RTDs are typically designed with three common configurations: two-wire, three-wire, and four-wire. In the two-wire configuration, as shown in Figure 1-1 below, the RTD is connected with two wires to either end of the RTD. In this configuration, the lead wire resistances cannot be separated from the RTD resistance, adding an error that cannot be separated from the RTD measurement. Two-wire RTDs yield the least accurate RTD measurements and are used when accuracy is not critical or when lead lengths are short. Two-wire RTDs are the least expensive RTD configuration.

In the three-wire configuration, as shown in Figure 1-2, the RTD is connected to a single lead wire on one end and two lead wires on the opposite end. Using different circuit topologies and measurements, lead resistance effects can effectively be cancelled, reducing the error in three-wire RTD measurements. Compensation for lead wire resistance assumes that the lead resistances match.

In the four-wire configuration, as shown in Figure 1-3, two lead wires are connected to either end of the RTD. In this configuration, the RTD resistance may be measured with a four-wire resistive measurement with superior accuracy. The RTD excitation is driven through one lead on either end, while the RTD resistance is measured with the other lead on either end. In this measurement, the RTD resistance is sensed without error contributed from the lead wire reacting with the sensor excitation. Four-wire RTDs yield the most accurate measurements, but are the most expensive RTD configuration.