The main aim of the extension of electrical cables is to transmit electric power between the components of electric power system, which include generating, distributing, transporting and investing the system with the best possible performance. In this paper, we will show the general principles which enable us to choose the suitable cable-section in accordance with the economic and technical feasibility. In this study, we develop two algorithms : the first algorithm takes into account the technical study, while the second concentrates on economic study and then the final section of the cable is selected as a result of both studies.