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A power transformer is designed to achieve maximum efficiency when operating near full load. This design consideration is crucial because transformers typically exhibit their best performance and efficiency levels in this range due to balanced voltage regulation and minimized losses.

At full load conditions, the copper losses (which are proportional to the square of the current) and core losses (which are constant, irrespective of the load) reach an optimal balance. If the transformer operates significantly below full load, the core losses become a larger proportion of the total losses, which can adversely affect efficiency. On the other hand, operating at peak load conditions can lead to increased losses from overheating and stress on the windings, which can be detrimental over time.

Additionally, transformers are not designed to be idle, as operating under no load conditions leads to unused capacity and inefficient energy usage. Similarly, half-load conditions do not take advantage of the full design capabilities of the transformer, resulting in a lower efficiency compared to near-full load scenarios. Thus, the design to achieve maximum efficiency is positioned around the conditions encountered near full load.