grounding rod query

[Theruse]

They will be pouring the slab on my 24x24 detached garage. I will be running 100a from the house by 2" schedule 40 conduit to the subpanel. I want to install the grounding rod in the slab below the location of the subpanel. Local code says 4" has to be above grade. Does grounding rod need to be attached to the mesh or rebar in slab ( a UFER) or is it to Ok to just make sure 8' is in ground?

 

[readhead]

When we prep for our metal buildings we put a bend in a 20’ bar and tie it to the other rebar. It has to be tied to something or it will flop around during the pour.

 

[mcspeed]

I believe two ground rods are required according to what I have read on this fine forum.

The UFERS I have observed had copper wire buried in foundation trenches. Didn’t have ground rods and were inspected by city. YMMV.


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[teamextreme]

Per NEC, if you have a ufer ground you don't need ground rods. You should check with your AHJ though, because different places enforce different rules on this.

 

[Solarphil]

I believe two ground rods are required according to what I have read on this fine forum.

Two ground rods are *not* required, but it’s a common misperception based on a functional reality. Code requires if using a ground rod as your GES then you must prove the ground resistance is below a given threshold, or else you *must* drive a second rod more than 6’ away from the first. Measuring the fall of potential on a grounding system is relatively expensive, ground rods are stupid cheap. Hence the belief that it’s a requirement, when really it’s just the easy way out.

None of which has anything thing to do with the OP’s question.

A Ufer ground is a great grounding system, far better than any number of ground rods. However I think you misunderstand its construction - there is no ‘rod’ involved. It’s a concrete encased electrode, and the electrode is the rebar itself. All of it. Typically you bend up a short stub where convenient, as readhead says, and connect your GEC to that.

In my neighborhood inspectors want to see the connection be accessible, so most foundation contractors stub it up in the middle of the wall near the service entrance location, and then the electrician installs a blank plate over a telcom ring (the orange outlet ‘boxes’ that have no back. Removing the blank cover lets you inspect the connection to the rebar.

 

[Solarphil]

I should add, for the record, code doesn’t say keep driving ground rods until the resistance is below that threshold, it just says drive a second and you’re done. Doesn’t mean you’ve established a good grounding system, just a compliant one.

 

[mcbane]

When I see local code requirements like 4” above grade I always wonder what the AHJ is trying to accomplish. Unless they watch the slab pour, how do they know someone didn’t just push a 8” rebar scrap half way in after screeding, as if it were a sill bolt?

In any case, I suggest asking your inspector to clarify whether that rule applies to ufer grounds or only to ground rods driven into soil. It is a much cleaner install to have just the copper ground wire come up out of the concrete.


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[readhead]

We simply take a picture or two of the rod in case anyone questions the UFER install later.

 

[bad_idea]

Looks like you got your answer, so please allow my tangent. I ran a 90a service out to my garage using MHF in 1 1/2" conduit. There was plenty of room in the conduit. I think 2" might be overkill and a waste of money. Keep in mind I am a DIY speaking on my first hand experience in my one install.

 

[mike93lx]

Looks like you got your answer, so please allow my tangent. I ran a 90a service out to my garage using MHF in 1 1/2" conduit. There was plenty of room in the conduit. I think 2" might be overkill and a waste of money. Keep in mind I am a DIY speaking on my first hand experience in my one install.

I used 2" for a buddy's barn with #2 MHF and it certainly made the pull easy. It wasn't a super straight run and we probably didn't lay everything out in the right direction, so I don't regret having the room. It also made dealing with the multiple LB's at the house easier

 

[larry4406]

When I built in Montgomery County MD we installed UFERS in both our houses and detached garages.

 

[andyvh1959]

For my 24x28 detached garage I installed one ground rod and 1-1/2 conduit inside the slab form before the concrete was poured near a corner of the slab where I knew I'd locate the sub-panel. Worked out great. Then the initial electrical inspection required a 2nd ground rod 6' from the first. So I drilled a hole through the slab and drove another ground rod down, connected a ground wire from that rod to the other rod and into the ground bus in the sub-panel, done. I used 1-1/2" conduit for the service cables, buried 24" and routed to the main panel in my attached garage, conduit all the way to the main panel. Electrical inspector just left, electrical is signed off completed.

