250.30(A)(6) - Grounding Electrode Conductor, Multiple Separately Derived Systems

[ThreeTenFifteen]

When using a wire type "common grounding electrode conductor" for multiple transformers - How does sizing this common GEC come into play when using ground rods as the electrodes?

250.66(A) - States that a grounding electrode conductor connected to (2) ground rods is not required to be larger than a #6CU or #4AL

250.30(A)(6)(a)(1) - States that a wire type common GEC is required to be a 3/0CU or 250KcmilAL minimum.

Would a 3/0CU GEC to the ground rods (2) be required if the intent is to use this as a "common" GEC for said XFMR's.

Any input is appreciated.

 

[LarryFine]

When using a wire type "common grounding electrode conductor" for multiple transformers - How does sizing this common GEC come into play when using ground rods as the electrodes?

This one:

250.66(A) - States that a grounding electrode conductor connected to (2) ground rods is not required to be larger than a #6CU or #4AL

 

[infinity]

Good question. I think the common electrode conductor rule "assumes" that there will be electrodes other then ground rods and does not directly address this scenario. I don't see any exception for the common grounding electrode conductor riser to be smaller then #3/0 regardless of what electrode it ultimately connnects to.

 

[jaggedben]

Note that 250.30(A)(6)(a) says that the common GEC shall be "permitted to be" 3/0 cu or 250 al. This is not a minimum requirement, it's more of a maximum requirement. Where 250.66 says a GEC 'shall not be required' to be larger than a given size, that would take precedent.

 

[infinity]

Note that 250.30(A)(6)(a) says that the common GEC shall be "permitted to be" 3/0 cu or 250 al. This is not a minimum requirement, it's more of a maximum requirement. Where 250.66 says a GEC 'shall not be required' to be larger than a given size, that would take precedent.

If you use a wire type common grounding electrode conductor the minimum size is #3/0 copper. There are other types on that list.

250.30(a)(6)(a)Common Grounding Electrode Conductor. The common grounding electrode conductor shall be permitted to be one of the following:
(1) A conductor of the wire type not smaller than 3/0 AWG copper or 250 kcmil aluminum

 

[augie47]

Are the ground rods the only electrodes for the building service ??

 

[ThreeTenFifteen]

Are the ground rods the only electrodes for the building service ??

In this particular situation, there is 4 services at the building. These transformers are fed from 2 of the 4 services and we believe those 2 services are utilizing ground rods only. I should note that these new transformers are located on a equipment island 300ft away from the building structure.

 

[jaggedben]

If you use a wire type common grounding electrode conductor the minimum size is #3/0 copper. There are other types on that list.

Disagree. It's not a minimum, it's not stated that way. Please reread my first post.

"Shall be permitted to be X size" here entails "Shall not be required to be larger than X size." So where both
250.66(A) and 250.30(A)(6)(a)(1) apply, and both effectively say "shall not be required to be larger than X", you can go with the smaller X.

 

[jaggedben]

In this particular situation, there is 4 services at the building. These transformers are fed from 2 of the 4 services and we believe those 2 services are utilizing ground rods only. I should note that these new transformers are located on a equipment island 300ft away from the building structure.

I think that description raises a lot of questions.

Are there other electrodes at the building and are all electrodes not bonded together as required by 250.50?
Are there actually four services or are they feeders? Where is the service point(s)?
Are the multiple supplies to the building actually compliant with 230.2 or 225.30?

Notwithstanding what I argued above I'm rather skeptical you can actually take advantage of what I'm arguing there. If you have other electrodes at the building, they must all be tied together and most likely all supplies must be grounded to all of them.

 

[wwhitney]

If you use a wire type common grounding electrode conductor the minimum size is #3/0 copper. There are other types on that list.

Disagree.

Seems like you are both partially right.

jaggedben is right in that the 2020 NEC has an exception to 250.30(A)(6)(a)(1) and (b) which states "If the only electrodes that are present are of
the types in 250.66(A), (B), or (C), the size of the common grounding electrode conductor shall not be required to be larger than the largest
conductor required by 250.66(A), (B), or (C) for the type of electrode that is present." So if only ground rods are present, the exception means the common grounding electrode conductor and the taps need be no larger than #6 Cu as per 250.66(A).

