The power rating of a transformer is determined mainly by its thermal limits, which depend on allowable temperature rise in windings and insulation. Conductor cross section, core size, cooling method, insulation class and permissible losses all influence how much current and voltage the transformer can handle continuously. Standards define maximum temperature rises for different cooling and insulation classes. Designers select core and winding dimensions so that, at rated kVA, losses and resulting temperatures stay within those limits, ensuring acceptable life expectancy under normal operating conditions.