A transformer has full load copper loss of 400 W. The copper loss at half full load will be
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Copper loss scales quadratically with the load factor ($x^2$).
- At $\frac{1}{2}$ load $\Rightarrow$ loss becomes $\frac{1}{4}$ of full load.
- At $\frac{1}{3}$ load $\Rightarrow$ loss becomes $\frac{1}{9}$ of full load.
- At $2$ times load $\Rightarrow$ loss becomes $4$ times full load.