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Background: 1950s single story ranch style house in California, late 1960s split bus electrical panel (100A service?), outlets in house are seemingly all 2-prong ungrounded. Electrical panel is on outside of unfinished garage. Garage upper part connects to attic space over adjacent kitchen/laundry room area, and that seems to be where cable has been run. Am currently in the process of installing 3x 8' copper grounding rods tied with 4 gauge bare copper as grounding electrode system to electrical panel to functionally replace old single galvanized pipe/10 gauge(?) copper wire.

Goal: Incrementally add "real" EGC to outlets (convert to 3-prong), starting with kitchen counter outlets, laundry room outlets, and refrigerator outlet.

Plan: I can't do a full-blown replacement of cable from the panel to each receptacle at this point. I do want to make the kitchen/laundry rooms safer, so I plan to run a single EGC grounding "bus" (6 gauge bare armored grounding wire) from the panel up and over, through the attic space along joists. This would connect to standard square steel junction boxes above each of the 4 outlets (2 kitchen counter, 1 refrigerator wall, 1 laundry wall). I would then run 10 gauge stranded green THHN wire from each junction box down through the wall cavity to the receptacle box and new 3-prong receptacle (existing hot/neutral + new ground wire). Kitchen counter receptacles would be GFCI, while laundry room (washing machine) and refrigerator outlets would be standard 3-prong.

Qs: Is this plan compliant with current code in the use of a common bare armored grounding wire "bus" to ground multiple circuits (2 I think)? Am I OK with 10 gauge THHN running from the grounding wire bus inside walls to the outlets (no conduit)? Does my grounding wire "bus" wire need to be unbroken, or can I make standard wire nut connections with the THHN inside the junction boxes? Are there safety issues I might be missing here? Also, should I somehow mark the outside of the armored cable to indicate it is an EGC - with green tape perhaps?

Bonus question: Laundry outlet is within a foot of the copper overpressure drain pipe from the water heater. Hot and cold water lines and black steel gas line are bonded together, but not connected to electrical ground system. Should I extend the gas/hot/cold bond to the copper overpressure pipe, and connect that with a 10 gauge THHN wire to the grounding "bus" to link the grounding systems?

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    Not an electrician, but, AIUI, there's no need to run #10 in the house. After all, those circuits are protected by 15 or 20A breakers and the circuits run on #14 or #12 wire. The ground only has to be as big as the circuit it protects. If you were going to replace the wire on a circuit with a modern NM-B cable, you'd get three #12 or three #14 in the cable... Sure, overkill is OK, but not necessary and not at today's copper prices. My comment specifically excludes the wire running to the grounding rods - I'm not sure at all about what's needed there.
    – FreeMan
    May 11 at 15:23
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    @FreeMan I've got a 100 foot roll of #10 stranded THHN, so I figured I'd use that. BTW, I've found your posts here helpful and knowledgeable - thanks.
    – Armand
    May 11 at 15:29
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    If you've got it, I guess there's no harm in using it. Of course, you might be able to exchange it and get some cash back... And thanks! :)
    – FreeMan
    May 11 at 15:30
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    NO black thhn on the grounding conductor NEC 250.119.A sizes 4 and larger can be reidentified as grounding conductor. This means your #10 needs to be green or bare.
    – Ed Beal
    May 11 at 16:28
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    @JimStewart I was thinking of running the #10 THHN into a receptacle box, then using a wire nut to connect that with a standard pigtail to the receptacle and a pigtail to the box. If the existing box is not metal, I would replace it with a metal one.
    – Armand
    May 11 at 17:25

3 Answers 3

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Only the main grounding conductor must be "unspliced or irreversibly spliced" (i.e. from the service entrance to the ground rods.)

As already noted #10 is overkill for circuit grounds (unless 25 or 30A circuits.) So's the #6, though I grasp your intent in doing that (make big heavy bus) but the fact is that the grounding conductor only needs to be upsized if the circuit conductors are upsized for distance/resistance - it's not normally carrying current so it's not getting hot in normal service, and when it is carrying current the idea is that the breaker should soon trip, so it does not heat up much. So 12 AWG ground is good enough for any 20A circuit running on 12 AWG wire. The "retrofit ground rules" are to connect to a ground of the same capacity or larger heading back to the same service box - so you are certainly legit, just a bit overkill from what you need to be.

The laundry outlet is also supposed to be GFCI these days (wet area) whether or not there is a sink there.

The one place I think you might be going a bit small is the bond to the water & gas lines at #10. That's a place that (IIRC) calls for 6 AWG.

Since you are using steel boxes, remember that you need to ground them using a pigtail to the threaded hole provided for a grounding screw (#10-32) (it does not need to be a "green grounding screw" though they are sold. It just needs to be a 10-32.)

Additional note: there's no reason to not start with putting in the GFCIs required now, if they are not currently present. They provide protection with or without ground - you just have to use the "no equipment ground" stickers until you get the ground wire run to them.

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Your basic idea is good but you must use GREEN wire if insulated yes all the way to #6 requires green insulation or bare is ok ( solid, stranded, insulated or bare), All splices in a box and after the panel you can basically ground just about everything with a 10awg from the service panel (if updating like you are I run 10awg to the splice boxes and yes you can use wire nuts. The line from the rods to panel must be continuous or have compression/welded connections.

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  • The ground rod to box conductor is bare 6 gauge copper, continuous through the various clamps on the rods and to the box. The #10 THHN is green. The #6 bare armored cable I want to run in the attic is bare inside the armor, but since all armor looks the same (I think?) I was thinking of wrapping it every 10 feet or so with green tape.
    – Armand
    May 11 at 17:19
  • It’s not required to mark the conduit but if you use metallic bonding is required.
    – Ed Beal
    May 11 at 18:35
  • What bonding do you mean here - armor to junction box? Grounding wire to junction box? I was planning on bonding the grounding wire to each metal junction box with a pigtail, and to attach the armor to the box knockout with a metal clamp that would be a bond.
    – Armand
    May 12 at 4:19
  • If you use metal boxes the connectors usually will create the bond but some inspectors require grounding lock nuts or bushings
    – Ed Beal
    May 12 at 13:19
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You are talking about retrofitting grounds. The 2014 NEC greatly expanded the freedom to do that.

A main ground backbone is a great idea. You only need #10 for the backbone, since that will suffice for any circuit up to 60A wire. Circuits are allowed to share 1 ground wire, since nobody expects two high-current ground faults to happen at the same time.

I am not comfortable with stranded or insulated THHN for the standalone grounds, however. THHN wire is not rated to be used in a house outside of conduit. However bare solid wire is routinely used without conduit.

Stranded bare wire is going to immediately "birdcage" so only solid bare wire makes sense to me.

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  • I guess I don't understand the reasoning behind bare wire OK but THHN not OK in a finished wall - any insight on that? Also, I thought I read somewhere that THHN in wall add-on grounding lines were an exception and were allowed, but I will look it up rather than relying on my memory :)
    – Armand
    May 12 at 4:23

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