Grounding the foundation rerod?

[Duffster]

So I am pouring a new foundation and find out I need to attach a ground wire to the rerod in the concrete walls. What is the theory behind this?

 

[buckeyefarmer]

The Ufer Ground

It makes a good ground better.

 

[framer]

Been around enforced by inspectors about 5 years now.Uses rebar as ground
wire.I do know now around here we have chiseled out concrete to find rebar
then weld a piece of rebar to it for electricians.

 

[dmccarty]

I wish I knew about this when we built the house.....

Later,
Dan

 

[69x11]

So I am pouring a new foundation and find out I need to attach a ground wire to the rerod in the concrete walls. What is the theory behind this?

is the house already grounded thru and electrode encased in concrete? If not there is no need to do this, it actually creates a parallel grounding issue, if the home is grounded thru a normal driven or buried electrode, then the new grounding would run back to existing service grounding (ie the rod) if the electrode is encased in concrete..ie the rebar, then the new grounding would need to attach to it. check out 2008 nec article 250. dang i cant remember...stand by

 

[69x11]

250.52 Grounding Electrodes.
(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm (2 in.) of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 6.0 m
(20 ft) of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (ｽ in.) in diameter, or consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 4 AWG.Reinforcing bars shall be permitted to be bonded together by the usual steel tie wires or other effective means.


Used as a Grounding Electrode Conductor

"250.66 Size of Alternating-Current Grounding Electrode Conductor.
(B) Connections to Concrete-Encased Electrodes.
Where the grounding electrode conductor is connected to a concrete-encased electrode as permitted in 250.52(A)(3), that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than 4 AWG copper wire.

 

[Duffster]

is the house already grounded thru and electrode encased in concrete? If not there is no need to do this, it actually creates a parallel grounding issue, if the home is grounded thru a normal driven or buried electrode, then the new grounding would run back to existing service grounding (ie the rod) if the electrode is encased in concrete..ie the rebar, then the new grounding would need to attach to it. check out 2008 nec article 250. dang i cant remember...stand by

I am not sure if I am following ya.

Right now there isn't a house or electrical service. Is the rebar ground done in lieu of the old (2 copper rods) way of grounding or is both supposed to be done?

Another thing. When grounding rebar in dairy barns for stray voltage the bar is all welded together but in the house it could be tied together. I don't see how the tied bar can be a good enough conductor.

 

[rjkobbeman]

If I am following this correctly, it sounds like you want to either ground the service through a properly-grounded-rebar ("foundation" ground) OR use the traditional copper rod driven down in the earth... but not both.

If you do both you can have potential between the two grounds.

 

[Duffster]

If I am following this correctly, it sounds like you want to either ground the service through a properly-grounded-rebar ("foundation" ground) OR use the traditional copper rod driven down in the earth... but not both.

If you do both you can have potential between the two grounds.

I have to check with the inspector to see if we have to do both.

I don't see how the foundation ground can be better than using copper ground rods.

 

[jonyyuma]

My house was built before this requirement, but several years ago, i added cable and burgular alarm, then I added a 10 foot ground rod to the other end of the house and a #8 solid ground wire to the gas pipe and bonded to the other end.Ie single ground for the whole house, my home does not have metal plumbing...This supposed to help the alarm system function correctly. Jy..

 

[bdeboer]

I am not sure if I am following ya.

Right now there isn't a house or electrical service. Is the rebar ground done in lieu of the old (2 copper rods) way of grounding or is both supposed to be done?

Another thing. When grounding rebar in dairy barns for stray voltage the bar is all welded together but in the house it could be tied together. I don't see how the tied bar can be a good enough conductor.

In 2006 here, the rebar attachment was done in lieu of the ground rods. The rebar was not welded. I haven't been around new construction since then and do not know what the current local practice is.

One of the concerns about going to this here, was what happens in the case of a lightning strike. Do you blow a hole in basement wall vs. a hole in the dirt? I am not sure if anyone has ever had this problem, but would rather replace some electronics than my foundation.

 

[Duffster]

I got the scoop on things. They still want 2 ground rods as well as the foundation rods.

 

[eepete]

In NC when I did my garage, since I put rebar in the pad I had to bring out a wire and attached that to my electrical ground and put in a ground round in the ground just outside the garage. The 4 wire feed from the house included my house ground, which tied into the ground system for the house.

Using a pad as a means of grounding sounds like a really bad idea. Concrete does conduct electricity, but not very well compared to wire. I like to think of my pad as a great big anti-static mat. When I was pulling up plastic on the pad I had down for painting, I got a nice static zap when I touched the pad surface.

Just for fun, I took an ohm meter and measured from my AC ground in the garage to a point about 4' in from the edge of the pad. I had to puncture the seal on the concrete with the point of the probe.

I initially got a resistance of 10 megohms. I mostened the contact point (ok, I spit on it :laughing and got a resistance of 2 megohms. So if you think of concrete with bonded/grounded rebar like a piece of wire, it's not very good.

There is a concept of "Ohms per square" when you have a sheet or surface material. If I exposed a 6 inch square of the concrete and put a metal plate on it and then measured the resistance, it would be much lower than the point contact measurement I made. Even lower still if the contact area (note: _area_ not _point_ contact) was wet (don't ask how that would happen :laughing.

The ohms per square concept is odd in that if you took a good sized area (like my 30'x40' pad) you would get the same resistance with the point probes 6" apart as you would 3' apart. Even though the probes are further away, there are now even more possible paths for the current to flow.

So if the pad were in contact with the ground, it would be a low enough resistance to pass some current, certainly enough to shock or kill someone touching a live wire. If the pad is on gravel, even if it's wet, there is still the ability to pass current but it's a pretty poor ground.

So a pad with rebar connected to your AC ground (and thus your ground rod(s)) is a good deal and it protect you from huge voltages developing along the pad if there was a lightning strike near by on the ground or on the building. Of course, this is lightning so all bets are off, YMMV, etc.

Pete