Rebar Grounding Electrode Question

ch0mpie

would grounding a sub-panel to the rebar in a slab-on-grade (not a footing) meet the NEC requirment for a grounding electrod assuming the rebar is #4-20' long minimum?

tgeb

You know I'm not an electrician, but I would say no.

A slab on grade is not necessarily "in" the ground, most are floating on a layer of stone which (to me) would not provide adequate conditions for an "ufer" ground.

Attached Files

ufergrnd.doc (28.5 KB, 134 views)

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Tom

www.gebcon.com

ch0mpie

Thats what I was thinking, but footings are often on stone or rock. The word doc you posted makes it seem like the properties of the concrete are more important than that of the soil.

griz

Not acceptable.

Where is 480 when you need him.
Who gave him the night off?

Static Design

NEC 250.52(3) An electrode encased by at least (2in) of concrete, located horizontally near the bottom or vertically, and within that portion of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 20ft of one or more bar or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of less the 1/2 in diameter.

NEC 250.58 where an ac system is connected to a grounding electrode in or at a building or structure, the same electrode shall be used to ground conductor enclosures and equipment in or on that building or structure.

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griz

Static, so it's ok the way chOmpie wants to do it?
Can you translate your response to English?

Kgmz

Yes and No.

Depends on where the sub panel is located.

If sub panel is in same building as main panel, then all grounds must be run to where main panel is grounded, which means 4 wires from main panel. And the neutral bus bar must not be bonded to sheet metal of panel and a ground bus bar must be added if there is not one in there.

If sub panel is in a detached building from where main panel is, then it is OK to put in a ground rod.

NEC 250.52(3) means it is OK to install ground rod that way.

NEC 250.58 means what I explained above about if sub panel is in same building as main panel.

ch0mpie

Panel is in a detached building and is getting 4 wires from the main panel. I know what the code says, but its a bit vague. A slab-on-grade is not a footing but has a considerable amount of direct contact with the earth.

Kgmz

Normally slab on grade will have a thickened edge, that thickened edge is the footing.

Code for slab on grade around here requires a 18" high thickened edge.

ch0mpie

Not the case, the building and footings are existing, but the floor slab was demo'd and is being replaced.

Kgmz

In that case I don't know, better ask the inspector.

Or is there any reason why you can't drive in 2 ground rods outside.

ch0mpie

Shallow rock but he found a spot where he was able to get the rods in on an angle.

Speedy Petey

Then it is FINE to be used as a Ufer electrode. NO other electrodes are/were required.

Static gave the correct code quote.

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tgeb

I don't see it that way, a typical slab on grade design almost isolates a slab from the earth.

The slab has 4" of clean stone below it, a layer of poly separating the concrete from the stone, then expansion joint along the perimeter keeping the concrete separated from the surrounding walls.

I would view a thickened slab section that does rest on soil as direct contact with the earth.
But then again.....I'm no electrician

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www.gebcon.com

Static Design

Think of the slab as the insulation of the wire and the re-bar as the conductor, the slab on grade isn't really the issue but the length or re-bar located in that concrete is.

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tgeb

Well that's just the thing Static.

If you are, as you say, insulating the conductor/electrode with an isolated slab does that not defeat the intent of providing a low resistance path to earth/ground.

The length of the rebar/electrode is not the concern, (to me), my concern is, does a slab on grade which is (essentially) floating on a bed of stone and separated from the stone with a sheet of plastic, considered to be in direct contact with the earth.

I can't see how this would qualify as an ufer ground. See disclaimer below.

I am not an electrician

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Tom

www.gebcon.com

Static Design

According to NEC 250.4(A)(5) The earth shall not be considered as an effective ground fault current path. The purpose of grounding is to provide a low impedance back to your gound (not just the earth). Technically a slab on grade is still touching the earth and they are normally poured with a thickened edge with reinforcing rods all tied together with the #4 rebar regardless of the poly or sand (what ever you use as a VB). If you have the length requirement then yes (on this particular case). I wish I had a code book in front of me, maybe I could make this clearer but if I remember when I get back home i'll grab the NEC and the Soars and try to turn mud into water haha.

Code is just the interpretation of one mans view, I may read it one way but someone else will another. Anything I am unsure of I always call my inspector as he is the one I gotta please. If I am wrong I am sure one of the veteran sparkies will correct me as I am just a kid.

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hedlok

I would say no...what you're describing sounds more like an equipotential bonding grid than a grounding electrode...also, won't there be a PVC moisture barrier separating the slab from the earth?

acrwc10

Think of the slab as the conductor also. With all the salts and chemicals in the concrete leaching into the soil you get a solid connection to Earth. If you have plastic in the way you have just cut that path to earth.
I know this post is much later and the slab is (hopfully) done, but information for the futer is important.

one man show

absolutely not!!
section 250.52 (3) says bottom of footing or foundation

(3) Concrete-Encased Electrode.

effective means