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Ufer (electrical grounding mechanism) not possible with floating raft slab?

Mark_Nagel | Posted in General Questions on

It might seem like a no-brainer but I felt I needed to ask (as I’m not an electrician).  I’m asking as pertains to US electrical code requirements: I suspect, however, that as this is a pretty primal thing that it would be in international code.

With a floating raft concrete and rebar foundation (on insulation) one would have to go with the old-fashion grounding mechanism of direct rod to earth rather than Ufer (electrical grounding through slab’s perimeter rebar).  Would this be correct?

NOTE: I’m just doing some preliminary planning/research.

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Replies

  1. Jon_R | | #1

    The slab wouldn't meet the "concrete in direct contact with the earth" requirement. But I assume you could lay bare wire under the foam, adding a lot of moist ground contact to what the still-required rod provides.

  2. charlie_sullivan | | #2

    Yes.

    Are you interested in Ufer because of problems getting a good ground in dry soil? Or just as an alternative? In most places a ground rod is inexpensive and works well, so it's not a problem but maybe you are dealing with very dry soil where it doesn't work well?

  3. Patrick_OSullivan | | #3

    As has been mentioned, clarification as to why you're asking would be helpful. The code requires that such a ground be used *if* it is present. In the case described, it is not present, for the reason Jon R stated. The code even has a clarifying remark: "Concrete installed with insulation, vapor barriers, films or similar items separating the concrete from the earth is not considered to be in 'direct contact' with the earth."

  4. Expert Member
    BILL WICHERS | | #4

    You can't just lay a ground wire under the footing and call it good. You CAN run a length of #2 copper 30" deep and use that though, I believe the minimum length is 20 feet (I'd have to check that, I typically put in rings around buildings where I'm always well over the minimum length). You could do this set a ways out from the foundation, probably in the same excavation, you just need earth ALL The way around the wire.

    If using rods, remember that you have to meet the minimum resistance measurement now. That usually means at least TWO rods will be needed. If you set these underground, I'd recommend making the connections by exothermic weld (that usually means "cadweld", which is a brand). I don't recommend using mechanical clamps underground.

    The Ufer ground only works when the concrete is directly contacting earth. As previously mentioned, any insulating barrier or vapor barrier prevents the Ufer ground from working.

    BTW, Ufer grounds are somewhat unique to the US in that they were developed here. You don't see them as often internationally.

    Bill

  5. Mark_Nagel | | #5

    Folks, thanks for the replies.

    I've done wiring for sub panels and for a garage build that I'd worked with my electrician on. The garage build was spec'd to have a Ufer ground: I installed that prior to the concrete pour. On my other sub panel/branches (pump house and a shed) I used ground rods: the pump house has one rod and the shed has two- I suspect the difference being that I did the pump house earlier on.

    No issues here (Pacific Northwest) with grounding to earth as there's a high water table.

    I suspect that I'm mixing up the grounding mechanics between branches and main and that I'd have to do grounding with rod(s) for the main service panel, which is what I currently have (detached main service as it's a manufactured home- main reason I'm looking to do a new "conventional" build; I'm tired of all the oddities of manufactured homes; my reference is therefore a bit on the non-standard side of things).

    My query here has more to do with literally managing the ins and outs of things through the building envelope. It appears that a ground wire will need to piece the building envelope. I will, for sure, work out the electrical code stuff/requirements with my electrician.

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