the shared conductor (common-impedance coupling) that appears as an offset moving with load. Give analog its own return.
No isolator across a ground-potential difference. Two grounds at different potentials drive circulating current through the loop — offset, noise, or damage. Break the loop with an isolator rather than hope the potentials match.
Mixing an active card with an active transmitter (or two passive ends). Either mismatch leaves the loop wrong: two passive ends give no power and no reading; two active ends double-power it. The fix is the same discipline every time — exactly one power source per loop.

Verification Checks

Loop-check before handing the point to the process (evidence-retaining sheets in templates):

Active/passive designation confirmed on both manuals; exactly one power source in the loop
Loop-resistance budget checked at 20 mA — supply covers transmitter minimum + sense + wire + barriers with margin
Loop calibration at 4 / 8 / 12 / 16 / 20 mA (0 / 25 / 50 / 75 / 100%); source, receiver, and displayed value agree within tolerance
Milliamp injection confirms the receiver independently of the transmitter
Shield landed at one end only; loop grounded at a single point
Analog run separated from power/VFD cabling per the routing plan
Terminal torques per the device manual, recorded
Completed loop sheet filed (cst loop-sheets); hand off to the commissioning templates loop-check workflow

Standards References

ISA analog signal practice — the 4–20 mA transmission convention and instrument-loop conventions are ISA-domain practice; cited as a category, not a specific clause.
NFPA 79:2024 — machine wiring-practice chapters: conductor identification, routing, and separation of signal from power (chapter-level citation).
IEC 60204-1:2016+AMD1:2021 — wiring practices for machine electrical equipment, the international counterpart; clause-level citation only.
Device manuals are the authority for terminal designations, minimum operating voltage, active/passive designation, and torque.

0–10 V analog signal wiring — the voltage alternative and when to prefer it (or not)
PLC I/O wiring — where the analog card lives
Panel grounding & bonding — shield-landing policy and the jobs of “ground”
Noise & EMC mitigation — separation classes and victim hardening; copper Ethernet for the network-side separation view
Commissioning templates — loop sheets and loop-check records

Trust Boundary — Engineering Judgment Required

This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.

Items marked Review pending await technical review against the identified edition. Items marked Planned have no reviewed source material behind them. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.