The conversion between the common and the differential modes is of great importance in the transmission of data over poorly balanced cables such as electrical wiring. In this paper, we have studied some of the mechanisms involved in this conversion, both experimentally and theoretically. The results show that along-the-line variations in the line parameters be it gradual or abrupt, are responsible for part of the common mode in electric cables. Asymmetries at the source are also responsible for part of the common mode conversion and they can be dominant in certain cases