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My home was built in the 1930s with knob and tube wiring. The previous owner(s) replaced or augmented it, a bit at a time, and when I purchased the home from the last one he said that there was no knob and tube wiring left, and from my trips into the attic I would say that is correct.

What there is, currently, is a hodgepodge of:

  1. Modern (ca. 2000) 12/2G NM cable.
  2. 1970s era 12/2G NM cable, but with a very light-gauge (estimate about 18G) ground wire.
  3. 1960s era fabric covered 12/2G Romex, again with a very light-gauge ground wire.

I believe that my current project will replace the last of the #3; I'm replacing the feeder to the back room which I use as a home office with 12/2G MC (and the appropriate boxes, clamps, etc....one "feature" of the previous installation was a joining of three cables with no wire nuts and a massive wrap of electrical tape). Downstream of there, though, are two receptacles fed with the #2 wiring which I can't go inside the walls to replace at this time. One feed is in decent shape electrically and I was just going to let it lie at present.

The other, though... well, I found when I first purchased the house (several years back) and tested the outlets that it had no ground. I went into the box and found the light-gauge ground broken off, with no slack to be able to pull enough into the box to tie onto. Since I wanted to use a computer on this outlet and needed a ground, I drove a listed ground rod just outside the wall and ran a green wire through the wall.

It has worked to quiet the electrical "noise" my system was seeing, but I know it's not code-approved. Now, though, with the other work I'm doing, although I'm still not ready to open the wall back there (1970's wood paneling which I can't match), my thought was to abandon the existing (#2) wiring in place, install an extender box on the current receptacle box, and drop a line of MC down from the ceiling as a temporary feed which will give me ground continuity back to the service entrance. When budget permits I plan to do a walls-out renovation of this room and I'll move the feed inside the wall at that time.

What should I do with the ground rod and wire I put in at this location earlier? Should I ignore and abandon it, or would it improve the grounding of the house as a whole if I tied it in to the new MC feed at this receptacle box? Looking for a recommendation as I move ahead.

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If you do want to retain that auxiliary ground rod and connect it to the whole system there are some rules to follow. One is that this "grounding electrode conductor" must be continuous from the electrode (rod) back to the system grounding point (likely meter-main, meter-disconnect, or main panel). This means the ground rod can't be attached with a pigtail to the ground conductor of the MC cable. Another rule is minimum gauge of the grounding electrode conductor. There are calculations for this, but the code also stipulates that (at least for a ground rod?) the GEC need not be heavier than #6 copper -- and so the common practice is "don't bother calculating; just pull a #6."

In summary: to keep that rod and properly complete its installation, pull some #6 solid bare copper along with the MC. Leave enough excess so that the #6 can be installed into the wall, poked through to the outdoors, and routed to reach the end of the rod without needing any splicing.

As for whether it's needful: this depends on what the home's grounding electrode system already includes. We'd need to know what combination of ground rod(s), copper water service line, concrete-encased electrode, etc the house already has. There's a good chance the home's system may not meet today's NEC requirements and of course there's no harm in bringing it up to or exceeding those.

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  • Thanks for the recommendation. The present grounding system is what looks like two pieces of steel rebar, driven 2" apart, with a #10 and a #8 wire back to the grounding/neutral bus bar. The problem is that there are no empty spaces on that bus bar; indeed, a couple of them were "doubled up" by the previous owner (it's a 16 space GE panel serving 22 circuits thanks to a handful of 1/2" breakers). I'm saving my pennies to have it replaced with a larger panel by a qualified electrician, but until then (and it'll be a while) I suppose I'll have to see about installing another ground bar.
    – ehbowen
    Aug 19 at 15:36
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    @ehbowen Some bus bars are designed for holding two or more conductors in a single hole. Supplemental bus bars are available too; you're not limited to only one. It does sound like additional grounding will be beneficial for your system. Leave enough excess grounding electrode conductor at the panel end too to ensure you have flexibility as to where the future upgraded panel will land.
    – Greg Hill
    Aug 19 at 16:57
  • Gotcha. Unfortunately at the moment I'm a little tapped out, buying the extra MC was all I could handle until I find new employment. But what I can do is, when I pull the MC, run an extra pull string along with it and leave it in place. Then, when I can come up with 100' of #6 (according to my measurement 51' is what I needed...rather have too much than too little), it should be a simple matter to pull it through the attic to the service entrance. Thanks for the help; marking it as the solution.
    – ehbowen
    Aug 19 at 18:11
  • My brain is just not working today. I will run the pull string for possible later use, but what I can do right now is pick up 15' of #6 and drive a new ground rod six feet away from the present rebar ground electrode. Duh.
    – ehbowen
    Aug 19 at 18:17
  • @ehbowen -- post a new question with photos of the labeling on the inside of your panel's door as well as the existing neutral bar situation re: the doubled-up holes in your panel -- we should be able to figure out something to get you by in the meantime Aug 19 at 23:25

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