This is a common and excellent question. The practice of using voltages that are multiples of 11kV (e.g., 11kV, 33kV, 66kV, 132kV, 220kV) is rooted in historical engineering practices and the practical challenges of early electrical power systems, not a strict technical rule.

The primary reason is related to voltage drop compensation. In early power transmission systems, which covered long distances, there was a significant voltage drop along the line due to the resistance of the conductors. Engineers designed the systems to transmit a voltage that was approximately 10% higher than the desired receiving-end voltage to ensure the load center received the target voltage.

For example:

• To deliver 10kV, the sending-end voltage was set to 10kV + 10% (1kV) = 11kV.

• To deliver 30kV, the sending-end voltage was set to 30kV + 10% (3kV) = 33kV.

• To deliver 60kV, the sending-end voltage was set to 60kV + 10% (6kV) = 66kV.

Another related factor, though often debunked as the sole reason, is the form factor of a sinusoidal AC waveform, which is approximately 1.11. Early system designers sometimes considered this in their calculations, but the voltage drop compensation model is the more widely accepted and historically accurate explanation.

It is important to note that this practice is not a universal law. Modern, high-voltage transmission systems use voltages like 400kV, 500kV, and 765kV, which are not multiples of 11. This is because at these extra-high voltage levels, line losses and voltage drop are significantly reduced, making the 10% compensation less critical and allowing for greater flexibility in voltage standardization.