I'm replacing an oven that was wired with 240V 3-wire 30A (Black, White, Red) and the new oven is 240V 4-wire 30A. Can I run a copper ground wire from a 15A grounded outlet two feet away?
No, you can't. That ground wire isn't nearly big enough!
You are allowed to retrofit just a ground, however it has to go
- back to the panel
- to non-flexible metal conduit or AC cable that goes all the way back to the panel
- to another appliance whose ground wire is at least 10AWG all the way back to the panel
- to the Grounding Electrode system of that panel, i.e. The copper wire that connects the panel with the ground rods/water pipe. You cannot use the water pipe.
- To some combination of the above
There is no need for the retrofit ground to be routed with the conductors, since it does not carry current except during fault conditions. Use any feasible route.