Grounding Ring

[TFT1980]

Hi all, for a 600-amp 208-volt 3 phase service what size GEC is required for a ground ring. The code states in 250.52 (A) (4) not smaller than 2 AWG. I'm pretty sure that according to 250.66 that I need to install a 2/0 copper conductor. Is this correct? Thanks in advance.

 

[kwired]

250.52(A)(4) says the ring itself must be no smaller than 2 AWG.

I don't see any requirement anywhere for it to be any larger either, though one may decide to go larger for design reasons I guess.

250.66(C) basically says the sole connection to a ground ring doesn't need to be larger than the conductor the ring is made of.

With that information I'd say you can use a 2 AWG for the ring and connect it with 2 AWG as long as you aren't also making any bonding jumpers from it to other electrode types.

I think the logic here is similar to ground rods, plates and concrete encased electrodes that have an allowed max size even if 250.66 otherwise says larger. The idea is the electrode likely would never dissipate any more energy than that size of conductor can supply to it (2 AWG in this case, 6 AWG for rods or plates, 4 AWG for CEE's.

 

[TFT1980]

This is for a pre-fabricated data center. This is pretty much our only means for grounding. There is no water pipe or building steel to ground to. It is a poured concrete structure that is being set on a slab foundation with footings. This is why I was thinking of sizing the GEC at 2/0 per 250.66. I would imagine that we are bonding the rebar of the foundation also but was just going to use a 4 AWG for that.

 

[Dennis Alwon]

I don't see it as Kwired does. The ground rod says it doesn't need to be larger than #6 but it certainly can be #8 for say a 100 amp service. The ground ring says it can't be smaller #2 but that implies it can be bigger

Rebar only needs #4

What are the sizes of your service conductors

 

[Tulsa Electrician]

I way I read it is. Based on the size of conductor used to form the electrode it's self which is #2 as a minimum. Once the electrode is installed. Size the GEC attached accordingly with 250.66 with (C) in mind.

So if you use a #2 cu than #2 cu is the largest required.
Nothing says you size the conductor per table 250.66 to form the electrode with #2 as a minimum.

So if you use a #2 cu ground ring.
Table 250.66 applies up to #2 cu then go no further for the GEC.

With that in mind I'm listening to the collective to change the way I read it. I'm open minded

If a set of engineered drawings say other wise I'll do it as long as the minimum is meet.

 

[TFT1980]

I don't see it as Kwired does. The ground rod says it doesn't need to be larger than #6 but it certainly can be #8 for say a 100 amp service. The ground ring says it can't be smaller #2 but that implies it can be bigger

Rebar only needs #4

What are the sizes of your service conductors

Parallel 350's

 

[kwired]

I don't see it as Kwired does. The ground rod says it doesn't need to be larger than #6 but it certainly can be #8 for say a 100 amp service. The ground ring says it can't be smaller #2 but that implies it can be bigger

Rebar only needs #4

What are the sizes of your service conductors

I see the conductor to a rod, pipe, plate same way AFAIK. No larger than #6 in any instance where there is no bonding jumper to any other electrodes, but it could be smaller if T250.66 allows it. For the ground ring I see it as that it does imply it can be bigger. I'm not seeing anything that says it must be bigger in any circumstances either though. So does that make it purely design decision if you want bigger and all that code actually requires is no smaller than 2 AWG?

250.66(C) does say the conductor to the ring doesn't need to be larger than the conductor that makes up the ring, and 250.52(A)(4) says the conductor that makes up the ring will never be smaller than 2 AWG. So for 3/0 ungrounded service conductors - can you run 4 AWG GEC to the ring that apparently must be made from at least 2 AWG??

I never installed a ground ring that I can recall so not really familiar with them, comments based on what I read.

So the first question I guess is when is the ring supposed to be larger than 2 AWG and/or where does it say this, all I am seeing that it is permitted.

 

[don_resqcapt19]

I see the conductor to a rod, pipe, plate same way AFAIK. No larger than #6 in any instance where there is no bonding jumper to any other electrodes, but it could be smaller if T250.66 allows it. For the ground ring I see it as that it does imply it can be bigger. I'm not seeing anything that says it must be bigger in any circumstances either though. So does that make it purely design decision if you want bigger and all that code actually requires is no smaller than 2 AWG?

250.66(C) does say the conductor to the ring doesn't need to be larger than the conductor that makes up the ring, and 250.52(A)(4) says the conductor that makes up the ring will never be smaller than 2 AWG. So for 3/0 ungrounded service conductors - can you run 4 AWG GEC to the ring that apparently must be made from at least 2 AWG??

I never installed a ground ring that I can recall so not really familiar with them, comments based on what I read.

So the first question I guess is when is the ring supposed to be larger than 2 AWG and/or where does it say this, all I am seeing that it is permitted.

Table 250.66 applies to the GEC run to a ground ring, but 250.66(C) sets the maximum required size of the GEC equal to the size of the ring.

