Common GEC for several transformers

[marcosgue]

Hi everyone, I usually run individual gec from the main groun bar to each dry type transformer SDS inside the premises, but 1. Can I run a common gec for several SDS and how to size this according to 250.66?
2. Is there some advantage to use individual gec or common gec for various SDS transformers?
Thank you and I appreciate some comments in this particular questions

 

[ptonsparky]

I would size the common one for the largest of that required by your largest SDS transformer(say 1/0) Each of the others would sized by its requirement. 1/0, 6,,4, 8, 8 etc.

The common would likely save some dollars. YMMV.
I know of no other advantage.

 

[infinity]

Yes you can run a common grounding electrode conductor for multiple transformers but it is required to be a minimum of #3/0. Take a look at 250.30(A)(6).

 

[ptonsparky]

Yes you can run a common grounding electrode conductor for multiple transformers but it is required to be a minimum of #3/0. Take a look at 250.30(A)(6).

The 3/0 GEC, would be from his 'main ground bar' to the GES.

I picture 5 transformers of various size in a common area. I would size for the largest required by 250.66 and run it to the 'main ground bar'. Others would tap to it with each sized as needed.

 

[augie47]

Yes you can run a common grounding electrode conductor for multiple transformers but it is required to be a minimum of #3/0. Take a look at 250.30(A)(6).

Was there not an exception in '23 that allowed a smaller common conductor IF none of the electrodes required one that large ?

 

[infinity]

Was there not an exception in '23 that allowed a smaller common conductor IF none of the electrodes required one that large ?

I believe that it was added to the 2020 NEC. I'm assuming that it wouldn't apply to the OP but you bring up a good point he didn't say what size GEC's are going to the electrodes.

 

[electrofelon]

I would use an aluminum GEC for the common GEC for cost savings.

Also does anyone find it worth it to use the EGC in the feeder supplying the transformer has the GEC?

 

[jaggedben]

Yes you can run a common grounding electrode conductor for multiple transformers but it is required to be a minimum of #3/0. Take a look at 250.30(A)(6).

I will point out that the code doesn't exactly say that. It says a 3/0 common GEC shall be permitted for multiple SDS, not required. One could argue that the method in 250.64(D)(1) could also be used, where if the combined circular mill area of all the transformer secondary conductors adds up to less than 1100, a smaller common GEC ought to be allowed per table 250.66.

Was there not an exception in '23 that allowed a smaller common conductor IF none of the electrodes required one that large ?

Yes, there is such an exception.

 

[infinity]

I will point out that the code doesn't exactly say that. It says a 3/0 common GEC shall be permitted for multiple SDS, not required. One could argue that the method in 250.64(D)(1) could also be used, where if the combined circular mill area of all the transformer secondary conductors adds up to less than 1100, a smaller common GEC ought to be allowed per table 250.66.

A wire type common grounding electrode conductor for multiple separately derived systems would be one that has more than a single transformer GEC connected to it. I don't see any way (other than what Augie mentioned) to use something smaller than #3/0.

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

 

[jaggedben]

A wire type common grounding electrode conductor for multiple separately derived systems would be one that has more than a single transformer GEC connected to it.

Right, and when I said the 'combined circular mill area of all the transformer secondary conductors', I meant all the transformers. Or as 250.64(D)(1) puts it "sized in accordance with 250.66, based on the sum of the circular mil area of the largest ungrounded conductors) of each set of conductors"

I don't see any way (other than what Augie mentioned) to use something smaller than #3/0.

As your quote confirms, it says "shall be permitted to be one of the following". That denotes that the 'following' are compliant, but not that they are necessarily the only ways to be compliant. You're reading it as if it only says "shall be one of the following." But it doesn't say that, so then perhaps there are other ways to be compliant, such as interpreting table 250.66 logically, as well as consistently with a very similar situation covered by 250.64(D) which I quoted above. To wit, if I have a common GEC for two service disconnects that each disconnect a set of 300kcmil copper, 250.66 and 250.64(D)(1) say I can use a 1/0 common GEC. But if I have two transformers that each have 300kcmil secondary conductors, I have to have a 3/0 common GEC instead of 1/0? That makes little sense, and my point is that there's also nowhere in the code that strictly and explicitly requires that.

Perhaps it's just another case of bad copy-editing, and the intent was simply for 250.30(A)(6)(a) to say "shall be one of the following" to create a simple rule. But that isn't what it says.