# GEC size: do (2) 100-amp services = (1) 200 amp?



## mikewillnot (Apr 2, 2013)

In the urban area where I frequently work, on doubles we usually install dual 100-amp services -- separate risers, meter enclosures, (discos), panels. This in lieu of one riser and a 200-amp double meter enclosure. Easier repair when the locked-out tenants take a crowbar to the meter box. 

I know the GEC to the ground rod is not required to be larger than #6. What about for the conductor to the supplemental electrode (the water main)? should it be sized as if the service is 100-amps, or 200? I've been running #4 for years now, and someone suggested that it's not required. Hmmmm.


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## papaotis (Jun 8, 2013)

i say a 100a service requires 100a wiring . 4 is not needed


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## HackWork (Oct 2, 2009)

papaotis said:


> i say a 100a service requires 100a wiring . 4 is not needed


True, but when they are sharing a GEC it is the same end result as if they were both connected to a 200A riser.

I have no idea about this, great question.


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## papaotis (Jun 8, 2013)

that would be true if they are sharing, but i have my doubts i makes any diff. a lightning bolt might melt it either way!:blink:


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## wildleg (Apr 12, 2009)

I believe 250.64 applies, in which case 250.66 note 1 applies (but I've been wrong before):


> 250.64(D)(2014) Building or Structure with Multiple Disconnecting
> Means in Separate Enclosures. For a service or feeder with
> two or more disconnecting means in separate enclosures supplying
> a building or structure, the grounding electrode connections
> ...


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## frenchelectrican (Mar 15, 2007)

mikewillnot said:


> In the urban area where I frequently work, on doubles we usually install dual 100-amp services -- separate risers, meter enclosures, (discos), panels. This in lieu of one riser and a 200-amp double meter enclosure. Easier repair when the locked-out tenants take a crowbar to the meter box.
> 
> I know the GEC to the ground rod is not required to be larger than #6. What about for the conductor to the supplemental electrode (the water main)? should it be sized as if the service is 100-amps, or 200? I've been running #4 for years now, and someone suggested that it's not required. Hmmmm.


That is good question there and I am not sure about the water lateral set up if you have single water lateral just run two set of #6's or run single #4 then split it with irreverseable split bolt or crimp.

the latter part is basied on single riser but senice you mention two risers so I have a feeling but not confirmed in your area. You can run either way. 

However if you do have two water laterals then just string it with #6 each.

Wildleg did bring up a good point and I belive it been written in there for a quite a while. 

Basically it the same way with 320 class services ( 2 X 200 amp services with single riser or lateral )


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## Dennis Alwon (May 9, 2009)

Table 250.66 is based on the size of the service conductor so if you use a #2 or #4 copper for 100 amps then a #8 copper grounding electrode conductor is required. For aluminum a #6.

You need to run a #8 copper or 6 aluminum to each service panel or meter base.


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## Dennis Alwon (May 9, 2009)

You can however run a #4 copper to the neutrals at the service drop and be done otherwise you need to run to each panel a #8 copper minimum.


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## HackWork (Oct 2, 2009)

Dennis Alwon said:


> Table 250.66 is based on the size of the service conductor so if you use a #2 or #4 copper for 100 amps then a #8 copper grounding electrode conductor is required. For aluminum a #6.
> 
> You need to run a #8 copper or 6 aluminum to each service panel or meter base.


The question here is, what if he only wanted to run one GEC to the panel area? He would have to run a #4 and then branch it into two #8's? Or just run one #8?

By your next post I believe you are saying that he would have to run a #4 in that situation, just like if it were a single riser service with 200A SEC's.


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## sbrn33 (Mar 15, 2007)

He would need 4 if he only wants to run one. Normally easier to run two #8's when it is 2 separate sockets as the connections have to be irreversible. IMO
Plus he can use 8 to the rod too as long as he protects it.


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## Drsparky14 (Oct 22, 2016)

HackWork said:


> True, but when they are sharing a GEC it is the same end result as if they were both connected to a 200A riser.
> 
> 
> 
> I have no idea about this, great question.




Depends, but you generally size the conductor based off of 250.66 table in the NEC. 

If it is a duplex you can connect them all together and size it off of 100amps based off of the size of largest service entrance conductor. 

The odds of both panels having ground faults at the same time are so slim that they allow both units to be tied to one set of electrodes and the sizing only to cover the max size of the largest service. 


Sent from my iPhone using Tapatalk


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## Dennis Alwon (May 9, 2009)

Drsparky14 said:


> Depends, but you generally size the conductor based off of 250.66 table in the NEC.
> 
> If it is a duplex you can connect them all together and size it off of 100amps based off of the size of largest service entrance conductor.
> 
> ...


Not sure what you are saying but you cannot just run one #8. A #4 to the meter and splice 2 #8 - one to each panel or meter. 

Here is a similar set up. Look at it with the trough


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## HackWork (Oct 2, 2009)

So you could run 2 X #8, or 1 X #4 and then branch it into 2 X #8 by the panels? Is that correct Dennis?


Then it's the same as when you use a single 200A riser. But in that situation, can you still use 2 X #8 since it specifies using a #4 sized to the 200A SEC's?


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## Dennis Alwon (May 9, 2009)

HackWork said:


> So you could run 2 X #8, or 1 X #4 and then branch it into 2 X #8 by the panels? Is that correct Dennis?
> 
> 
> Then it's the same as when you use a single 200A riser. But in that situation, can you still use 2 X #8 since it specifies using a #4 sized to the 200A SEC's?


Yes, that's it but of course this all depends on the size of the service conductors. For instance if you used 2- 200 amp meter base with 200 amp wire up to the drop for future upgrade then the grounding electrode conductor will be sized differently even if you use #4 tor #2 to the panel.


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## mikewillnot (Apr 2, 2013)

[OP] In my original [recurring] scenario we're currently using #2 SEU for each riser, with a single GEC of #4 copper run from the first panel to each electrode, rod/pipe, with split-bolt taps and equal-sized conductors to the second panel. 

I'm just wondering if the #4 might be overkill. I'm generally carrying #6 around in the truck as it works for 100 and 150-amp services, but the #4 definitely costs more, especially when the panel's at one end of a long house and the water meter's at the other.


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