I have a detached garage that has a 3 wire feed, powered from a separate 100amp disconnect from the main panel. Originally the old box had the ground and neutrals together on the same bar. I discovered this upon installing a new box and assumed it was wired wrong due to everything I've read about sub panels and how they shouldn't be bonded.
Upon further research I have seen others say that since my sub panel isn't connected to my main panel in the house, that my sub should have ground and neutral bonded. I'm without power for 3 days now.
Best Answer
Since it's a subpanel, neutral and ground should be put on separate bars. That's always the case.
If you are certain you have an ungrounded 3-wire feed, then you can jumper neutral and ground at the panel. However you should be prepared to easily remove that jumper.
Not all grounds are wires. Metal conduit is a grounding path.
As soon as feasible, run a ground wire. It doesn't need to be in the same cable or route as the feed. If it's conduit, it can be fished with some difficulty. It also doesn't need to enter the house, if the house has a ground rod, it can go there instead. As soon as you have retrofitted, remove the neutral-ground jumper.
Remember the detached outbuilding also needs its own ground rod.