I’m remodeling a 100-year-old house that has knob and tube electrical wiring (14 gauge). I’m running two new 20 Amp circuits to the attic with 12-3 w/ground Romex. Can I run a ground wire from the new Romex circuits to the existing knob and tube receptacles to ground them and allow me to replace the ungrounded receptacles with grounded ones? This, in essence, would be a “shared” ground. I know that a “shared” neutral is improper and will trip Arc Fault Circuit Breakers; is a “shared” ground also improper?
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What do you mean "run a ground wire"? If you can run a ground wire why not just run new romex?
No you cannot ground outlets in this way. The only way to run a 3 prong receptical on an ungrounded circut is to make it a gfi circuit.
I agree with Mike. Why in the world would you add a ground wire to an ancient knob and tube circuit when it would be just as easy to run new romex?
Agreed. Only way to do it is to rewire. Or you can install GFCI plugs which will shut plug off if there’s an issue. They will work w/ 2 wire, but if you use a tester on them it will read a fault w/ ground. This is b/c the testers are made to trace down a ground wire. While the GFCI will not be grounded, it will still shut the plug off, so no threat of shock. If no power is running through plug, grounding is a mute point.