Minimum recommended ground ring size?

[greenspark1]

Hi all. NEC 250.52 requires a ground ring to be no smaller than #2 copper. However I see many engineers specifying 2/0 or 4/0 CU. I have been unable to get a clear answer from them as to why they use such large conductors. Usually something along the lines of "to provide a good ground". Anyone have an explanation or good reasoning behind this practice? What size do you typically use for a ground ring? Thanks for helping explain/debunk this practice.

 

[texie]

Hi all. NEC 250.52 requires a ground ring to be no smaller than #2 copper. However I see many engineers specifying 2/0 or 4/0 CU. I have been unable to get a clear answer from them as to why they use such large conductors. Usually something along the lines of "to provide a good ground". Anyone have an explanation or good reasoning behind this practice? What size do you typically use for a ground ring? Thanks for helping explain/debunk this practice.

I see this often as well. I guess they feel that the larger conductor gives more surface area. That then begs the question, can you then connect to the oversize ground ring with a #2 as the GEC? I think there was thread on this recently. Not sure of the conclusion.

 

[don_resqcapt19]

I see this often as well. I guess they feel that the larger conductor gives more surface area. That then begs the question, can you then connect to the oversize ground ring with a #2 as the GEC? I think there was thread on this recently. Not sure of the conclusion.

If the ground ring is larger than #2, then the GEC to the ground ring has to equal the size of the ground ring or be sized per Table 250.66...you would use the smaller of the two.

 

[gadfly56]

Hi all. NEC 250.52 requires a ground ring to be no smaller than #2 copper. However I see many engineers specifying 2/0 or 4/0 CU. I have been unable to get a clear answer from them as to why they use such large conductors. Usually something along the lines of "to provide a good ground". Anyone have an explanation or good reasoning behind this practice? What size do you typically use for a ground ring? Thanks for helping explain/debunk this practice.

In the Beginning was The Project, and The Project had The Spec.
The Spec was with The Senior Engineer,
And The Senior Engineer saw that it was Good.
All Proper Things were in The Spec, and there was nothing
Outside The Spec that was Proper.
And The Spec said, "Thy ground ring conductor shall be 2/0,
and no size smaller shall be Proper."
And so it was, and it was Good.

It came to pass that a Junior Engineer was tasked with Another Project,
And he strove mightily against The Deadline.
In his despair, he cried out to The Senior Engineer;
"The EC's are rebellious! Lo, they clamor for Knowledge
on every detail and I am sore pressed to answer them!"
And The Senior Engineer said; "Fear not! For see, I have The Spec,
and herein shall you find what you seek."
And doubt entered the heart of The Junior Engineer, and he said;
"Is this not The Spec from another Project? Will it suffice?"
And The Senior Engineer said; "Well may thouest calculate every
Trivial Quantity and watch thy Project Manhours flow like a river
in flood and thy Profit Margin dwindle away. Or takest thou
The Spec and brandish it before the rabble."
And The Junior Engineer said; "Meh, good enough."

And so The Spec became The Boilerplate, and was handed down
from The Senior Engineer to The Junior Engineer, and when The Junior
Engineer passed to Senior status, so did he likewise pass it to
the next generation. And so it has been, and so shall it be,
until the end of Time.

 

[J.P.]

This forum needs a like button.

 

[jim dungar]

... I guess they feel that the larger conductor gives more surface area. ...

Many times the surface area is not due to 'ohms resistivity' as much as it is to 'corrosive resistivity'.
The larger the hunk of copper in the ground, the longer life span we expect it to have. It is kind of nice when the grounding electrode matches the life of the facility.

 

[kingpb]

Because #4/0 AWG is the smallest size soft drawn copper conductor that can safely dissipate 40kA for a duration of 0.5sec assuming 60Hz, X/R of 20, and 40 deg C ambient.

While also factoring in some rough handling and mild corrosion.

Hopefully that gives some insight as to why us "Sr. Engineers" use it in our specs.

 

[don_resqcapt19]

Because #4/0 AWG is the smallest size soft drawn copper conductor that can safely dissipate 40kA for a duration of 0.5sec assuming 60Hz, X/R of 20, and 40 deg C ambient.

While also factoring in some rough handling and mild corrosion.

Hopefully that gives some insight as to why us "Sr. Engineers" use it in our specs.

So are you expecting that the ground ring would have a low enough impedance so that 40kA would actually flow under fault conditions?

 

[dereckbc]

Many times the surface area is not due to 'ohms resistivity' as much as it is to 'corrosive resistivity'.
The larger the hunk of copper in the ground, the longer life span we expect it to have. It is kind of nice when the grounding electrode matches the life of the facility.

Ditto. I do a lot of Rings for Telecom and there are two reasons.

1. As mentioned life span.
2. Fault Current Capacity. A large telecom facility can have up to 69 Kv service and need a larger conductor size to handle the higher possible fault current in the event of primary to secondary fault in transformers.
3. We sleep better at night and own stock in copper mining companies. :huh:

FWIW a POCO sub-station Ground Grid is constructed with up to 750 MCM with 20 foot on cent grid with 20 foot ground rods. But that is a whole other discussion.

 

[PatM]

In the Beginning was The Project, and The Project had The Spec.

I sent this to the Engineering Manager here, he laughed his tukas off.

 

[greenspark1]

In the Beginning was The Project, and The Project had The Spec.
The Spec was with The Senior Engineer,
And The Senior Engineer saw that it was Good...

Enjoyed that one.

Re: corrosion issues, wouldn't this be factored into the NEC? I would think that they would anticipate corrosion and upsized accordingly. They aren't exactly known for cutting corners at the expense of cost.

This is a 480V 3200A service, so no special MV or HV equipment here.