A formula for calculating mutual coupling between diversely routed, horizontal earth-return conductors that measure grounding system impedance is derived by using the complex ground plane concept used to approximate the Carson equation. Each potential conductor section is approximated with equal-length segments that parallel one of the current conductor sections. By repeating the parallel segmentation of each potential conductor section with each subsequent current conductor section and summing the segment mutuals, overall mutual couplings between diversely routed conductors is obtained. This process is useful for calculating mutual to the potential conductor and for comparing possible potential conductor routings so as to minimize the overall mutual coupling. >