Impedance and circulating Current Calculations for UD Multi-Wire Concentric Neutral Circuits

Two different methods are presented for calculating the impedances of UD multi-wire concentric neutral circuits. In the simplified method, the concentric neutral is represented as a uniform sheath and in the detailed method each wire of the neutral maintains its identity. It is shown that both methods essentially give the same results for both single-phase and three-phase circuits. The parameters affecting the division of current between the earth return path and the concentric neutral in single-phase circuits are defined. The effects of conductor spacing, concentric neutral radius, and concentric neutral resistance on earth-neutral circulating currents in three-phase circuits are presented. For single-phase circuits, it is shown that reducing concentric neutral size reduces neutral current but may cause an increase in neutral watts-loss.