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A house that is being constructed is being built on ground that is only 18" deep before it hits bedrock. The rebar can't be used as the grounding electrode because there is a vapor barrier below the concrete slab. There are no copper pipes to use either. We can't use plates or horizontal ground rods because those need to be at least 30" deep.

Is there a grounding method that would work for this case? Will drilling into the bedrock be acceptable? I've been looking through section 250 of the NEC and haven't found anything that could help.

  • There are probably two critical factors--resistance and security. If you can demonstrate that you've accounted for both, you'll probably get a pass. – isherwood Sep 30 at 17:53
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    One wonders that a building permit was issued without approved documentation on how the required grounding stick would be installed. – Carl Witthoft Sep 30 at 18:59
  • Is the vapor barrier under the footings as well as the slab? Does your AHJ have any local amendments to the NEC? – ThreePhaseEel Sep 30 at 22:58
  • Also, how far into construction are you? Is the slab cured already, or has it yet to be poured? – ThreePhaseEel Oct 2 at 1:58
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You need to talk to your AHJ

(Authority Having Jurisdiction, i.e. The guy who issues the permits).

This is provided for in the Electrical Code by giving AHJs latitude to set grounding standards to suit local conditions. You are like the 10,000th person in their jurisdiction to have that problem. They have compared notes with other AHJs nationally with similar geology. They know how to solve it.

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