This paper considers the harmonic supply voltage evaluation of non linear distorted loads, such as diode bridges, arch furnaces and so on, connected to medium voltage cable line. The connection to the MV grid allows a cost reduction of the measuring and protection apparatus, but it introduces problems such as the low short circuit power that can cause a high harmonic voltage distortion. The paper presents a frequency evaluation of the equivalent impedance of the distorted loads supply line and a harmonic analysis of the supply voltage and absorbed current. The analysis has been carried out in different load conditions and it considers also the possible presence of LC tuned filters.