Why cable resistance reads as temperature
An RTD measurement is a resistance measurement, and the instrument cannot tell sensor ohms from cable ohms. In a 2-wire circuit, both conductors sit in series with the element, so their full resistance is added to the reading — and interpreted as extra temperature:
where the slope is the RTD's sensitivity — about 0.385 Ω/°C for a PT100 near ambient (this tool computes it exactly from the Callendar–Van Dusen coefficients at your process temperature), and ten times that for a PT1000.
Worked example
PT100, 2-wire, 100 m of 1.5 mm² copper (12.1 Ω/km) at 50 °C:
- Lead resistance = 2 × 0.1 km × 12.1 = 2.42 Ω
- Slope at 50 °C = 0.3851 Ω/°C
- Error = 2.42 ÷ 0.3851 = +6.3 °C — the reading is 6.3 °C high, always high
The same run as 3-wire with 2% lead mismatch: 0.05 Ω residual → +0.13 °C. As 4-wire: zero. As PT1000 2-wire: +0.63 °C — one-tenth, same cable.
Field notes
- 2-wire error is always positive (reads high) and drifts with cable temperature — copper's resistance rises ~0.4%/°C, so a sun-heated cable tray adds error that wasn't there at commissioning.
- 3-wire compensation assumes matched leads: same length, same gauge, same temperature, ideally the same cable. Splices and repairs break the match.
- Transmitter-at-the-sensor beats everything: a head-mounted temperature transmitter converts to 4–20 mA right at the RTD, making cable length irrelevant — usually the cleanest fix for long runs.
- Convert the resulting resistance to temperature precisely with the PT100/PT1000 calculator, and see the sensor comparison in our RTD vs thermocouple guide.
Frequently asked questions
How much error does lead wire cause on a 2-wire PT100?
Both leads add in series, so error ≈ (2 × one-way lead resistance) ÷ 0.385 Ω/°C. Just 1 Ω of total lead resistance reads about 2.6 °C high — and 100 m of typical instrument cable can add several ohms.
Does a 3-wire RTD eliminate lead wire error?
It cancels the lead resistance only to the extent the leads are matched. The residual error equals the mismatch between conductors — usually 1-2% of the lead resistance — which is why 3-wire is the industrial default but not laboratory grade.
Why does a 4-wire RTD have no lead error?
Current flows through one pair while voltage is sensed through the other, which carries virtually no current — so no voltage drops across the sense leads and cable resistance becomes irrelevant.
Is a PT1000 better for long cable runs?
Yes: the same lead resistance is ten times smaller relative to the sensor resistance, so the same cable produces one-tenth the temperature error. PT1000 2-wire is common in HVAC for exactly this reason.
Provided for reference and education. See our disclaimer.