I have a subpanel that is wired with 6-3 and no ground. I understand the code changes and why its important to add the ground directly back to the main. My question is this. Can I add a SEPERATE #14 green sheathed wire back to the main or does it have to be ran with the existing 6-3. Additions have been added since this was originally done and it is not possible to run the ground next to the original wire. What is the max distance away from the supply if any that I can run the ground.