Our Professional Answer

The use of voltages in multiples of 11 kV (such as 11 kV, 33 kV, 66 kV, and 132 kV) is a historical convention, not a modern technical requirement. This practice originated in the early days of alternating current (AC) power systems. Early electrical engineers designed transformers to step down high voltages to usable levels, and a common practice was to have a 10% voltage margin to account for line losses. This meant that a nominal 10 kV system would be operated at 11 kV to ensure a stable 10 kV supply at the user end. Over time, as technologies improved and line losses became more predictable, this 10% margin was often standardized, leading to the widespread adoption of 11 kV and its multiples.

Today, while many legacy systems around the world still operate on these standards, modern high-voltage transmission systems often use other voltages chosen for optimal technical and economic efficiency, such as 400 kV or 765 kV.