According to the new conception of electrification, the medium voltage 20 kV networks are to be extended. To what degree it is advisable to do so, depends on many factors: characteristics and design of the networks and step-down transformers, etc. In medium voltage networks as compared with low-voltage 0.4 kV networks, voltage and power losses can be neglected. Therefore the wire diameter can be diminished, calculating it only from the condition of its heating by load currents. Later, it must be checked if the wire diameter corresponds to mechanical requirements. In supplying the energy via low-voltage lines, greater expenses must be taken into account than in delivering this energy via medium voltage lines. Evaluating the transition from low-voltage lines to medium-voltage lines, we must know how the cost of small step-down transformers increases in the medium-voltage version.