Formula of 2 Wire RTD, 3 Wire RTD & 4 Wire RTD

2- Wire RTD :

where

• Rpt is Resistance of RTD
• R2 is Resistance of first lead wire (Extension cable used to connect RTD)
• R3 is Resistance of second lead wire (Extension cable used to connect RTD)
• L2 & L3 are extension cable leads

The wire resistances R2 and R3 are inevitably included in the measured value. Wire resistance can be cancelled by calibration, but just at one specific temperature of the wire (often room temperature).
3- Wire RTD :

4- Wire RTD :

The current source will maintain the excitation current constantly (0.1 – 1 mA). Following the Law of Ohm, R = U / I, the resistance can be calculated. The digital volt meter, DVM, has an input impedance of at least 10 Mohms.
Thus just an extremely small current will pass through the DVM which means that the voltage drops over the wire resistances R2 and R3 will be practically none.