The practice described herein provides instruction on the testing of distribution transformers as voice-frequency noise generators, to measure the degree to which they may contribute t o electrical noise in communication circuits that are in parallel with the power-supply circuits serving the transformers. Transformers have a characteristic common to other iron-core devices of causing harmonic currents at voice frequencies to flow in supply circuits t o the transformers. The magnitudes of these currents, and of interference that may result, vary according to design of the transformer as well as excitation voltage. The tests described in this practice provide a standard method for use by the transformer manufacturer, user, and others in industry for the purpose of better evaluating the design choices available and moving toward industry objectives as to levels of harmonic exciting current expressed as I*T (current X telephone influence factor [TIF])