While in the first stage of electricity supply low voltage is used, the second and third stages exploit higher voltages; therefore, it is of importance in these cases to find the least per unit costs. The next task is to determine the scale of nominal voltages. To solve these tasks, the fed zone should be optimized by size and stage voltage. The author derived an expression for such a zone which contains zone size and stage voltage dependent and independent terms as well as terms increasing and decreasing with zone size increasing. At the optimum zone size and stage voltage the influence of these terms is balanced. Under certain circumstances it is expedient to install step-down transformers directly at the customer.