Rates of development of the Latvian economy are closely connected with continuous development of existing cities and settlements of city type and increase in total number of city dwellers. The cities are important consumers of the electric power because not only the most part of the population of the country lives in them, but industrial enterprises are placed there. With development of the cities the volume of power consumption changes because of building of new microdistricts, trading and other objects, growth of the expense of the electric power for household needs. In the cities there are objects of social and municipal character the electric loads of which are comparable to similar indicators of the industrial enterprises. With the development of the cities there is a necessity to make changes in networks and build new lines and substations, to provide reconstruction of existing substations. Power consumption in big cities makes two thirds of annual consumption in the country. The biggest user of the Latvian power supply system is its capital city – Riga. The power supply system of Riga is a component of the country power supply and, as well as the whole power supply systems, it is in a process of continuous development. The whole power supply system and its parts alike should be rationally constructed and development stages should be economically proved. For creation of reliable and economically effective system of power supply, the general principles of construction of networks should be developed. It is allows to generate rationally power supply system, to expand networks without radical changes in the future, to make placing of future substations and sources of supply, and also to define sequence of building of new elements of system. It is necessary to consider that the solution of the problems of development at early design stages is carried out in the conditions of incompleteness and uncertainty of the information as well as unstable economic situation in the country in the given period. This makes it difficult to take decisions about the tendencies of development of power supply systems. The purpose ot this Thesis is elaboration of a system approach and uniform principles of formation of power supply systems for the cities and choosing of networks parameters at early design stages of development projects.