Pure soft copper in power-transformer windings may not be strong enough for units of low impedance or high rating. Increased strength may be obtained by alloying or cold working. Cold-worked copper will soften with time and temperature, so it is important to predict the effects of overloads and short circuits. A method of using Arrhenius plots to do this from laboratory measurements is described which shows that, for normal power transformers, conductor softening will not occur in normal service