we can calculate the voltage level of overhead line by seeing the insulator. How?

Question

it is related to electrical engineering.

Answer 1

You can get an idea on counting the discs on the insulator string and multiplying by 12kV/disc.
Again it is only a general rule of thumb and is not accurate.

Source 50 years as an Electrical Engineer

Answer 2

You can only get an "idea" of the amount of voltage by the style of the insulators. It goes by class. For example (and not probably correct) 220 to o1200 have 3 inch insulators, 1200 to 2000 5 inch insulators. The average overhead in a rural community could be 13,000 volts, stepping down to 220 for the drops.