A loop-powered transmitter draws its operating power from the same two wires that carry its 4 to 20 mA signal. A field-powered transmitter has its own power supply and uses separate wiring for the measurement.
This is a reference for the engineer choosing between the two, or reading them off a P&ID and onto an I/O list. It covers the difference, when each is used, the loop voltage budget that decides it, and what changes on the I/O list and in the field wiring. It is not an intrinsic-safety or cable-sizing guide.
Key takeaways
- Loop-powered (2-wire) shares power and signal on one pair. Field-powered (4-wire) has separate power and signal.
- The choice is set by the device's power draw. A simple transmitter runs on the loop; a device that needs more needs its own supply.
- On the I/O list both are one analog input channel. The difference is the power source, the fuse, and the terminal count.
- A 2-wire loop only works if the voltage budget holds the transmitter's minimum terminal voltage at full current.
- A 4-wire device needs a field power feed on the drawings and a slot on the electrical load list, which a 2-wire device does not.
The two arrangements
In the 2-wire arrangement the transmitter sits in series with the loop and regulates the current through it between 4 and 20 mA. That current is both the device's power and its signal. A pressure transmitter PT-101, a differential-pressure flow transmitter FT-101, and most temperature transmitters TT-104 are built this way, because their electronics draw little enough to run inside the 4 mA the loop always carries.
In the 4-wire arrangement the device has a dedicated power feed, commonly 24 V DC or an AC supply, and the measurement leaves on a separate signal pair or an isolated output. The power circuit and the signal circuit are independent, so the device can draw whatever it needs without starving the signal. A three-wire variant exists, sharing a common return between power and signal, and it sits between the two in wiring and behaviour.
When each is used
The device's power appetite decides the arrangement.
| Loop-powered (2-wire) | Field-powered (4-wire) | |
|---|---|---|
| Typical devices | Pressure, DP flow, level, basic temperature transmitters | Magnetic flowmeters, Coriolis meters, analyzers, radar with heaters |
| Power draw | Fits inside the loop current | Exceeds what the loop can supply |
| Local display, heater, active sensor | Usually not, or a low-power display | Common; the reason for the separate feed |
| Wiring | One instrument pair | A power pair and a signal pair |
A device with a heater, a backlit display, or a sensor that has to be energised cannot generally live on loop power. The magnetic flowmeter is the classic example: it has to energise its coils, which the loop cannot do, so it is field-powered as a rule.
The power budget that decides it
For a 2-wire loop the deciding number is the voltage budget. The supply has to hold the transmitter's minimum terminal voltage after every drop around the loop, at the worst case of 20 mA.
V(supply) - I x (R(sense) + R(cable) + R(barrier) + ...) >= V(min at transmitter)
A common loop runs from a 24 V DC supply through a 250 ohm sense resistor, and the transmitter needs perhaps 12 V at its own terminals to operate. Subtract the drop across the sense resistor, the cable, and any intrinsic-safety barrier at 20 mA, and what remains has to stay above that minimum. When it does not, the loop will read correctly at low current and then fall over near full scale, which is a hard fault to chase in the field. A device that cannot be made to fit the budget is field-powered instead.
What changes on the I/O list
The signal class is the same either way. Both a 2-wire and a 4-wire transmitter are one analog input, so both count as one AI channel when you classify the signal. The difference lives in the wiring and power columns.
| Aspect | Loop-powered (2-wire) | Field-powered (4-wire) |
|---|---|---|
| Signal class | AI | AI |
| Channels | One | One |
| Power source | From the loop, via the I/O card or marshalling | Dedicated field power feed |
| Terminals | Two | Four |
| Electrical load list | Not listed | Listed as a powered field device |
| Field cable | One instrument pair | Power pair plus signal pair |
The row that catches people is the electrical load list. A 4-wire device is a powered load, so it needs a circuit, a fuse, and a place on the load list that the electrical engineer builds the panel schedule from. A 2-wire device generates none of that. Carrying a power-type field on the I/O list is what keeps a 4-wire device from being counted as one AI channel and then quietly left off the power distribution.
How it reconciles with the rest of the set
The arrangement ties the I/O list to two other documents. On the wiring and termination side, the terminal count and the extra power pair of a 4-wire device change the cable and the cabinet layout. On the electrical side, the 4-wire device appears on the load list where the 2-wire device does not. Reconciling the two is what catches the field-powered instrument that was counted as a single analog input and then had no power circuit drawn for it, which is a gap that only shows up at commissioning when the device will not energise.
A starting point
If you are deciding the arrangement or reconciling a list, the 4 to 20 mA, HART, and fieldbus reference covers how the signal itself is carried, and the signal classification reference covers why both arrangements are still one analog input. Record the power type on the I/O list from the start: it is a small column that decides whether a device also lands on the electrical load list, and adding it back after the list is issued means revisiting every transmitter.