RTD Wiring Configration

Wiring configurations for RTD’s are 2, 3 or 4 wire single and dual circuitry.

The basic 2 wire configuration is used when accuracy is not overly important, although still repeatable. The wire resistance is measured in conjunction with the RTD element and not deducted from the circuit resistance total value. If the wiring in the circuit is of a large wire size or the length is relatively short the inaccuracy would be minimal.

The 3 wire configuration is probably the most commonly used. It has a third wire to compensate for the lead wire resistance.
There are effectively two circuits: one circuit incorporates the RTD element (one white, one red) while the second circuit is designed to measure the resistance of the two common wires (red, red). This resistance value is then deducted through the measuring instrument to produce a resistance value across the RTD element only. The lead wire size or length has no influence on the actual resistance reading across the RTD element as long as all the wiring is identical in diameter (gauge size) and length.

The 4 wire configuration goes one step further than the three wire by compensating each wire joined to the RTD element (two white on one lead wire of the RTD element and two red on the opposite lead wire). There are actually now three circuits that are measured for resistance.

The two compensating circuit resistance values are totalled, and then divided to show the average resistance on each compensating circuit. This value is then deducted as with the three wire from the overall resistance in the circuit to give an actual resistance value across the RTD element. This configuration is the most
accurate and usually supplied in conjunction with high accuracy or calibration checked RTD elements. Applications can include pipeline custody transfer, research laboratories, refineries, and chemical or pharmaceutical plants.

An uncommonly used but alternative 4 wire configuration is the 4B. 

This design has two independent circuits: one including the RTD element (one red at each end) while the other is a loop (white throughout the loop). The loop has the identical wire size and length as the circuit that includes the RTD element. The end result is almost the same as the 3 and 4 wire configurations with the loop circuit being deducted from the circuit that includes the RTD element.