PF 0.8 at full load is commonly used. It is a fair guess for most normal motors and it also makes the math very simple since 0.8^2 + 0.6^2 = 1. Hence magnetizing current is 60 percent when "torque" current is 80 percent of rated current. And that does not mean that total current is 140 percent ;-)
If you are about to apply static power factor corection, do not apply it based on an assumption. You must be very careful not to correct to more that 80% of the magnetising current if you are permanently connecting the capacitors to the motor circuit and using the same starter to control both.
For bulk correction and voltage drop calculations, then the assumption is fine.
The power factor can vary considerabley depending on the size and speed of the induction motor.
Larger motors tend to have a much lower magnetising current (down to 20%) than small motors (up to 60%). Two pole motors have a lower magnetising current than 8 pole motors and submersible pump motors tend to have a high magnetising current.
Just to add to skogsgurra's good response, voltage doesn't matter. What might matter is whether the motor is synchronous or induction. 0.8 is a good number for induction motors at reasonable loads. With synchronous motors, it's usually a controllable factor.
For actual induction motor manufactures, consider the following guidelines:
4160 Volts is used for large motors, 300 HP and above. Use PF 0.85 to 0.90
600 Volts is used for medium size motors 50HP to 500 HP, use PF 0.85 to 0.88
