how overhead transmission voltage is decided?

Based on MW transmitted & length of line,of course economy as well.

voltage of line is depends on voltage drop down, power losses and other cases that sinnadurai said.

Transmission line voltage selection is based on distance over which power is to be transmitted and amount of power to be transmitted. You can use below empirical formula for calculation of economical transmission voltage:

V = 5.5 SQRT[(L/1.6)+(3P/100)]
V is economical line voltage in KV
L is length if transmission line in KM
P is per phase power to be transmitted in KW

somewhere voltage is 132 KV somewhere it is 400 KV .Presently HVDC with even high voltage of 765 KV /800 KV is under use. I want to know , is there any standard formula for using these specific voltages?Along with please mention what are the factors and how those factors like conductor spacing are decided during installation of high voltage transmission line?

Pawan,

After considering everything that everyone above has said, then you have to consider the local "tradition" and legacy conditions since there is no point in specifying a new line at 145kV if the existing lines are 132kV. The cost of interconnecting equipment will far exceed any other gain, then there's the equipment sparing costs, development of new standards, ready availability of equipment at the new voltage, etc., etc.

On the other hand as new lines are placed in new territories there is the opportunity to take advantage of new technologies such as HVDC which have greater latitude when interfacing to the older systems.

At the risk of sounding redundant, economics, standards, local conditions, and the "we've always done it that way" philosophy tend to guide and limit the choices.

Dear Pawan,
HVDC systems is under use, because the power electronic equipments are very expensive and use of HVDC in short distance lines is not economical.
On the other hand, voltages of lines is limited. because we need standards to equipments manufacturing, Implementation and maintenance in all of the world for economical reasons, and use of arbitrary voltages is impossible. for example we have 132,230,400kV in Iran and we cant design a 160kV line!
I hope my answer helps you.