The article is devoted to the development of methods for theoretical substantiation of the permissible supply voltage asymmetry coefficient for an asynchronous electric motor operating in the steady state under nonnominal operating conditions. The core of the technique lies in the fact that the heat loss power of the electric motor operating in the steady state with the actual: load/utilization level, asymmetry degree, supply voltage is calculated. The calculation is carried out using an L-shaped equivalent circuit of the electric motor, which is connected separately to the voltage components of the forward and inverse phase sequences with the subsequent imposition of the calculation results produced by the algorithm proposed by the authors and developed on the basis of the program in the MathCAD system for each of the components. Provided that we predetermine the motor torque under voltage u, the degree of asymmetry of voltage k and angular velocity ω -М(u,k,ω); the moment of resistance as a function of degree of load z and angular velocity ω -М с (z,ω); and the steady angle speed ω y (z,k) by numerically solving the equation М(u,k,ω) -М с (z,ω) = 0. Then, we find the power of heat loss ∆Р у (u,z,k) for this angular velocity. The permissible asymmetry of the voltage k d (u,z,t) is determined by equating these losses to losses permissible under heating conditions, which are a function of the temperature t of the ambient temperature ∆Р d (t), and solving the resulting equation for the asymmetry factor of the supply voltage.As an example, we made the calculation of the parameters of the steady state of the 4A355S6U3 electric motor. According to the calculation results, the curves of the dependence of the power of heat losses on the magnitude and degree of voltage asymmetry at different values of the motor utilization/load rate were plotted. An acceptable voltage asymmetry factor is determined for these conditions.