An indirect method for determining the effectiveness of the transformers operating in a distribution network with variable load schedules, according to th e parameter of their thermal load as a ratio of heat accumulated by transformer oil to the maximum permissible heat is proposed provided the thermal stability of the insulation. a mathematical model is developed which allows establishing analytically the f orced temperature value and time invariable of heating or cooling on the section of the time dependence of the oil temperature, as well as an algorithm for calculating the amount of heat accumulated by the oil and the coefficient of heat transfer from the oil to the transformer tank is compiled. approbation of the indirect method indicates that the divergence between the transformer oil heating time invariable which was obtained by solving the differential equation of thermal balance and calculated using th e part of temperature dependence of its heating for two minutes, is 3.6%, and at determination of the forced temperature value 0.8%.