In an electrical system of 3 phase at incomer in R phase voltage is 252 V and both other phases voltage is balanced. Why is it so?

I am not sure I understand your question, but here in the U.S.A. the 3 phase multiplier is the square root of 3.

So for example a single phase would be 120 * 1.732 = 207.84 (rounded up to 208). This the "High Leg" voltage.