I discovered our old garage/rec-room subpanel is fed with 3-wire 220V (no ground), with N-G bonded locally. (This is an attached garage, soon to have a laundry.) If I'm reading the tables correctly, this 50A feed requires a #10 Cu grounding conductor to the service. Before talking to the inspector, I'd like to run my plans past you for reasonableness.
1) The existing feed is buried in walls, so the new ground won't be able to run beside it. (Code requires the 4 wires to run together, right?) I'll instead take a fairly-direct route, and label it well. Is this likely to be acceptable as the best fix for a bad situation?
2) This grounding wire needs to be either protected (cable, conduit, etc) or #6 or larger. Instead of running 75' of #10 Romex, it appears that two #12's would cover the current rating of one #10, so I wonder about substituting both conductors of 12/2 Romex. There's a grounding bar at each end, so no splicing would be required. (It'd be necessary to mark with green tape at both ends, and probably green on the outer sheath wouldn't be a bad idea.) Is this reasonable and safe? (No point asking the inspector something that'll offend him if I can avoid it.)
Of course I'll isolate N from G, adding a ground bar to this subpanel.
Thanks much for your advice!