I'm moving my water heater to another corner of the room it is in, to allow for more usable space in the room. However, the grounding wire is only 14 ga instead of the required 10, and it's not in the same cable as the power conductors. I need to upgrade the ground wire or run a new 10/2 w/ground cable, but I'm not sure which will be simpler.
This answer about grounding an oven receptacle states that you can use "any achievable route" to retrofit a ground. Does that mean you can run THHN outside of conduit, even though you'd normally never be allowed to?