The DC load current in copper electrochemical plants is normally supplied by two or four similar transformer-rectifiers connected in parallel, 6 pulses each (12 or 24 pulse systems respectively). An accurate determination of losses in transformer-rectifiers is very important, because it establishes the true value of efficiency and hence, the cost of copper production. The efficiencies of the transformer - rectifiers declared by the manufacturers are not always checked in the plants, once the transformer-rectifiers are fully operating. A true measurement of losses must consider the actual waveform of AC voltages and AC currents, including harmonic effects. However, at the factory the tests are focused to measure losses at rated sinusoidal voltage and reduced current or losses at rated sinusoidal current and reduced voltage, but not both rated variables simultaneously. In this paper, a method to measure in the factory the losses at rated voltage and rated current is proposed. © 2011 IEEE.