The current rating of a power transformer is mainly limited by temperature rise in windings and core. As current increases, copper losses (I²R) grow rapidly, causing heating. If the temperature exceeds insulation class limits, insulation ages faster, reducing life expectancy or causing failures. Cooling capability, ambient temperature, winding design, conductor size and oil or air circulation all influence allowable current. Standards define maximum hot spot temperatures and winding gradients. Short term overloads may be allowed if thermal models show that cumulative insulation aging stays within acceptable limits over the transformer’s lifetime.