 

[Theruse]

I am also in Montgomery County and tried to call Dept. of Permitting Services to ask an inspector. Had to schedule appt. and 1st available was June 22nd. A no go. My electrician retired. I used to do the work and he would pull the permit. So will double check with another electrician but am pretty sure the UFER may work. Only issue I ran into with NEC guidelines is the slab will be insulated and NEC says insulated does not constitute a ground. But the slab is tied to the footings which are not insulated, so still some confusion.

 

[Jim greengo]

When I see local code requirements like 4” above grade I always wonder what the AHJ is trying to accomplish. Unless they watch the slab pour, how do they know someone didn’t just push a 8” rebar scrap half way in after screeding, as if it were a sill bolt?

In any case, I suggest asking your inspector to clarify whether that rule applies to ufer grounds or only to ground rods driven into soil. It is a much cleaner install to have just the copper ground wire come up out of the concrete.


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The inspector checking the rebar in footings checks the ufer ground before footing is poured around here anyway.

 

[ilikedirt]

I'm in Virginia and had a #4 20ft rebar tied into my #5 in the footer for a UFER ground. That's what the county likes here.

As far as your "insulation" goes, I think the UFER will be fine as long as your footer/concrete comes in direct contact with the earth. Meaning no vapor barrier or foam under the footer.

 

[larry4406]

I am also in Montgomery County and tried to call Dept. of Permitting Services to ask an inspector. Had to schedule appt. and 1st available was June 22nd. A no go. My electrician retired. I used to do the work and he would pull the permit. So will double check with another electrician but am pretty sure the UFER may work. Only issue I ran into with NEC guidelines is the slab will be insulated and NEC says insulated does not constitute a ground. But the slab is tied to the footings which are not insulated, so still some confusion.

Use #4 copper wire tied to the footing rebar for a minimum length of 20’. Continue the wire by turning up to where your panel will be. Then the only junction is where the #4 wire is connected to the ground lug in the panel.

Don’t forget your 4-wire feed and separate the ground and neutral bars in your sub panel.

 

[mcbane]

The inspector checking the rebar in footings checks the ufer ground before footing is poured around here anyway.

If that is the case, I don't see the value added by having the rebar stick up out of the slab. Around here, the inspector accepts a listed direct burial clamp to secure a copper wire to the rebar. That connection is encased in concrete and the only thing sticking out of the slab or footing is the copper wire.

 

[mike93lx]

If that is the case, I don't see the value added by having the rebar stick up out of the slab. Around here, the inspector accepts a listed direct burial clamp to secure a copper wire to the rebar. That connection is encased in concrete and the only thing sticking out of the slab or footing is the copper wire.

the value comes from them passing you instead of failing you.

I'd get a confirmation ahead of time that it will be ok.

 

[Theruse]

The #4 copper wire secured to the footing rebar seems to be a cleaner method. I will have 9' concrete sidewalls so I will connect the wire to the exposed rebar with the direct burial clamp from the footings before they install the forms for the concrete walls. Better hustle since the forms are expected to go up tomorrow. Was scheduled for today, but a no-show by the crew. I will take pictures for when the electrical permit inspector comes. The next inspection is for the poured concrete wall (initial inspection was prior to the footing pouring. Around here, I will have to pull three permits, foundation/construction, electrical and plumbing (2).

 

[larry4406]

The #4 copper wire secured to the footing rebar seems to be a cleaner method. I will have 9' concrete sidewalls so I will connect the wire to the exposed rebar with the direct burial clamp from the footings before they install the forms for the concrete walls. Better hustle since the forms are expected to go up tomorrow. Was scheduled for today, but a no-show by the crew. I will take pictures for when the electrical permit inspector comes. The next inspection is for the poured concrete wall (initial inspection was prior to the footing pouring. Around here, I will have to pull three permits, foundation/construction, electrical and plumbing (2).

If you are using the typical aluminum concrete wall forms that are prevalent in our area, you can run the copper wire up inside the wall forms and then come out one of the snap tie locations at a proper height to coordinate with your panel location and mounting height.