Dear Suresh,

In India, transmission voltage levels are expressed in terms of 11 and its multiples (such as 11 kV, 33 kV, 66 kV, 132 kV, 220 kV, 400 kV, and 765 kV) mainly due to historical and technical reasons. When the power sector was first established during the British era, electrical equipment like transformers and switchgear were designed with a 10% insulation margin above the nominal voltage. For example, instead of designing for exactly 10 kV, engineers standardized it at 11 kV to account for safe operation. This practice continued for higher voltages, making them multiples of 11 for consistency and ease of equipment manufacturing. Over time, this became an industry norm across the country, ensuring compatibility, uniformity, and reliability in the design, operation, and maintenance of transmission systems. Thus, the use of 11 and its multiples in India is a legacy of both engineering safety margins and historical standardization.