Additional Grounding Electrodes at Subpanel

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wwhitney

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Hello,

I have a residence with an exterior main disconnect feeding an interior subpanel 30 feet away via a 4 wire cable, SER #1/0 Al. At the main disconnect is the usual grounding electrode system consisting of a Ufer ground, a driven ground rod, and the metallic water service. At the subpanel there is an additional Ufer ground available. Can this additional Ufer ground be connected to the subpanel's ground bar? Or can it only be utilized by running a separate bonding conductor back to the main disconnect? FWIW, the subpanel Ufer ground is part of a basement footing, while the main disonnect Ufer ground is part of a shallow footing.

Thanks, Wayne
 
This thread had been temporarily closed, due to concern that the question may have been an attempt by a Do It Yourself person to obtain assistance in performing electrical work. In an exchange of Private Messages, I have been assured that the OP is working under the applicable laws and rules of his jurisdiction, and that this work is part of his intended path toward licensing as a General Contractor. I judge that that falls within the Forum rules, and am therefore reopening this thread.

I offer the OP an apology for the delay and inconvenience.
 
OP here, thanks for reopening the thread. To clarify, my electrician connected the second Ufer ground to the subpanel (main distribution panel) ground bar, and I'm trying to figure out if this was proper. A few more comments:

Is there any point, in terms of reducing the resistance of the GES, to utilizing the second Ufer ground? They are both 20' #4 Cu wires in different parts of the same monolithic concrete foundation, about 30' apart. As I mentioned, the foundation is deeper at the MDP. I'm not sure how well the Cu wires were attached to the foundation rebar,. but the rebar is continuous (but spliced) between the two points.

If the second Ufer ground is useful, the question is whether attaching it to the MDP ground bar is code-compliant. Does 250.54 bear on this? If the second Ufer ground is "supplementary", then attaching it to the EGC is OK.

If 250.54 does not apply, I think the question boils down to whether the #2 EGC in the #1/0 Al SER feeding the MDP can do double duty as the bonding jumper connecting the second Ufer to the GES. This was discussed in the January 2007 thread "Combo GEC and EGC", but there didn't seem to be a definitive answer there.

Thanks for your help. Cheers, Wayne
 
PS I'm very sorry about the newbie mistake of duplicate posts, I was getting some "invalid post" and "invalid forum" error messages that I thought meant my post didn't go through. I'm chagrined.
 
Wayne, you're kinda all over the place here, but I think I understand what you got.

wwhitney said:
Is there any point, in terms of reducing the resistance of the GES, to utilizing the second Ufer ground? They are both 20' #4 Cu wires in different parts of the same monolithic concrete foundation, about 30' apart. As I mentioned, the foundation is deeper at the MDP. I'm not sure how well the Cu wires were attached to the foundation rebar,. but the rebar is continuous (but spliced) between the two points.
Click to expand...

If there is foundation rebar, and it qualifies as a CEE per 250.52(A)(3), then the #4 conductor can be used as a Grounding Electrode Conductor. If there is a second #4 conductor, you need not do anything with it; if desired, you could use the CEE (Ufer) itself to connect two electrodes together, and that could be a use for the second conductor.

If the CEE has been connected to the service as required, I do not know of a prohibition from connecting a subpanel to the electrode as well, unless you bring 250.6 into play. But there is no real circuit path added by doing this that I'm aware of.

Now, am I with you so far?
 
georgestolz said:
Wayne, you're kinda all over the place here, but I think I understand what you got.
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George, thanks for fixing my duplicate posts. To put it simply, what I've got is two different #4 Cu conductors sticking out of a monolithic foundation in two places 30 feet apart. Each one is embedded 20 feet in the concrete and may or may not be tied to the rebar. So under 250.52(A)(3) are these two separate CEEs?

georgestolz said:
If there is a second #4 conductor, you need not do anything with it;
Click to expand...
If the second #4 conductor is a separate electrode, then I would think that 250.50 would require it be connected to the GES, unless it is "supplementary" per 250.54. What does "supplementary" mean here?

georgestolz said:
If the CEE has been connected to the service as required, I do not know of a prohibition from connecting a subpanel to the electrode as well, unless you bring 250.6 into play. But there is no real circuit path added by doing this that I'm aware of.
Click to expand...
When you say there is no circuit path added by doing this, you mean as far as generating objectionable current as per 250.6? I would think that if the second #4 electrode is electrically connected in the foundation to the first #4 electorde via the foundation rebar, that could make a circuit via the subpanel EGC. Is that an objectionable current issue? I'm not very clear on 250.6.

