Pt100 wiring — 2-, 3- and 4-wire connections
A Pt100 sensor can be connected with two, three or four wires. The 3-wire connection compensates for lead resistance and is the industrial standard; the 4-wire connection eliminates lead resistance entirely and gives the highest accuracy.
Why the number of wires matters
A Pt100 measures temperature as resistance — about 0.385 Ω per °C. The leads between the sensor and the instrument have resistance of their own, and the instrument reads sensor and cable as one: every ohm of lead resistance reads as roughly 2.6 °C. The three connection types differ in one thing only — how they deal with that lead resistance.
2-wire — the shortest path
The two leads carry both the measuring current and the signal, so the cable's resistance adds directly to the element's — and appears in the reading as extra temperature. A 2-wire connection therefore suits short cable runs, and installations where a transmitter sits directly in the connection head, so the leads are only centimetres long.
A Pt1000 changes the arithmetic: with ten times the element resistance, the same ohm of lead resistance reads as roughly 0.26 °C — a tenth of the Pt100 error. Lead resistance still counts, but it rarely decides — which is why Pt1000 elements are usually connected with two wires.
An older variant adds a closed loop of the same cable alongside the measuring pair: the instrument measures the loop's resistance and subtracts it from the reading.
3-wire — compensation
The third lead lets the instrument measure the cable resistance separately and subtract it from the reading. The compensation assumes the leads are alike — same length, same cross-section, same temperature — which wiring from the same cable gives in practice. That balance of accuracy and simplicity is what has made 3-wire the industrial standard for process installations.
4-wire — the measurement stands alone
Four wires separate the jobs completely: one pair carries the measuring current, the other pair reads the voltage over the element. Lead resistance drops out of the measurement entirely, whatever the cable length. The 4-wire connection is the choice where the accuracy requirement is tightest — laboratory work, calibration and narrow process windows.
Connection diagrams
The diagrams show the possible combinations for inserts with one, two or three Pt100 elements in 2-, 3- and 4-wire connection, with the terminal numbering used in the KP connection heads. The same numbering appears on the replaceable sensor inserts.
One Pt100 element
Two and three Pt100 elements
With and without a transmitter
Without a transmitter, the element's leads run from the insert's terminals through the cable to the instrument — and the connection type decides how much the cable affects the reading, as above. With a head-mounted transmitter, the element is wired to the transmitter inside the connection head, and the signal leaves the sensor as 4–20 mA on two wires — a current signal that stays accurate over long cable runs regardless of lead resistance. The diagrams below show both arrangements; the transmitters themselves are covered in the temperature transmitter programme.
Frequently asked questions
Why does a Pt100 have 3 wires?
The third wire lets the instrument measure the resistance of the cable separately and subtract it, so the reading reflects the sensor element rather than the leads. It compensates fully when the three leads are alike — same length, same cross-section, same temperature.
How much error does a 2-wire connection add?
About 2.6 °C per ohm of lead resistance on a Pt100: the cable's resistance adds directly to the element's, and the instrument reads the sum as temperature. That is why 2-wire connections are kept short — or paired with a transmitter in the connection head.
When is a 4-wire connection needed?
When the accuracy requirement is tightest — laboratory measurements, calibration work and narrow process windows. Separate current and measuring pairs remove lead resistance from the measurement entirely, so the reading is independent of cable length.
What is a jumper connection?
A jumper is a small metal bridge between two terminals in the connection head. In the KP connection diagrams it appears in the 2-wire arrangement of a 3-terminal insert: the jumper joins the two terminals that share one end of the element, so the insert connects with two wires.
Why is a Pt1000 usually connected with two wires?
A Pt1000 element has ten times the resistance of a Pt100, so the same lead resistance disturbs the reading ten times less — roughly 0.26 °C per ohm instead of 2.6 °C. That makes the simple 2-wire connection practical for most Pt1000 duty; where the accuracy requirement is tight, 3- and 4-wire connections work for a Pt1000 exactly as for a Pt100.
Why specify a sensor with two Pt100 elements?
Redundancy and role-splitting: one element serves the control loop, while the second serves safety monitoring, an independent recorder — or waits as an installed spare. Two elements in one insert give a second measuring point at the same spot in the process, with a single process penetration.
Related
- Temperature sensor types and ranges — tolerance classes and temperature ranges for Pt100 and thermocouples
- Pt100 resistance table — the resistance curve behind the measurement
- Temperature transmitter guide — when the signal leaves the head as 4–20 mA
- Knowledge Center — all reference guides and tools