Due to the form factor=1.11, so output voltage must be multiply of form factor, so we get 11, 22, 33 the answers mentioning the form factor 1.1 are correct. Just one clarification... 1.1 is form factor of a Sine wave...hence ac voltage being a sine wave holds Form Factor of 1.1 2. FORM FACTOR of sine wave is 1.11rms/avg=(im/root2)/(2*im/pi) = 1.11 FF of square wave is 1.00 n triangular is 1.15 so I think form factor is not the only logic behind 11kv/33kv but while the generator o/p formula suggest there are certain terms multiplied with the form factor(1.11) n it is forcibly made to a standardized value by changing certain parameters.. 3. First let us define form factor before we go into the question: The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is approximately 1.11. Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V. Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11 does not arise.