Why do we use multiples of11 as voltage in transmission lines ?

Answer 1:

Transformer comes into picture when you talk about transmission.Transfomer Induced voltage equation contains 4.44 factor.

E=4.44*f*T*phi
E -Induced emf per phase
T -number of turns
f -frequency
phi -maximum flux per pole

From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11.
So always transmission voltage is multiple of 11.

Answer 2:
The reason is some thing historical. In olden days when the electricity becomes popular, the people had a misconception that in the transmission line there would be a voltage loss of around 10%. So in order to get 100 at the load point they started sending 110 from supply side. This is the reason. It has nothing to do with form factor (1.11). Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V.
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