The deferment of reinforcement of a bulk supply point or primary substation, which is above firm capacity has substantial economic benefit. As the rating of power transformers is largely limited by the temperature of the winding insulation, one study before deferment which can be achieved is to determine if the individual transformer can thermally operate at loads above the nameplate rating. This article describes how a thermodynamic model for power transformers is derived and the benefits of the application achieved.