The role of wiring system in an electric vehicle is very important for the correct performance of electrical drives. The transmission of power and the functionality of the onboard systems depend by the correct choice of the cables used, which permits to reduce the overvoltage during the transient and the rapid ageing of the insulations. An important aspect in cable sizing is the determination and verification of ampacity, which is influenced by many factors as the temperature, the installation, and the type of voltage supply. In the paper are analyzed the thermal effects of current ripples introduced by the DC-DC converter during the discharge phase of the battery storage system. A numerical approach based on magnetic and thermal finite element analysis is used with the aim to analyze the cable performance.