What is the maximum allowable operating temperature of transformer oil?

The allowable operating temperature of transformer oil depends on transformer design, insulation class, and manufacturer guidance. In practice, operators monitor top oil temperature and winding hot spot temperature rather than oil alone. Sustained high oil temperature significantly accelerates aging of both oil and paper insulation. Utilities follow loading guides that define temperature limits to maintain acceptable aging rates. Persistent operation near or above these limits triggers investigation of cooling systems, oil condition, load profile, and internal transformer health.