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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?

  • CONDUIT OR ROMEX? – JACK Sep 12 at 22:42
  • Can you post photos of the inside of the oven junction box please? – ThreePhaseEel Sep 12 at 23:20
  • Romex, no conduit or grounded box right there in the wall, just a junction box nailed to the wall. – Gregory Brouillette Sep 13 at 21:09
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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.

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    ... and, it needs to travel with the power lines, right? (You can't have your line/neutral go one side of the room, and the ground the other, right?) – Daniel Griscom Sep 12 at 21:29
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    AFIK a retrofit ground does not have to follow the path of the existing cable. The thinking is that a ground only carries a current in case of a fault expected to quickly trip the breaker. Hence the current in the isolated ground only flows very briefly, not long enough to cause significant inductive heating in any metal it passes near or through.. – Jim Stewart Sep 12 at 21:53
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    @DanielGriscom -- it does not need to follow the path of the rest of the conductors in a retrofit situation, see 250.134(B) Exception 1 for details – ThreePhaseEel Sep 12 at 23:23
  • Ah; it's the line and neutral which always have to travel together. (Interesting: thanks.) – Daniel Griscom Sep 12 at 23:55
  • Thanks. My intuition was that it wasn't big enough. Regarding the 4th option (Grounding Electrode), if my panel grounding wire is connected to the water pipe, why can't I ground to the same water pipe twenty feet closer to my access point? Why do I need to run it all the way to the panel wire rather than the water pipe it's also connected to? – Gregory Brouillette Sep 13 at 21:03

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