What happens to the efficiency of a transformer as it approaches full load?

As a transformer approaches full load, its efficiency typically increases due to a decrease in relative losses. Transformers are designed to operate most efficiently near their rated capacity. At lower loads, the proportion of losses—such as core losses (which remain constant regardless of load) and copper losses (which grow with load)—tends to be less favorable in terms of efficiency.

As the load increases toward the transformer’s rated capacity, the copper losses rise, but they do so at a rate proportional to the square of the current. Consequently, while copper losses do increase, the fixed core losses remain constant. This dynamic results in a greater efficiency of energy transfer as the transformer approaches full load. Therefore, under full load conditions, the transformer's ability to convert input energy to output energy remains at a maximum effectiveness, which is why efficiency is observed to be higher at this point.