In order to reliably determine the supply voltages (Ui) of the individual phases (L1, L2, L3) of a load (4) in a multiphase supply network (2), in particular a three-phase supply network, a measuring module (6) is provided and is used to determine the supply voltages (Ui) from measuring voltages (Ui,mess) with the aid of a matrix operation. The matrix operation is used, in particular, to compensate for potential differences or potential shifts between the measuring system and the supply network (2) without the need for hardware measures, for example a voltage transformer.