This paper analyzes the existing method of selecting cross-sections of wires by the method of economic intervals, substantiating the the necessity of modernization for power transmissions with a voltage of 220 kV and above, performed with a split phase. The proposed algorithm for determining the current load in which, on the basis of minimum reduced cost, additional reactive power sources should be abandoned and split phase wires should be used. A series of intersecting curves, allowing not only to choose the best wire size, but also to determine the load at which the wire phase should be split into several components.