A great diversity of the nominal voltages in power networks can be explained based on the sizes and shape of the fed zone. To find the reason of such diversity, some assumptions should be made: the variation in parameters is continuous, the voltage is constant, the electricity consumers are spread uniformly, etc. To pass from continuous load to its discrete location, a discreteness factor is calculated for various shapes of the fed zone. The expression obtained for specific costs of the network shows that they decrease in the following cases: with voltage increasing as far as the costs of line construction are not rising too rapidly; with the load density and specific conductance of phase conductor increasing; when the consumers are located more sparsely. To reduce specific costs, an appropriate current density in phase conductors should be chosen. In the work it is shown that the choice of optimal sizes for the fed zone should be made with due consideration for the step-down transformer.