What is the formula for calculating the full load current of 300 HP motor?

One horsepower electric is defined as 746 watts, so 300 horsepower is 224 KW. You don't say what the power factor is, so lets assume 0.85. Divide 224 KW by 0.85, and you get 263 KVA. You are running three phase, no doubt, so divide by 3 to get 88 KVA per winding. Lets assume 480 three phase delta, so divide by 480 to get 183 amperes per winding. Finally, look at motor efficiency. Lets assume 0.95, giving 192 amperes per winding.

To translate between winding current and phase current in balanced three phase delta, multiply by the square root of 3, giving a phase current of 333 amperes per phase.

That's a lot of current, requiring very large windings and conductors, and it would be a very large motor. I would use a motor with a higher voltage, perhaps 4160. This would bring the current down to around 22 amperes per winding, or 38 amperes per phase, a more reasonable value for such a large motor.