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Thread: slab grounding

  1. #1
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    slab grounding

    does a slab need to be grounded for a residential addition? can i get a code reference?the existing service is already grounded and far from the new addition.

  2. #2
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    A concrete slab can't be used as a grounding electrode so there is no need to connect to any steel in that slab.

    Chris

  3. #3
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    the GC had a slab inspection today , He called me , saying that the inspector said that the slab needs to be grounded ....?? is it in the NEC ? or what code?

  4. #4
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    He is mistaken. He is probably talking about a CEE (concrete encased electrode) which has to be in the footer not the slab itself.

  5. #5
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    I beg to differ.

    My first intimation that I might disagree with the above opinions comes from local Northern Nevada experience, where they've been requiring ALL slab home foundations - even those without footers - to have the "Ufer" ground.

    Reviewing 250.52(A)(3), I do not see any depth requirement for the slab at all, or even and requirement that the slab have rebar in it. (IRC does not require rebar in house slabs) You're only required to have 2" of concrete around the 'electrode.'

    From that, I conclude that yes, even a surface-laid slab can qualify as a grounding electrode. (This only makes sense, as that it exactly the sort of foundation that Mr. Ufer was dealing with during the war, when he invented this sort of electrode).

    Which returns us to the original question: Does the NEC require such? Well, if we back up to 250.50, the 'exception' tells us that we need not tie in the slab if the rebar is not accessible.

    Mind you, my answer is referencing the 2011 code, where this section (250.52) has been extensively edited. I strongly recommend review of this section, as the new text is quite at variance with many opinions that I have heard over the years.

  6. #6
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    Quote Originally Posted by wireperson View Post
    the GC had a slab inspection today , He called me , saying that the inspector said that the slab needs to be grounded ....?? is it in the NEC ? or what code?
    It is not the slabs or footers that are being grounded. It is the services that are being grounded using the UFER method. The old service is already grounded and needs nothing more. No UFER grounding is required for the addition. A new service being built on an old house also does not require UFER grounding.
    Unlimited Electric Contractor/Standard Electric Inspector/Traffic Signal Inspector/Highway Lighting and Level One Traffic Signal Installer.

    I know you believe that you understand what you think I said but I'm sure you realize that what you heard is not what I meant.

  7. #7
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    what is the rest of the story?

    perhaps there is more to the story...slab for a hot tub? whirlpool?

  8. #8
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    Quote Originally Posted by Cavie View Post
    It is not the slabs or footers that are being grounded. It is the services that are being grounded using the UFER method. The old service is already grounded and needs nothing more. No UFER grounding is required for the addition. A new service being built on an old house also does not require UFER grounding.
    Slabs never qualify as a uffer.

    However, if the footings of the addition contain 20' or more of 1/2" or larger rebar connected together than you must connect it to the service.


    250.50 Grounding Electrode System. All grounding electrodes
    as described in 250.52(A)(1) through (A)(7) that are
    present at each building or structure served shall be bonded
    together to form the grounding electrode system......

  9. #9
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    Iwire, that's why I stressed checking the latest code edit.

    250.50 exception says there's no need to connect the rebar if the rebar is not accessible.

    The 250.52 section I cited sure looks to me as if every slab that's in contact with the ground can be used- there's no depth requirement at all. Just give me a 4-1/2" chunk of concrete that has either rebar, or a copper wire in it, and you're good to go.

    I think we need to re-open the entire topic of 'what is a Ufer.'

    (BTW, I agree completely with you as to service changes, etc.)

  10. #10
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    Quote Originally Posted by iwire View Post
    Slabs never qualify as a uffer.

    However, if the footings of the addition contain 20' or more of 1/2" or larger rebar connected together than you must connect it to the service.
    I wanted add that any electrode available is supposed to be connected to the service. So if they create one (I.E. the example above) you must use it.

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