The choice of lines’ cross-sections for electric networks' rational designing is necessary to fulfill technical and economic requirements. The existing method of a choice on economic considerations using economic density of a current does not work in the conditions of free market prices. The aim of this paper is working out the modified method of economic intervals for choice lines’ optimum cross-sections under market conditions. The method is based on search of minimum total annual costs for lines’ construction. By well-founded mathematical transformations it is possible to exclude some components from function of costs, which are ambiguously defined in the conditions of market economy. The method allows to execute economically well-founded choice of 20-110-330 kV overhead lines. Developed nomograms have universal character for given voltage lines. The creation of user interface for method’s realization by Matlab environment is proposed.