# sizing grounding electrode conductor.



## jkandrot

Hi, I got a question about sizing the grounding electrode conductor. The job is a 400 amp single phase 120/240 overhead service on a comercial bldg, a church hall to be exact. We have a cls 320 meter socket and double lugs going to 2 seperate 200 amp panels within the building. The service entrance conductors from the weather head are (2) 500MCM coppers and (1) 250 MCM CU ungrounded conductor. On the load side double lugs (2 seperate 200 amp panels) we used 4/0's hots and 2/0 neutrals copper. (I know, they could of been AL. but the supplier made a mistake and we decided to use them anyway). We used 2 - 5/8" ground electrodes 10 ft. apart ( PP&L requirement). Now comes the hard part. The NEC in art. 250.66 has a chart that states you go by the largest service entrance conductor, which we used 500MCM. The chart states 1/0 cu. I was all ready to do this when my friend says I was wrong. He claims it's like a 200 amp service which required a #4 cu from each panel to both rods. He says something about the service will never see a 400 amp overload because the cb's are 200 amp., so therefore why size that conductor for 400 amps? I kinda see his point, but there's nothing in the NEC that says anything like that. What do I do? I have been trying to get a hold of my inspector, but he's hard to get and the suspence is killing me, and holding up my job. I just joined & this is my first posting. Great site! Thank you all. Joseph Kandrot:icon_confused:


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## backstay

250.66(A) GEC to ground rod only is #6 CU


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## old jim

*gn rods, 250.66A requires #6cu. only*



jkandrot said:


> Hi, I got a question about sizing the grounding electrode conductor. The job is a 400 amp single phase 120/240 overhead service on a comercial bldg, a church hall to be exact. We have a cls 320 meter socket and double lugs going to 2 seperate 200 amp panels within the building. The service entrance conductors from the weather head are (2) 500MCM coppers and (1) 250 MCM CU ungrounded conductor. On the load side double lugs (2 seperate 200 amp panels) we used 4/0's hots and 2/0 neutrals copper. (I know, they could of been AL. but the supplier made a mistake and we decided to use them anyway). We used 2 - 5/8" ground electrodes 10 ft. apart ( PP&L requirement). Now comes the hard part. The NEC in art. 250 has a chart that states you go by the largest service entrance conductor, which we used 500MCM. The chart states 1/0 cu. I was all ready to do this when my friend says I was wrong. He claims it's like a 200 amp service which required a #2 cu from each panel to both rods. He says something about the service will never see a 400 amp overload because the cb's are 200 amp., so therefore why size that conductor for 400 amps? I kinda see his point, but there's nothing in the NEC that says anything like that. What do I do? I have been trying to get a hold of my inspector, but he's hard to get and the suspence is killing me, and holding up my job. I just joined & this is my first posting. Great site! Thank you all. Joseph Kandrot:icon_confused:


nec250.66A #6 cu.


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## Dennis Alwon

As others have said you never need more than a #6 to the ground rod. You can either come from the meter base to the first rod and then add a bonding jumper to the second rod or you can run a grounding electrode conductor from both panels to both rods.

250.66 requires a 1/0 to the other electrodes and for bonding water pipes etc.

May I ask why you used 4/0 copper for the panels?


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## backstay

Dennis Alwon said:


> As others have said you never need more than a #6 to the ground rod. You can either come from the meter base to the first rod and then add a bonding jumper to the second rod or you can run a grounding electrode conductor from both panels to both rods.
> 
> 250.66 requires a 1/0 to the other electrodes and for bonding water pipes etc.
> 
> May I ask why you used 4/0 copper for the panels?


I think someone forgot to put AL on the order.


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## wildleg

just curious, but what is the calculated load ?


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## jkandrot

*AL was on the order all right!*



backstay said:


> I think someone forgot to put AL on the order.


AL was on order. so was SINGLE PHASE! But delivered was a 3 phase 320 meter socket! The supplier made the mistakes. I should have known because the girl taking down the order can write faster than I can talk; It's a wonder the rest was right!


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## jkandrot

*CU to panels.*



Dennis Alwon said:


> As others have said you never need more than a #6 to the ground rod. You can either come from the meter base to the first rod and then add a bonding jumper to the second rod or you can run a grounding electrode conductor from both panels to both rods.
> 
> 250.66 requires a 1/0 to the other electrodes and for bonding water pipes etc.
> 
> May I ask why you used 4/0 copper for the panels?


I asked for AL. 4/0 AL would of been right for 200. The CU is better so we installed it. We also asked for a single phase meter socket! But we got 3 phase. We wasted over an hour and a half driving back to the supplier to exchange that. We wern't goining back to exchange th e cu for all!


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## jkandrot

*CU instead of AL*



jkandrot said:


> AL was on order. so was SINGLE PHASE! But delivered was a 3 phase 320 meter socket! The supplier made the mistakes. I should have known because the girl taking down the order can write faster than I can talk; It's a wonder the rest was right!


Oh yea. The 4/0 and 2/0 conductors were wrong. CU instead of AL that I asked for! We installed it anyway.


