Transferred voltages during earth faults on subtransmission lines contribute to the primary side risk at a zone substation. A method of computing transferred voltages throughout sub-transmission networks which employs phase coordinates and linear algebra is presented. The mutual impedance between one or more phase conductors and overhead earth wires is considered. Self and mutual impedances can be evaluated using modified Carsonโs equations which are reproduced herein. The application of the method was demonstrated for a range of situations using an example sub transmission network. Opportunities to simplify models were also assessed, including the application of a single-phase equivalent model.