1. I have two devices which are are very far order of 10's of meters. Final distance depends upon installation site.

2. Now what I read is Rs485 is three wire(not 2 wire communication which is misconception), so common gnd should be there.

3. Now both devices are powered by their own power supply(or battery in case power goes)

4. How do I do common gnd for them. What I read running 3rd wire parallel to A&B of Rs485, large gnd loop will form? So what's the solution for it?

 

It is not necessary for a third conductor (GND) for RS485 transmission to function.
It will work quite well without.
Max.

 

1. I have two devices which are are very far order of 10's of meters. Final distance depends upon installation site.

2. Now what I read is Rs485 is three wire(not 2 wire communication which is misconception), so common gnd should be there.

3. Now both devices are powered by their own power supply(or battery in case power goes)

4. How do I do common gnd for them. What I read running 3rd wire parallel to A&B of Rs485, large gnd loop will form? So what's the solution for it?

RS-485 is full differential which allows for a considerable common mode ground difference between two nodes. There are other systems (CAN in particular) that are quasi-differential with respect to a common ground. CAN transceivers do allow some common mode differences, but it is more limited than RS-485.