This article deals with the problem of calculating the additional real-power losses in double-wound supply transformers with voltage class 6 (10)/0,4 kV, caused by unbalanced active inductive load connected in a star connection with an insulated neutral. When solving the problem, authors used the theory of electric circuits, method of balanced components, field and comparative experiment, modern devices for the analysis and synthesis of electric circuits. With the research results authors obtained the functional dependence, allowing calculating the additional losses in the transformer caused by unbalanced load, which differ from the similar ones due to the use phase resistance. In order to confirm the obtained functional dependence, researchers measured current, voltage and real power for each phase of "distribution transformer-unbalanced load" module. The experiment results allowed making a conclusion that the real-power losses, calculated according to the classical formula, should be adjusted in accordance with the developed functional dependence. The application of functional dependence is possible in organizations involved in the design, replacement and upgrading of transformer substations of urban and industrial distribution networks of electric power systems in order to increase energy savings in them.