Abstract: In this study, researchers conduct an experimental investigation of the changes of active power losses of power three phase two-winding transformer in the idle mode when changing the temperature of its magnetic system. The object of the study was the three-phase two-winding transformer of the brand RP3-380/220V-2, 5 with nominal power of 2.5 kW·A, voltage on the high voltage winding of 380 V, low voltage winding of 220 V. To perform the experiments, a stand was assembled consisting of a personal computer, laboratory autotransformers, measurement unit, universal eight-channel temperature meter, three phase two-winding transformer, analyzer of power quality and interface devices and measuring devices with a personal computer to synchronize measurements. Experimental studies were carried out with conventional phase voltages at the LV winding of the test transformer are respectively 200, 210, 220, 230 and 240 V in accordance with the requirements of GOST 3484.1-88. "Transformers Electromagnetic methods of testing". The study presents graphs corresponding dependencies confirming the compliance of completed experimental studies of the above-mentioned standard. By results of processing of experimental data managed to draw the following conclusions. With increasing temperature the magnetic system the total losses of active power idle in all three-phases of the test transformer are reduced. With increasing temperature the magnetic system when testing three-phase transformers with three-phase excitation transformation ratio is practically unchanged.