But infinity is right in that a separate exception is required. Without it, 250.30(A)(6)(a)(1) would require a common GEC of the wire type to be #3/0 Cu as the smallest size. Nothing says that you have to use a 250.30(A)(6) common GEC, or use one of the wire type. You are free to run individual GECs for each SDS, and then 250.66(A) would obviously apply to each such GEC. But if you choose to use a common GEC, and choose to make it of the wire type, 250.30(A)(6)(a)(1) clearly requires that common GEC to be at least #3/0 Cu to take advantage of the allowance in 250.30(A)(6) to use a common GEC.

Except that we have this exception, so it's not an issue.

Cheers, Wayne

 

[jaggedben]

But infinity is right in that a separate exception is required. Without it, 250.30(A)(6)(a)(1) would require a common GEC of the wire type to be #3/0 Cu as the smallest size. ...

On it's face 250.30(A)(6)(a) doesn't require anything, it only permits stuff. So I'd stick with my argument under the 2017 NEC. But the exception certainly clears it up, I'd guess that's why they put it in.

But perhaps this is irrelevant to the OP if there are actually other electrodes present.

 

[wwhitney]

On it's face 250.30(A)(6)(a) doesn't require anything, it only permits stuff.

OK, but if you aren't using the permission of 250.30(A)(6) and its section (a), you can't use a common GEC with taps at all. In that 250.30(A)(5) requires a GEC for each SDS, and that GEC has to be unspliced, so no taps. I guess you could run a single #6 to the ground rods and cad-weld legs to each SDS if you wanted, as I see nothing in 250.30(A)(5) saying the GEC can't be shared. [BTW, that's my argument that a single unspliced GEC can be run along through all the enclosures of the the SDS MBJs to the GES, as each SDS individually complies with 250.30(A)(5).]

So if you are going to use the allowances of 250.30(A)(6) and its section (a), then you have to make your wire-type common GEC at least 3/0 Cu. Unless your argument is that the main paragraph of 250.30(A)(6) just says "A common grounding electrode conductor for multiple separately derived systems shall be permitted," without imposing a requirement that you comply with the subsections (a) and so forth. Which is maybe technically true, but the implication that the global allowance of 250.30(A)(6) is subject to the limitations in its subsections is pretty clear.

I guess I agree there is some sloppy writing and possibly inappropriate use of the "shall be permitted" language, but the latter is rife through the NEC (see in particular 3xx.10 and 3xx.12 for wiring methods).

Cheers, Wayne

 

[jaggedben]

OK, but if you aren't using the permission of 250.30(A)(6) and its section (a), you can't use a common GEC with taps at all. ...

I don't follow. The OP has identified another section one can use to size the GEC. I agree it's sloppy writing. One might have thought there'd be a reference to 250.66 in there somehow.

 

[wwhitney]

I don't follow. The OP has identified another section one can use to size the GEC.

250.64(C) says "Except as provided in 250.30(A)(5) and (A)(6), 250.30(B)(1), and 250.68(C), grounding electrode conductor(s) shall be installed in one continuous length without a splice or joint." So if you aren't following 250.30(A)(6), you need a continuous GEC. I.e. you can't use a common GEC with reversible taps to each SDS. You'd need to use irreversible taps.

Cheers, Wayne

 

[jaggedben]

Because finding an alternative to 250.30(A)(6)(a) means you can't use 250.30(A)(6)(c)? Why?

Fwiw, continuing on the theme of sloppy writing, I don't understand why 250.30(A)(6) doesn't just refer back to 250.64(D), but the requirements largely overlap. Although for some reason it seems that sizing the common GEC based on the circular mil sum if conductors from the different systems got left out as an explicit option. I don't see an obvious reason why those requirements should be different.

 

[wwhitney]

Because finding an alternative to 250.30(A)(6)(a) means you can't use 250.30(A)(6)(c)? Why?

I guess I'm saying that from context, the first sentence of 250.30(A)(6) is implicitly "A common grounding electrode conductor for multiple separately derived systems shall be permitted in accordance with 250.30(A)(6)(a) through (c)." So the allowances come as a package.

But that's sloppy, that language should be explicit. And this debate is academic, as 2020 NEC added the necessary exception. Of course, the existence of the exception implies the intent is what I describe, as per your interpretation, no exception is required.

Cheers, Wayne