As far as the size of the ring conductor, it is required to be larger than 2 AWG when the design documents require it to be.

All of the industrial ones I have installed have been 4/0. The typical application is 4/0 ring, 10' ground rods at each corner, and for larger buildings no more than 100' apart, and a 2/0 jumper to building steel at the location of each ground rod. Grounding electrode conductors from the ground ring as required and sized per Table 250.66, but in this case, they would never be larger than 3/0 as that is the maximum required GEC size.

 

[kwired]

Table 250.66 applies to the GEC run to a ground ring, but 250.66(C) sets the maximum required size of the GEC equal to the size of the ring.

As far as the size of the ring conductor, it is required to be larger than 2 AWG when the design documents require it to be.

All of the industrial ones I have installed have been 4/0. The typical application is 4/0 ring, 10' ground rods at each corner, and for larger buildings no more than 100' apart, and a 2/0 jumper to building steel at the location of each ground rod. Grounding electrode conductors from the ground ring as required and sized per Table 250.66, but in this case, they would never be larger than 3/0 as that is the maximum required GEC size.

Makes sense. What about my example of 3/0 ungrounded service conductors - 250.66 GEC only needs to be 4 AWG, if you had a ground ring it must be 2 AWG or larger, can the GEC to that be 4 AWG? The way I read things I think it certainly could be. Even if designer wants 4/0 for the ring conductor, I think the GEC would still be permitted by 250.66 to be 4 AWG. Kind of no different than having qualifying structural steel electrode - #4 GEC is all that would be needed with this size of service conductors even though same steel would require a larger GEC if the service conductors were larger.

 

[Dennis Alwon]

Makes sense. What about my example of 3/0 ungrounded service conductors - 250.66 GEC only needs to be 4 AWG, if you had a ground ring it must be 2 AWG or larger, can the GEC to that be 4 AWG? The way I read things I think it certainly could be. Even if designer wants 4/0 for the ring conductor, I think the GEC would still be permitted by 250.66 to be 4 AWG. Kind of no different than having qualifying structural steel electrode - #4 GEC is all that would be needed with this size of service conductors even though same steel would require a larger GEC if the service conductors were larger.

I don't think so. The conductor to the ring must not be smaller than #2. Imo, for a 3/0 copper conductor you would need a #2 grounding electrode conductor to the ground ring.

 

[kwired]

I don't think so. The conductor to the ring must not be smaller than #2. Imo, for a 3/0 copper conductor you would need a #2 grounding electrode conductor to the ground ring.

Maybe, but read wording carefully in 250.66 along with wording in subsection (C).

Shall not be less than....table 250.66 except as permitted .... (A) through (D).

(C)....... shall not be required to be larger than the conductor used for the ground ring.

Does not say anything about matching the ground ring conductor anywhere.

I get that in many cases the GEC will likely end up being same size as the ring conductor, I also get that if a ring is used there is likely more thought behind performance of this thing than there is for those that just end up driving a rod (or two) because that is cheap and happens to fulfill minimum requirements.

 

[LarryFine]

All of the industrial ones I have installed have been 4/0. The typical application is 4/0 ring, 10' ground rods at each corner, and for larger buildings no more than 100' apart, and a 2/0 jumper to building steel at the location of each ground rod. Grounding electrode conductors from the ground ring as required and sized per Table 250.66, but in this case, they would never be larger than 3/0 as that is the maximum required GEC size.

That sounds more like a lightning-protection system.

 

[don_resqcapt19]

Makes sense. What about my example of 3/0 ungrounded service conductors - 250.66 GEC only needs to be 4 AWG, if you had a ground ring it must be 2 AWG or larger, can the GEC to that be 4 AWG? The way I read things I think it certainly could be. Even if designer wants 4/0 for the ring conductor, I think the GEC would still be permitted by 250.66 to be 4 AWG. Kind of no different than having qualifying structural steel electrode - #4 GEC is all that would be needed with this size of service conductors even though same steel would require a larger GEC if the service conductors were larger.

Yes, the only modification to the sizes in Table 250.66 that is made by 250.66(C) is to say the GEC to the ring is never required to be a larger size conductor than what was used to create the ring. It does not prohibit the use of a smaller GEC to the ring where such smaller GEC is permitted by Table 250.66.
Edit to add left out word ....not

 

[don_resqcapt19]

That sounds more like a lightning-protection system.

Very typical petro-chemical facility specification.

 

[kwired]

Yes, the only modification to the sizes in Table 250.66 that is made by 250.66(C) is to say the GEC to the ring is never required to be a larger size conductor than what was used to create the ring. It does prohibit the use of a smaller GEC to the ring where such smaller GEC is permitted by Table 250.66.

been away from here for a little bit, but don't recall seeing any wording that prohibits this.

 

[don_resqcapt19]

been away from here for a little bit, but don't recall seeing any wording that prohibits this.

I left out a word.... it should have said does not prohibit. Will edit my previous post.