Voltage Selection - Multiples of 11

Answer; it's all in your mind...

I think this post may make this the winner of the most often asked inane question in this forum.

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Get outta here. Conductor sizing is the king.

Unless we lump all stupid car questions in.

It's so that when you compute electricity per acre (43,560 ft², which is divisible by 11²), the numbers come out even.

its due to form factor of transformer (1.1)

It is true only with respect to AC power systems. The first known man-made source of electricity is a cell, which is DC in nature. But, after realizing the disadvantages of the DC electric equipments, AC Electricity generating machines were invented. When these AC Machines were developed, the power of these machines was to be compared with the already available DC electricity, as it is human tendency to compare anything new with the existing ones. As in the case of steam engines.

When steam engines were invented, the power of the steam engines was compared with that of the horses, which were the power sources before the invention of the steam engine.

So, a value called RMS Value for AC Electricity was derived which compared the effectiveness of the AC Electricity with that of the DC Electricity. This value is the Effective Value of AC Electricity. As we were more interested in knowing the effect of AC electricity, all measuring instruments were and are designed to measure only the RMS value of AC electricity – may it be Voltage, Current, Power, etc.

But, for the designer sitting in the design lab, more than the effective value, the average value over a period on one sinusoidal cycle of AC Electricity was important. So, he designed an AC electric Generator, which would produce, on an average, a voltage over a period of one cycle, of say, 10kV (10 is a round figure, you know).

But, when this machine was built to the design and put to operation and when the output voltage was measured, it was found to be 11kV, as the meter was measuring NOT the average value but the effective or the RMS value. This relation existed for any voltage. So, a factor was arrived at – relating the RMS value and the average value, called Form Factor, which is the ratio of RMS value to the Average value, which for a sinusoidal wave form was about 1.1. Then, when the voltage was to be transformed, it was easy to have a whole number for the turns ratio of the transformer and hence all subsequent AC voltages became multiples of 11.

How does one explain 25kV overhead electrification for railways in those terms?

In the primative days of power transformation, it was considered prudent to have only whole numbers as transformer turns ratio. But, when electric traction was developed, the power transformation technology was already developed into an advanced stage and it was possible to have fractional transformer turns ratios too. Thus, it was possible to have, let us say, 110kV/25kV Transformers for electic traction.

The above is also true for AC Power Transmission Voltages of 400kV & 765kV, which are not multiples of 11, but again, they are made possible due to technological advancements.