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Let's say we have 9'x25' monolithic concrete slab of 5" thickness with a 12" deep by 12" wide "haunch" footer around the perimeter. At 2-1/2" inches below the surface of the slab there is the #4 rebar grid set at 24" on-center. Now, running through one of the 25' footers is a 20' length of #4 rebar that is sitting on chairs that rest directly on the soil. It is encased in the footer but is not attached to the rebar grid above. Can that 20' length of #4 concrete-encased rebar serve as the UFER electrode that has the copper wire attached to it? slab rebar diagram

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    Is there a layer of poly (plastic) between the slab and the underlying bed/subsoil? May 28 at 20:04
  • There won't be a layer of poly between the footer and soil though possibly a layer of poly between the inner "field" of the slab and the stones. Not sure if that would be a waste, since the moisture could enter the slab from the footer.
    – mr blint
    May 28 at 20:07
  • You appear to be asking this question at a point where you could simply connect the two. 20 foot is minimum, so you don't have to, but more is good if available, and if you haven't poured the concrete yet you can tie the stuff together... For that matter, with a typical "haunched footer" you can tie a rebar ring all the way around the footer. 4 chunks of rebar, 4 bends for the corners, plenty of overlap to tie them together... Steel is cheap, concrete is expensive.
    – Ecnerwal
    May 28 at 21:18
  • @Ecnerwal: I am only asking if the minimum 20' length encased down in the footer MUST be attached to the rebar grid that is a couple of inches below the surface, in order to be NEC compliant. That is a Yes/No question.
    – mr blint
    May 29 at 1:09
  • Yes, you can do the absolute bare minimum. / No, it's not the best approach. ;^)
    – Ecnerwal
    May 29 at 1:59
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#4 or 1/2” rebar can be used as the concrete electrode 20’ is the minimum and you will need to stub it up or connect a #4 awg copper wire to it with an approved clamp usually if connecting to the rebar the wire in the concrete is required to be sleeved as it exits but then the connection is not required to be accessible, if connecting to a stub of rebar that attaches to the 20’ piece again #4 awg copper can be used but the connection has to be accessible (I cad weld these so they don’t require access but not many electricians have the blocks to cad weld any more. But yes you can use #4 rebar. Normally the lower pieces connect to the upper ones or there may be 2 parallel bars the wire ties that hold them in place count as connecting. The length of copper if connected in the concrete also counts.

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  • You say "normally" the electrode rebar is attached to the rest of the grid, but is it a violation of the NEC to have that minimum 20' piece of rebar be unattached to the rest of the grid?
    – mr blint
    May 28 at 20:15
  • I watched a youtube video on cadwelding. The process seems straightforward but I would prefer to avoid spending $150 for a tool that I would only use once (the form) . A lot less money to use the clamps that are rated for being encased in concrete. I am not sure what you meant here: "....if connecting to a stub of rebar that attaches to the 20’ piece again #4 awg copper can be used but the connection has to be accessible". Would that be where the 20' piece of rebar took a turn and exited the slab so that the copper would be attached to it outside of the slab?
    – mr blint
    May 28 at 20:20
  • Code only requires the 20’ but the entire 20’ will have to be at the bottom of the pour it must be encased, you would then use an additional piece to stub up from that piece from the bottom or tie the copper to the rebar and sleeve it these are the 2 choices. You could not bring the 20’ piece up and out because it would not be at the bottom and not be 20’ long. why would you not want your entire footing or whatever is continuous to be tied together ? you would only improve the ground electrodes contact with ground verses some what reduce its ability to do be at ground if short Ties are cheap.
    – Ed Beal
    May 28 at 21:40

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