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My workshop project has a 1200 sqft slab with 40 cu-yards of concrete! and 900 lbs of rebar in it. Unfortunately, I didn't set up to put in a proper concrete encased grounding connection...
is there any way to do this after the fact? ( I cant actually get to any of the exposed rebar as its all got at least 2" of concrete on it per code) I do have a "normal" earth ground consisting of an 8' piece of 5/8 copper rod buried at 36" in the dirt outside the foundation and tied in with #2 Cu wire... And my water comes in via PVC, so that's not tied in...

I cant be the first person to finish the slab before I got the electrical planned fully... enter image description here

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    Do you really need a concrete encased electrode? It'd be a lot easier to just hammer in another 8' copper ground rod and call it good.
    – Nate S.
    Sep 1 '20 at 22:36
  • I don't think one is required by code in Tucson, I just wished i'd added one when it was easy and all the rebar was exposed. Just that 200' of 1" rebar in the tie-bars would make a great earth ground... Also, I don't think I need a second copper ground rod, I know some people use three, but I cant find anything in the code showing a need for more than 1.
    – mark f
    Sep 1 '20 at 23:09
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    You only need one if you measure its resistance to earth and it's under 25 ohms... but the device that does that test is expensive, so it's rarely done. Instead, you can just add a second one, and then code says you don't need to test. And yeah, that rebar would have been a great ground, but I wouldn't want to jackhammer down to attach to it unless you are planning to use really sensitive equipment that really needs an excellent ground. If your current ground isn't good enough for what you need, the cheapest, easiest option at this point would be to keep adding more ground rods until it is.
    – Nate S.
    Sep 1 '20 at 23:16
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    @NateS., I think you stated that backwards. Resistance needs to be less than or equal to 25 ohms, so you'd need a second rod for measurements over 25 ohms.
    – isherwood
    Sep 2 '20 at 15:56
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    @CarlWitthoft you don't get "concrete encased electrode" a.k.a. "Ufer" ground. The whole dang slab participates, and it's one of the best grounding methods available, particularly in "desert" conditions where dry soils reduce the effectiveness of traditional rods. It was developed to help keep munitions stores from blowing up via static, if I recall correctly.
    – Ecnerwal
    Sep 2 '20 at 17:20
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Basically not without chiseling (or drilling and chiseling) in to find rebar (and it would have to be a 20 foot chunk unless it was all tied properly for use as a ground even though you forgot that part at the time.) Covered by 2" concrete is not actually inaccessible, particularly if the concrete is relatively fresh/green. You can do that, and patch where you chisel out, or you can drive a few extra ground rods (one more will technically do, but they are not that expensive...)

Note if you have (code acceptably, but practically dubious) buried your rod horizontally at 3 feet, do consider actually driving it and any friends you give it in. I have 5 driven from the bottom of a 3 foot trench (so they go down to 11 feet) myself. My concrete contractor ignored local code and did not provide a ground connection as is required in all new concrete foundations here.

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