This article discusses the loading limits of high-voltage power transformers. It looks at the effect of cyclic loads and ambient temperature. It covers two types of transformer, those designed to British Standard (BS)171 and those designed to Electricity Supply Industry (ESI) specification 35.2 for emergency rated system transformers. The article attempts to clarify the different performance characteristics of these transformers which, from time to time, have led to some confusion amongst power engineers.