The paper approaches the optimization of water distribution networks supplied from one or more node sources, according to demand variation. Traditionally, in pipe optimization, the objective function is always focused on the cost criteria of network components. In this study an improved linear model is developed, which has the advantage of using not only cost criteria, but also energy consumption, consumption of scarce resources, operating expenses etc. The paper treats looped networks wich have concentrated outflows or uniform outflow along the length of each pipe. An improved model is developed for optimal desing of new or partially extended water distribution networks, which operate either by means of gravity or a pump system. The model is based on the method of linear programming and allows the determination of an optimal distribution of commercial diameters for each pipe in the network and the length of the pipes which correspond to these diameters. Also, it is possible to take into account the various functional situations characteristic found during operation. This paper compares linear optimization model to the some others, such as the classic model of average economical velocities and Moshnin optimization model. This shows the good performance of the new model. For different analyzed networks, the saving of electrical energy, due to diminishing pressure losses and operation costs when applying the developed model, represents about 10 ... 35 %.