Thanks, Wayne
 
wwhitney said:
George, thanks for fixing my duplicate posts. To put it simply, what I've got is two different #4 Cu conductors sticking out of a monolithic foundation in two places 30 feet apart. Each one is embedded 20 feet in the concrete and may or may not be tied to the rebar. So under 250.52(A)(3) are these two separate CEEs?
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Well, it sounds as though you might have three. Mustn't forget the rebar, you know. :)

Under the 2005, yes, all three would be required to be used, IMO. However, in practice, one used will generally pass inspection, and in 2008 the language of 250.52(A)(3) will be added to:
Where multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.
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So if you're given too much grief, you can always point to the 2008 ROC as proof that the AHJ could relax a bit.

If the second #4 conductor is a separate electrode, then I would think that 250.50 would require it be connected to the GES, unless it is "supplementary" per 250.54. What does "supplementary" mean here?
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Supplementary is undefined, so it falls on the AHJ to determine (90.4). Generally, these are considered "extra" ground rods required by equipment manufacturers for certain equipment. It would be a stretch to call these "supplementary" electrodes, but it's possible.

Is that an objectionable current issue? I'm not very clear on 250.6.
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It's simplest to just imagine a light bulb. If you connect it to a hot on one side of the filament, and connect the neutral and grounding conductor on the other side of the filament together, then neutral current will flow on both conductors.

This is basically what 250.6 is getting at. If you set up a second path for neutral current to get back to the utility (not counting the earth itself), then this is to be avoided.
 
Just to make sure I understand the question, Wayne is asking if it's permissable to use a feeder's EGC to double as a jumper between electrodes, right? If not, then kindly disregard the rest of this post. :rolleyes:

We've always been told that more grounding is better, but I could see the objection that a fault seen by a circuit in the sub-panel would send some current through the rebar to the neutral via the main bonding jumper.

But then, other electrodes, such as building steel, are often used as electrode interconnections. I haven't seen examples of rebar used this way, nor have I seen it prohibited. Not the same as paralleling a neutral.

How about a real-world similar example: Let's suppose we upgrade a service, using the existing panel as a sub, separating the EGC's and neutrals. If there was a water-pipe or other GEC, must it be removed?
 
georgestolz said:
This is basically what 250.6 is getting at. If you set up a second path for neutral current to get back to the utility (not counting the earth itself), then this is to be avoided.
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OK, so we keep the neutral conductors separate from the EGC and GEC, with the single connection at the service entrance. But what about having a loop within the EGC/GEC system? Does that create the possibility of objectionable currents under 250.6?

Thanks, Wayne
 
Not quite following but I wonder if the # 4 are one codnuctor stub up more than 20 feet apart to indicate that there is at least 20 feet of #4 copper in the footing and one is just a reference piont because it may not have been inspected.
 
Bea said:
Not quite following but I wonder if the # 4 are one codnuctor stub up more than 20 feet apart to indicate that there is at least 20 feet of #4 copper in the footing and one is just a reference piont because it may not have been inspected.
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That would be nice, but I saw the forms before the pour, they are two separate 20' lengths of #4 copper. Wayne
 
wwhitney said:
That would be nice, but I saw the forms before the pour, they are two separate 20' lengths of #4 copper. Wayne
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If that's the case, I would say you have 2 separate grounding electrodes & they need to be bonded together per 250.50. JMHO.
 
LarryFine said:
Just to make sure I understand the question, Wayne is asking if it's permissable to use a feeder's EGC to double as a jumper between electrodes, right?
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Yes, after exploring my situation it has boiled down to this question. Any takers?

Thanks, Wayne
 
rcarroll said:
If that's the case, I would say you have 2 separate grounding electrodes & they need to be bonded together per 250.50. JMHO.
Click to expand...
I agree, which brings us back to my boiled-down question:
LarryFine said:
Wayne is asking if it's permissable to use a feeder's EGC to double as a jumper between electrodes, right?
Click to expand...
wwhitney said:
Yes, after exploring my situation it has boiled down to this question. Any takers?
Click to expand...
I'd say yes, it's okay, and a good idea.
 
I disagree. If the EGC for the feeder begins in the service, then it is not a bonding jumper (as governed by 250.53(C)); it is a GEC (as governed by 250.64). Therefore, it must abide by 250.64(C), it must be continuous.

If it is terminating in a subpanel en route to the GE, it is not a continuous conductor.
 
George ,250.53(C) does not include the continuous requirement found in 250.64.

It reads
250.53(C)
Bonding Jumper ;The bonding jumper(s) used to connect the grounding electrodes together to form the grounding electrode system shall be installed in accordance with 250.64 (A)
,(B) and (E)....."

250.64 (C) has been left out and I think that is where it says "shall be installed in one continuous length without a joint or splice"
 
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whoops , George I misread your post sorry about that. But I 'm not sure why the feeder EGC can't also seve to bond the electrode?
 
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A bonding jumper starts at an electrode and ends at an electrode. This conductor starts at the service and eventually gets to an electrode. Isn't this a GEC?
 
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