The paper considers multi-winding transformers for resistance spot welding. The primary windings of such transformers have several sections for adjusting the secondary voltage and welding current. The article gives a method for calculating currents and losses in these sections for any series-parallel connection of sections. The method is based on load independence of the ratio of sections currents to the primary current. To analyze the accuracy of the method, a numerical experiment was carried out on a three-winding and six-winding model transformers. The advantage of the developed method is shown in comparison with the use of the equivalent circuit of a multi-winding transformer. An example of calculation of currents and losses in a serial transformer for resistance spot welding using the developed method is given.