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Redoing my kitchen and installing a new range while also moving the location of the range outlet (thus, I am required to bring it up to code). Existing range is 8/3 with no ground and a 3 prong outlet I will be changing to 4.

There are no other grounding sources nearby that can meet the requirements so I want to retrofit a ground back to the breaker box, which I can do fairly easily as my floor joists have an open structure.

I know that retrofitting a ground is acceptable practice. I know I need a 10AWG ground. I know that the NEC says I can run it unprotected through the sealed floor cavity. The problem is that I can't figure out exactly which type of wire to use.

Local HI stores only have THHN and bare copper. As far as I can tell, THHN can never be run without conduit and after extensive searching I can't find any other single-conductor wiring rated for in wall use. Do I really just run a bare ground 50ft through my floor? Is unprotected THHN acceptable in grounding applications (can't find an NEC reference)? I know it's not dangerous since it's just a ground but it just seems counterintuitive to not protect the wire itself from any basic threats (corrosion, etc.)

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As long as it stays in the joist/stud cavities, a bare or THHN wire is acceptable for a separate grounding run

Separately run grounding wires for receptacles and branch-circuit extensions can be bare copper or (green) THHN of the appropriate gauge, provided they are run within building wall and joist cavities that protect them from physical damage. This is set out by NEC 250.134(B) Exception 1:

(B) With Circuit Conductors. By connecting to an equipment grounding conductor contained within the same raceway, cable, or otherwise run with the circuit conductors.

Exception No. 1: As provided in 250.130(C), the equipment grounding conductor shall be permitted to be run separately from the circuit conductors

and 250.120(C):

(C) Equipment Grounding Conductors Smaller Than 6 AWG. Where not routed with circuit conductors as permitted in 250.130(C) and 250.134(B) Exception No. 2, equipment grounding conductors smaller than 6 AWG shall be protected from physical damage by an identified raceway or cable armor unless installed within hollow spaces of the framing members of buildings or structures and where not subject to physical damage.

Of course, if you're paranoid about this, you could simply order some 8AWG Bare Armored Ground cable in and use it instead. Note that where you are running these separate grounding wires into boxes, you'll need to use a compression or setscrew type grounding strain relief fitting ("Kenny clamp") to ensure that no strain is transmitted to the grounding connections inside the box.

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  • Would you prefer insulated wire so that it is less likely to be subject to corrosion? I can imagine a slow leak in a kitchen that wouldn't be discovered for years, or just water getting through from usual floor cleaning. Aug 8, 2021 at 11:04
  • +1 For Kenny clamp. I haven't heard that in a long time.
    – JACK
    Aug 8, 2021 at 12:29
  • when you say: "to ensure that no strain is transmitted to the grounding connections inside the box", do you mean panel box or receptacle box or both?
    – Matt
    May 2 at 18:44
  • @Matt I'm referring to whatever boxes it runs into (whether it be a panel box, a receptacle box, a switch box, or some other junction box somewhere) May 2 at 23:44
  • 0 If it's ok to post link here, I'd be interested to see where I can buy single or a couple of Kenny Clamps for my needs #6 or #8 bare copper). All i see is trade store that sell in quantity of a 100
    – Matt
    May 3 at 0:51

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