Voltage ratings are determined by insulation design, creepage and clearance distances, bushing ratings, and core/winding configuration. The transformer must withstand continuous operating voltage plus overvoltages from switching and lightning, so basic insulation level (BIL) is carefully selected. System voltage class and grid standards dictate nominal ratings. Internally, turn insulation, barriers, and oil or solid dielectrics are designed to coordinate with external bushings and clearances, ensuring safe performance at the specified voltage levels.