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## jkandrot

*Load calc/*



wildleg said:


> just curious, but what is the calculated load ?


 I really did'nt have enough info to do the load calc. I'm sure we'll be way over the required amp rating. The biggest loads are lighting and air conditioning. Of course the kitchen will have a bit added. However, there is no equipment there (gutted under construction) and a new gas service was just installed. The owners assured me everything that could be gas will be. So that lightens up the electrical load. I should try to guess and at least get a estimated load calc. just to be sure.


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## Dennis Alwon

Actually 4/0 alumin is only good for 180 amps so unless the load is 180 amps or less then it would be a violation installed on a 200 OCPD.


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## B4T

Dennis Alwon said:


> Actually 4/0 alumin is only good for 180 amps so unless the load is 180 amps or less then it would be a violation installed on a 200 OCPD.


Just want to add that if this was a *residential* service.. 4/0 AL is rated for 200 amps.. 310.15B.7


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## Dennis Alwon

B4T said:


> Just want to add that if this was a *residential* service.. 4/0 AL is rated for 200 amps.. 310.15B.7


Not when you have 2- 200 amp panels it is not.


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## B4T

Dennis Alwon said:


> Not when you have 2- 200 amp panels it is not.


I'm not seeing where having more than one panel changes the rating..

(7) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders. For individual dwelling units of onefamily,
*two-family, and multifamily dwellings,* conductors,
as listed in Table 310.15(B)(7), shall be permitted as
120/240-volt, 3-wire, single-phase service-entrance conductors,
service-lateral conductors, and feeder conductors
that serve as the main power feeder to each dwelling unit
and are installed in raceway or cable with or without an
equipment grounding conductor. For application of this section,
the main power feeder shall be the feeder between the
main disconnect and the panelboard that supplies, either by
branch circuits or by feeders, or both, all loads that are part
or associated with the dwelling unit. The feeder conductors
to a dwelling unit shall not be required to have an allowable
ampacity rating greater than their service-entrance conductors.
The grounded conductor shall be permitted to be
smaller than the ungrounded conductors, provided the requirements
of 215.2, 220.61, and 230.42 are met.


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## Dennis Alwon

Read this carefully



> For application of this section,
> the main power feeder shall be the feeder between the
> main disconnect and the panelboard that supplies, either by
> branch circuits or by feeders, or both, *all loads *that are part
> or associated with the dwelling unit. The feeder conductors
> to a dwelling unit shall not be required to have an allowable
> ampacity rating greater than their service-entrance conductors.


Neither panel supplies all the loads associated with the dwelling so B(7) does not apply


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## B4T

Dennis Alwon said:


> Read this carefully
> 
> 
> 
> ...........


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## Dennis Alwon

Do you see what I mean?


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## B4T

Dennis Alwon said:


> Do you see what I mean?


Yes... I had to read it a few times for it to sink in.. :thumbsup:


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## jkandrot

*Don't understand*



Dennis Alwon said:


> As others have said you never need more than a #6 to the ground rod. You can either come from the meter base to the first rod and then add a bonding jumper to the second rod or you can run a grounding electrode conductor from both panels to both rods.
> 
> 250.66 requires a 1/0 to the other electrodes and for bonding water pipes etc.
> 
> May I ask why you used 4/0 copper for the panels?


I gotta be missing something here. Where do you guys get # 6 from? And what OTHER rods? And PP&L doesn't allow ground rod conductors in meters. They must come from the panels GRND bus. Also, 250.66 is a chart for sizing electrode conductors, right? Is not the largest service entrance conductor 500MCM? Doesn't the chart state that for 500MCM cu a 1/0 cu grounding conductor is required for the ground rod? :confused1:


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## Dennis Alwon

Yes 250.66 is for the grounding electrode conductor however look at 250.66(A)


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## Dennis Alwon

The NEC requires that a ground rod be supplemented by another rod or some other electrode unless you can show 25 ohms on one rod. Look at 250.53(A)(2) exception


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## jkandrot

*I think I got it now*



Dennis Alwon said:


> As others have said you never need more than a #6 to the ground rod. You can either come from the meter base to the first rod and then add a bonding jumper to the second rod or you can run a grounding electrode conductor from both panels to both rods.
> 
> 250.66 requires a 1/0 to the other electrodes and for bonding water pipes etc.
> 
> May I ask why you used 4/0 copper for the panels?


I just found a good answer about sizing the GEC on Mike Holts site. And I think it was by you! You stated an obvious explanation. "The electrode is only as good as #6 cu." so why go with a bigger GEC. Now that makes sense. Thanks.:thumbsup:


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## jkandrot

*I Got it!*



Dennis Alwon said:


> Yes 250.66 is for the grounding electrode conductor however look at 250.66(A)


 Thanks Denise, now it makes sense.:thumbsup:


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## Dennis Alwon

jkandrot said:


> Thanks Denise, now it makes sense.:thumbsup:


You're welcome
LOL-- the name is Dennis not Denise-- that's a women's name :laughing:


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