In three phase power calculations, the square root of three is used. Why is this so and how was it determined?

Has to do with the geometry of 3 phases that are 120 degrees to each other. From trig the line voltage between any two phases is twice the max of the voltage of any one phase times sin(120 degrees), or Sqrt(3) times the phase voltage.

Here's an explanation:

Re: Why is the Square root of Three a factor in 3 phase power calculations?