The paper treats looped networks wich have concentrated outflows or uniform outflow along the length of each pipe. An improved model is presented 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 exploitation.