The conversion is one division. What is usually missing is not the ratio but the rest of the arithmetic: which gauge, over what length, and which part of the error a third wire or a commissioning trim actually removes.

This page gives the number, the table, and the two cases where the usual rule of thumb is wrong.

The one division you need

A platinum RTD changes resistance at a rate set by IEC 60751. At 0 °C that rate is 0.3908 Ω per kelvin for a PT100 and 3.9083 Ω per kelvin for a PT1000. Divide the loop resistance by that slope and you have the error in degrees.

Slopes calculated from the Callendar-Van Dusen coefficients in IEC 60751. The slope falls slightly as the sensor heats, so the error a given cable causes grows a little with process temperature. Full values are in the PT100 and PT1000 resistance tables.

The slope is why the factor of ten exists and why it never goes away. It is not a tolerance, a grade, or a manufacturing difference. A PT1000 simply moves ten times further for the same degree.

What your cable actually adds

On a 2-wire connection both conductors are in series with the element, so the loop is twice the one-way length. Copper resistance at 20 °C from the standard AWG table gives the numbers below.

Every solid line is a PT100. The whole PT1000 range fits inside the shaded band at the bottom.

Two-wire connection, copper at 20 °C, both conductors counted. Read the three numbers in each cell as AWG 18, AWG 22 and AWG 24. In metric terms those are roughly 0.82, 0.33 and 0.20 mm².

A 10 °C error on a Class A PT100 is roughly seventy times the element's own tolerance at 0 °C. At that point the sensor grade you paid for has stopped mattering.

Why 3-wire cancels less than people assume

A 3-wire connection does not remove lead resistance. It measures one conductor and subtracts it from the other two, which works only while all three conductors are identical.

They usually are, to within a fraction of a percent, when the cable is a single homogeneous run. The mismatch shows up somewhere else: a crimp that is not quite the same as its neighbour, a junction box where one conductor was extended, a terminal that has oxidised on one leg only.

This is the failure that gets mistaken for a bad sensor, because it is stable, plausible and temperature dependent. The 2-wire vs 3-wire vs 4-wire comparison covers the wiring schemes themselves.

First, the part your commissioning engineer already removes

A fixed lead resistance is an offset, and most RTD transmitters and PLC input cards let you trim an offset at commissioning. Where that trim exists and was used, the static term in the table above is already gone.

That changes which numbers matter. The 43 °C in the bottom-right cell is what an untrimmed 2-wire run would report, not what a commissioned loop reads. Treat the table as the size of the correction, not as live error.

Two cases where the trim is not available: a controller with no offset parameter, and a sensor that gets swapped for one on a different cable length without anyone re-running the trim. Both are common in retrofits.

The term nobody puts in the budget: the cable's own drift

Copper has a temperature coefficient of about 0.393 % per kelvin, close enough to a platinum RTD's own coefficient to be ironic. The lead resistance you measured on a cold bench is not the lead resistance you have in a hot cabinet.

Ten metres of AWG 22 is 1.06 Ω of loop at 20 °C. Put that harness in a 60 °C enclosure and it becomes 1.23 Ω. The extra 0.17 Ω is another 0.43 °C on a PT100.

On a 2-wire installation that drift is uncorrectable and it moves with the seasons, which is exactly the signature people report as "the reading creeps in summer". On a 3-wire run it cancels with everything else, provided the conductors still match.

Work out your own case

Enter the loop resistance if you have measured it, or a cable length and gauge if you have not.

The calculator applies the copper coefficient to whichever ambient you enter, so a measured loop taken on a bench can be projected to the temperature the cable will actually see.

When to add a wire and when to change the element

Adding a third conductor fixes the matched part of the error. Moving to a PT1000 divides every term by ten, including the parts a third wire cannot reach: the imbalance and the copper drift.

The last row is the one that gets forgotten. A 4-20 mA transmitter mounted at the probe removes the problem entirely, because a current loop does not care about conductor resistance.

Whether that is worth the cost depends on whether the cable run is fixed. Our Class A PT1000 3-wire probes cover the middle two rows.

If you are choosing between the two elements for other reasons as well, the PT100 vs PT1000 comparison covers the rest, and the RTD guide covers how the element works. Mounting affects a different error budget entirely, which the sensor mounting guide deals with.

Common questions

How much is 1 ohm of lead resistance in degrees?

2.56 °C on a PT100 and 0.256 °C on a PT1000, at 0 °C. The figure grows slightly as the sensor gets hotter, reaching 2.64 °C per ohm at 100 °C on a PT100.

Some sources quote 2.60 °C. That comes from using the average coefficient 0.385 Ω/K over 0 to 100 °C rather than the derivative at 0 °C given by IEC 60751, which is 0.3908 Ω/K. Both are right about different things; this page uses the derivative throughout.

Does a 3-wire RTD remove lead resistance completely?

No. It cancels the part that is matched across the conductors. Any imbalance between them passes straight through. A 0.05 Ω mismatch is 0.13 °C on a PT100.

How long can a 2-wire PT100 cable be?

It depends on the accuracy you need. For 1 °C or better, roughly 4 m of AWG 22 or 9 m of AWG 18. For 0.1 °C, a 2-wire PT100 is not the right choice at any practical length.

Why does my reading drift with the seasons?

On a 2-wire run, copper lead resistance rises about 0.393 % per kelvin. A harness that warms from 20 °C to 60 °C adds roughly 16 % to its own resistance, and on a PT100 that shows up as a fraction of a degree to a few degrees depending on length.

Is PT1000 always better for long cable runs?

For lead error, yes, by a factor of ten. It is not automatically better overall: self-heating is higher for the same excitation current, and not every controller accepts both elements.