size bonding jumper to ground ring

[malachi constant]

911 call center, new construction.
We have a ground ring at the generators and another one at the building sized the same. Question is what size the bonding jumper needs to be.
I think it is pretty straightforward. 250.53(C) tells you to size the grounding electrode per 250.66. It is a 2000A 480V service with largest ungrounded conductors being (8) sets of #350 AL. Equivalent area is 8x350=2800 kcmil. Our grounding electrodes are copper, so 3/0 is required - correct?

Unfortunately 2/0 was specified and is already installed, the trenches have all been backfilled. So a new trench would be required to get a 3/0 installed. I am a very by-the-book engineer but wonder if this something worth approaching the electrical inspector about. I know enough about grounding to know what is code, but not enough to make a judgment call as to whether the system is intrinsically safe as it is currently installed. Any thoughts on this? I don't want to propose something to the inspector that is a bad idea.

For what it's worth, the owner only cares that the system meets Motorola R56. We started with a 4/0 but as part of VE this was reduced to a 2/0, which still met R56...but no one bothered to check and see that it still met NEC.

Thanks!

 

[petersonra]

250.66(C)

 

[malachi constant]

250.66(C)

"Where the grounding electrode conductor is connected to a ground ring as permitted in 250.52(A)(4) that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than the conductor used for the ground ring."

I forgot to mention the ground rings are 4/0. This is not a code requirement, or a Motorola R56 requirement - both those allow #2, but at the Owner's request we arbitrarily increased their size.

So I have a dilemma. We could have put in 2/0 rings and a 2/0 bonding jumper and been fine. Or #2 rings and 2/0 jumper. Or 4/0 rings and a 4/0 jumper (which we started with, but VEd down to 2/0 - whoops). But we put in 4/0 rings and a 2/0 jumper, so we technically have a code violation. Is this grounding system robust enough there are no issues? The Owner knows grounding and Motorola inside and out and has no issue with the installation, but the NEC aspect of this got overlooked during VE. Again, I know enough about grounding to know what is code, but not enough to make a judgment call as to whether the system is intrinsically safe as it is currently installed. Thoughts?

 

[jumper]

"Where the grounding electrode conductor is connected to a ground ring as permitted in 250.52(A)(4) that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than the conductor used for the ground ring."

I forgot to mention the ground rings are 4/0. This is not a code requirement, or a Motorola R56 requirement - both those allow #2, but at the Owner's request we arbitrarily increased their size.

So I have a dilemma. We could have put in 2/0 rings and a 2/0 bonding jumper and been fine. Or #2 rings and 2/0 jumper. Or 4/0 rings and a 4/0 jumper (which we started with, but VEd down to 2/0 - whoops). But we put in 4/0 rings and a 2/0 jumper, so we technically have a code violation. Is this grounding system robust enough there are no issues? The Owner knows grounding and Motorola inside and out and has no issue with the installation, but the NEC aspect of this got overlooked during VE. Again, I know enough about grounding to know what is code, but not enough to make a judgment call as to whether the system is intrinsically safe as it is currently installed. Thoughts?

Personally, I know nothing about ground rings and only once in a fit of madness looked at R56, but if a #2/0 jumper is good for a #2/0 ring-why would it matter if the ring is oversized to #4/0?

 

[malachi constant]

Personally, I know nothing about ground rings and only once in a fit of madness looked at R56, but if a #2/0 jumper is good for a #2/0 ring-why would it matter if the ring is oversized to #4/0?

I'm with you, but I am a stickler for following the letter of the law. I just got off the phone with the electrical inspector, seems he is also. We both are on the same page that trenching in a 3/0 seems unnecessary, but code doesn't allow any exception, and each agreed to think about it for a while.

If the code had read "the grounding electrode shall not be required to be larger than the minimum required conductor used for the ground ring", or something like that, there would not be an issue. As it stands we have a violation of code, but one that given the circumstances should, maybe, one could argue, be excepted this one time. (Oh, that makes me cringe.) It is hard to justify spending $5K on trenching in a 3/0 when a 2/0 is (I assume) perfectly fine.

 

[jumper]

I'm with you, but I am a stickler for following the letter of the law. I just got off the phone with the electrical inspector, seems he is also. We both are on the same page that trenching in a 3/0 seems unnecessary, but code doesn't allow any exception, and each agreed to think about it for a while.

If the code had read "the grounding electrode shall not be required to be larger than the minimum required conductor used for the ground ring", or something like that, there would not be an issue. As it stands we have a violation of code, but one that given the circumstances should, maybe, one could argue, be excepted this one time. (Oh, that makes me cringe.) It is hard to justify spending $5K on trenching in a 3/0 when a 2/0 is (I assume) perfectly fine.

I will probably get banned for saying this: but I think you are fine here, code is code-but code allows an AHJ leeway to decide what is suitable - IOW, if the inspector and owner are okay, I do not see a problem.

Heck, if R56 is satisfied I would take the money and run.

I may be wrong, but I think you are seeing ghosts where none exist.

 

[david luchini]

I'm with you, but I am a stickler for following the letter of the law. I just got off the phone with the electrical inspector, seems he is also. We both are on the same page that trenching in a 3/0 seems unnecessary, but code doesn't allow any exception, and each agreed to think about it for a while.

If the code had read "the grounding electrode shall not be required to be larger than the minimum required conductor used for the ground ring", or something like that, there would not be an issue. As it stands we have a violation of code, but one that given the circumstances should, maybe, one could argue, be excepted this one time. (Oh, that makes me cringe.) It is hard to justify spending $5K on trenching in a 3/0 when a 2/0 is (I assume) perfectly fine.

You could always dig up the ground ring where the grounding electrode connects and replace some of the 4/0 ring conductor with 3/0. Seems like that would make you "legal."

 

[malachi constant]

You could always dig up the ground ring where the grounding electrode connects and replace some of the 4/0 ring conductor with 3/0. Seems like that would make you "legal."

Interesting idea. Might be the fallback plan.

A new wrinkle - it was revealed to me this afternoon that the bonding jumper was not in fact VE'd from 4/0 to 2/0. It was kind of the other way around. The whole system was 2/0, but during the addendum period our engineers noticed the lightning protection had 4/0 verticals, and decided to bump up the ground rings to match the lightning protection system.

So we've got 4/0 coming from roof to ground. A quarter-mile of buried 4/0 running around the building. A hundred feet away some buried 4/0s running around the generator. And those two 4/0 ground rings connected by 2/0 instead of 3/0 (or 4/0).

I don't think we have any issues here. But gonna have to get out NFPA 780 to get up to speed on lightning protection. This is not how I wanted to spend my spare time this week. Sigh.

 

[ActionDave]

I will probably get banned for saying this: but I think you are fine here, code is code-but code allows an AHJ leeway to decide what is suitable - IOW, if the inspector and owner are okay, I do not see a problem.

Heck, if R56 is satisfied I would take the money and run.

I may be wrong, but I think you are seeing ghosts where none exist.

This topic and your post in particular bring to memory some wise words given to me by a Good Old Boy I had the pleasure of knowing and working with, "If it don't matter, it don't matter."

 

[malachi constant]

It don't matter if it REALLY don't matter. Was on site this morning and found the Owner VE'd the ground rings to 2/0, that's what got installed. I thought we had 4/0 rings per the plans. So non-issue. Contractor just thought a 3/0 jumper was required due to Table 250.66, and didn't see the exception, that's what got this all going.

Well, I learned a little more about grounding. That's about the only good thing I can say about all this. Thanks all.