The paper presents a study of the relationship between the measurement error of the parameters of the classical transformer equations, the stiffness of these equations and the error in the calculation of currents in the windings. The mathematical formulation of the error estimate is given. The study was performed on the example of a linear two-winding transformer. The load of this transformer is the active resistance. The currents in the windings are calculated using the results of the calculation of a three-dimensional non-stationary magnetic field of a transformer with an attached electrical circuit and using an equivalent circuit of the transformer. A comparison of the results of calculation and measurement is given. The calculation error increases with decreasing transformer load and reaches a maximum value for short circuit mode. The increase in error is due to an increase in the number of conditionality of the transformer equations. It is shown that to obtain a satisfactory accuracy of the primary current calculation in short circuit mode the measurement error of the transformer model parameters should be up to 10–3 %.