Electric supply shortages experienced last summer and expected again this summer have renewed interest in energy conservation. Traditionally, utilization equipment (such as lights and motors) has been the only part of a building's electrical system for which efficiency was given major consideration in design. Losses in the electrical distribution system itself were generally assumed to be negligible, and the substantial impact of transformer efficiency was often overlooked. New government guidelines and product offerings are changing the way transformer efficiency is viewed and energy-efficient transformers are specified.
Load loss is associated with full-load current flow in the transformer windings, due primarily to the resistance of the winding material. Because transformers traditionally used copper windings, load loss is also referred to as "copper loss." Following Ohm's law for power in a resistor, P=I2R, copper loss varies with the square of the load current.
For a given transformer, the manufacturer can supply values for no-load loss, PNO LOAD, and load loss, PLOAD. The total transformer loss, PTOTAL, at any load level can then be calculated from:
PTOTAL = PNO LOAD+ (% Load/100)2 x PLOAD