4–20 mA wiring reference
By Saad, practising automation engineer · Updated 2026-09-25
2-wire, 3-wire, 4-wire, RTD, and thermocouple conductors. This page is a wiring reference, not a generated loop diagram and not a voltage-drop calculator.
2-wire transmitter
A 2-wire 4–20 mA transmitter is loop-powered: the same pair carries supply and signal.
| Conductor | Role |
|---|---|
| + / brown | Loop supply from the analog input or barrier, typically 24 VDC |
| − / blue | Return to the analog input. The current is the process signal |
The loop sheet shows a loop-powered transmitter, two cores, and the AI channel. No separate L+ power pair.
3-wire transmitter
A 3-wire transmitter has a dedicated supply and a signal high; 0 V is shared.
| Conductor | Role |
|---|---|
| L+ / brown | 24 VDC supply |
| Signal / white or black | 4–20 mA into the analog input |
| 0 V / blue | Common return for supply and signal |
The loop sheet must show three cores and a powered transmitter, not a 2-wire ISA bubble.
4-wire transmitter
A 4-wire transmitter has an independent mains or DC supply plus a isolated 4–20 mA pair.
| Conductor | Role |
|---|---|
| L / brown | Supply live (or L+) |
| N / blue | Supply neutral (or 0 V) |
| I+ / black | Analog output high to the AI |
| I− / white | Analog output low to the AI |
The loop sheet shows a mains-fed or separately powered device and a signal pair. Do not draw it as loop-powered.
RTD
An RTD is a resistance measurement. Lead-wire compensation needs 3-wire or 4-wire, not a 4–20 mA pair.
| Conductor | Role |
|---|---|
| 2-wire | Lead resistance adds to the reading. Rare on plant RTDs |
| 3-wire | Two leads on one end cancel most copper resistance — the usual field RTD |
| 4-wire | Kelvin sense. Highest accuracy, extra cores |
The loop sheet shows an RTD element and an RTD card or temperature transmitter. A transmitter in the head is 4–20 mA, not RTD wiring.
Thermocouple
A thermocouple is a millivolt junction. Extension cable must match the type (K, J, T). It is not 4–20 mA.
| Conductor | Role |
|---|---|
| Positive leg | Type-coloured (e.g. green for IEC type K) |
| Negative leg | White on IEC type K |
| Screen | Usually earthed at one end only, per the project standard |
The loop sheet shows a TC element and a TC card or a head-mounted transmitter. Do not number TC extension as instrument pair cores unless the head outputs 4–20 mA.
Frequently asked questions
What is the difference between 2-wire and 4-wire 4–20 mA?
2-wire is loop-powered on one pair. 4-wire has a separate supply plus an isolated signal pair. Mixing them on the sheet is a common drafting error.
Does an RTD need lead compensation?
Yes if you measure resistance at the card. Use 3-wire or 4-wire. A head-mounted transmitter turns the RTD into 4–20 mA and the field cable is then a current loop.
Where is the cable screen earthed?
Follow the project standard. Typical practice is a single earth, often at the marshalling or barrier, not both ends. SchematicForge does not invent bonding.
Generate the loop sheet from an IO list on the homepage.