PT100 with 2, 3 or 4 wires: differences and when to use each
How lead resistance affects a PT100 reading with 2, 3 and 4-wire connections, with a numerical example, and which connection to choose in the field and in the lab.
The instrument reads the PT100 temperature from its resistance. Between the sensor and the instrument there are leads, and they have resistance too. The connection (2, 3 or 4 wires) decides how much of it gets into the reading.
2 wires
The resistance of both leads adds to the sensor's, and the instrument shows a higher temperature than the real one. Example: with 10 m of 0.5 mm² copper cable, each lead has about 0.34 Ω. That is 0.69 Ω extra, which at 0 °C becomes an error of about 1.8 °C. That is why a 2-wire connection only suits very short leads or a transmitter mounted in the sensor head.
3 wires
The instrument measures the resistance of one lead and subtracts it. It works well when the three leads are equal: same gauge, same length, no different splices and at the same temperature. Any difference between them becomes an error. It is the most common connection in industry.
4 wires
One pair of leads carries the current and the other measures the voltage right at the sensor. The lead resistance practically does not enter the reading. It is the connection for the lab, for calibration and for high-accuracy measurements.
Which one to choose
| Connection | When to use |
|---|---|
| 2 wires | Transmitter in the sensor head or very short leads. |
| 3 wires | Industry standard, with the three leads equal. |
| 4 wires | Calibration, laboratory and high accuracy. |
Tolerance classes AA and A of IEC 60751:2022 require a 3 or 4-wire connection.
Signs of a wiring problem
- Reading always a few degrees above expected: 2 wires with a long cable, or unequal leads on 3 wires.
- Fluctuating reading: loose terminal, bad splice or moisture.
- Reading at full scale or a sensor alarm: broken lead.