Comparison

Two-Wire vs Four-Wire Transmitters

Two-wire transmitters are powered by the loop. Four-wire devices have separate power. Compare wiring, loop compatibility, isolation and supply voltage.

Controls & Instrumentation · Last reviewed October 17, 2026

Short answer: two-wire transmitters are the default for simple measurements, because one pair carries both power and signal. Four-wire transmitters suit devices that need more power than a loop can supply, such as many analyzers, magnetic flowmeters and devices with displays and relays.

	Two-wire (loop-powered)	Four-wire
Power	From the 4–20 mA loop, so the device must run on less than 4 mA	From a separate supply, AC or DC, on two wires
Signal	On the same pair as the power	On a second pair, an active 4–20 mA output
Power available to the device	Small, about 3.5 mA at the lowest point of the loop	As much as the supply provides
Typical devices	Pressure, temperature and level transmitters	Magnetic flowmeters, analyzers, devices with displays, relays or heaters
Wiring	One cable pair per device	Two pairs, or one cable with four conductors
Isolation	The loop supply and the receiver set the isolation. Isolators are added where needed	Power and signal are often isolated from each other in the device
Supply voltage	Needs enough loop voltage at the terminals (for example 10 to 12 V minimum)	Supply voltage set by the device, for example 24 V DC or 120/240 V AC
Cost	Lower wiring cost	Higher wiring cost

Loop compatibility

A two-wire transmitter needs a loop supply that provides the minimum terminal voltage at 20 mA, after the voltage drop across the receiver and the cable.
A four-wire transmitter with an active output supplies the current to the receiver itself, so the loop must not also have a supply. Connecting a loop supply in the way of an active output damages or confuses it.
Some four-wire devices have a passive output that needs an external loop supply. The wiring diagram of the maker shows which.
A three-wire device shares a common between the power and the signal, which can make isolation and ground loops harder.

How to choose

Check the power the device needs. Anything above a few milliwatts rules out a two-wire design.
Check the supply available at the location: DC from a panel or AC from the area.
Check the receiver: is the input active or passive, isolated or not.
Compare wiring cost and the number of conductors.
Plan isolation and grounding to avoid ground loops.

Common mistakes

Putting a loop supply into a four-wire active output.
Using a loop supply voltage that is too low for the transmitter and the receiver resistance.
Forgetting that a loop-powered device with a backlight draws more than the loop can give at 4 mA.
Not checking whether the output of a four-wire device is active or passive.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Why are most pressure transmitters two-wire?

They need very little power, and a single pair is cheaper to install and works over long distances.

Can I power a four-wire transmitter from the loop?

No. The loop supply cannot provide enough power, so the device needs its own supply.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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