The paper deals with the calculation of Joule's losses on overhead transmission lines. The presented method allows taking into consideration the influence of a current asymmetry caused by various conductors layout and their inductive and capacitive couplings. The losses evaluation has been carried out with respect to the total transmitted energy, data given by year and day load diagrams were used. The various properties of a load including its asymmetry connected to the transmission line could be involved. The proposed method enables to determine operating costs respecting all important factors, to accomplish an economic analysis for an assessment the whole service life costs and to compare economic advantages of a transposed or non-transposed transmission lines.