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Engineering Toolbox/Pt100 Lead-Wire Error

Temperature Instrumentation

Pt100 Lead-Wire Error

Estimate how RTD lead resistance can shift the apparent Pt100 temperature in a 2-wire circuit, or evaluate the residual effect of unequal leads in an idealized 3-wire compensated measurement.

RTD wiring condition

Enter resistance for each relevant conductor, not the cable-loop resistance unless you divide it correctly between leads.

Field use

Useful when an RTD reads consistently high or wiring resistance is suspected.

A 2-wire resistance measurement includes both conductor resistances. A properly implemented 3-wire measurement compensates lead resistance by relying on matched leads; mismatch can leave residual error.

Calculation basis

2-wire: Rmeasured = RPt100 + Rlead1 + Rlead2
3-wire estimate: Rresidual = Rlead1 − Rcompensation

The apparent resistance is converted back to temperature using the same IEC 60751 Pt100 nominal characteristic used by the AccessRA Pt100 calculator.

Three-wire compensation is instrument-dependent and assumes appropriately matched leads. Four-wire resistance measurement is preferred where the highest resistance-measurement accuracy is required. This tool does not include transmitter input accuracy, self-heating, sensor tolerance or thermowell error.

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