Wiring a 3 Wire RTD - Thermometrics Corporation 3 Wire RTD Wiring Diagram. L1 and L3 carry the measuring current while L2 acts only as a potential lead. No current flows through it while the bridge is in balancee L1 and L3 are in separate arms of the bridge,resistance is canceled. This circuit assumes high impedance at Eo and close matching of resistance between wires L2 and L3. 3 Wire RTD Sensor| Wiring a 3 Wire RTD | 3 Wire RTD Probe 3 Wire RTD Circuit In a three-wire circuit two constant current sources are used, in order to compensate for the disadvantages described above for the two-wire circuits. Similar to the two-wire circuit the current source IK2 is used to measure the temperature dependent resistance RT including the lead and terminal contact resistances.