In a three-phase transformer, the load losses are generally similar between phases, with some small variations due to the proximity of the tank on lateral phases. However, when a three-phase test configuration is used, the measured active losses in each phase are significantly unbalanced and do not reflect the actual losses in each phase. To provide a physical explanation for this phenomenon, measurements were taken on small magnetically coupled reactors and the experimental results were used to validate a simulation model based on the magnetic-circuit theory. The simulation model successfully reproduced the apparent load loss unbalance in a three-phase 47-MVA transformer. It is concluded that, even if the apparent losses are unbalanced, the total load losses in the transformer are correctly obtained by the summation of losses in each phase. The authors point out that since the FRSL (frequency response of stray losses) method is based on the comparison of losses to diagnose winding displacements, the three-phase test configuration cannot be used for this purpose