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#92688 - 04/02/05 12:42 AM Grounding 2 service panels  
AdamsAtoms  Offline
Joined: Mar 2005
Posts: 24
How do you ground 2 service panels on a house? I have a 320A meter base that feeds a 100 amp panel and a 200 amp panel. I planned on installing 1 ground rod and running a #6 from the rod to each panel. What Im not sure about is the cold water ground. Do I run a ground wire from each panel to the cold water ground sized from table 250.66? Thank you.

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#92689 - 04/02/05 03:54 AM Re: Grounding 2 service panels  
e57  Offline
Joined: May 2003
Posts: 2,876
GEC for SEC size from table 250.66 to the Meter Base neutral, and for wire size of SEC to each Main Panel from the same table from the ground of the meter base, applicable conduits, cabinet of main, and neutral bus.

Or, you can get an "All in one" unit, 320A Meter/Main Combo. What you loose in mat. cost you gain in labor reduction.

As for water, and gas if applicable to your area AHJ, I usually like to go from rod(S), (one way to gas and rod) to water and then meter base, or meter main, with a single conductor.

Mark Heller
"Well - I oughta....." -Jackie Gleason

#92690 - 04/02/05 04:05 AM Re: Grounding 2 service panels  
e57  Offline
Joined: May 2003
Posts: 2,876
Oh, "Do I run a ground wire from each panel to the cold water ground sized from table 250.66?" See 250.66 (A) not nessesary to be larger than #6. So long as it is not (Solely, or in the path) connected to a neutral, if so it need to be sized for the supply conductors.

Mark Heller
"Well - I oughta....." -Jackie Gleason

#92691 - 04/02/05 06:30 AM Re: Grounding 2 service panels  
shortcircuit  Offline
Joined: Jun 2004
Posts: 608
The way I ground the service installation you are describing is to run a common GEC from the metal water pipe electrode to the larger of the 2 service panels ground bar where the service grounded conductor terminates, sized based on the service entrance conductors feeding the 320 meter...then TAP the common GEC (with a burndie) with a tap conductor based on the size of the 2nd panels service entrance conductors feeding that panel and connect the tap in that panels ground bar where the grounded service conductor in that panel terminates. (Bond each panel enclosure)

I have been told that you can run separate grounding electrode conductors sized based on the service entrance conductors feeding each panels service disconnecting means to the metal water pipe electrode.

IMO... this method seems to conflict with the requirement of 250.104(A) which requires the bonding jumper to metal water piping systems to be sized according to table 250.66 (This is a gray area that the code does't specifically address or prohibit.)

Unless you can prove 25ohms or less to that single ground rod, you better add a 2nd...


[This message has been edited by shortcircuit (edited 04-02-2005).]

#92692 - 04/02/05 02:11 PM Re: Grounding 2 service panels  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
We're talking about one meter here, right?

In that case, what you will have is one "main" and one "sub" panel, or a "service disconnect" with two "sub" panels.

Your ground rod, and water bond, usually go to the service disconnect or main panel. I say "usually," because I see no reason the water bond cannot go to the ground bus on the sub-panel.
Your sub-panels have a ground wire, sized to the amps serving that panel, that goes back to the service disconnect/main panel.

Perhaps you are feeding "through" one panel, to the next, without using a breaker...this is possible with some panels. You should, in that case, have a main disconnect that will kill power to both panels; you shouldn't have to pull the meter to shut down the panels. In that case, you separate your neutral from your ground at, and run your ground wire to, the disconnect.

#92693 - 04/02/05 03:08 PM Re: Grounding 2 service panels  
Redsy  Offline
Joined: Mar 2001
Posts: 2,056
Bucks County PA
What you are doing is commonly done around here, with the 400-amp (320-amp continuous rating) meter socket fitted with double barrel lugs which feed (2) 200-amp panels.
It is usually sold as 400-amp service, but the GEC to the water main is actually sized like a 200-amp service(#4 copper) based on the size of the SE cable.
I connect it to one of the panels and run a #4 tap to the other panel.
The portion that goes to the ground rod is done the same way but need not be larger than #6 copper

#92694 - 04/02/05 03:59 PM Re: Grounding 2 service panels  
shortcircuit  Offline
Joined: Jun 2004
Posts: 608
renosteinke...multiple disconnects are allowed according to 230.40 exception #2 and no more than six in one location. The GEC must connect to the service equipments grounded conductor at an accessible point... see 250.24(A)(1)

Redsy...each seperate service disconnecting means enclosure must have its own GEC tap connected to the common grounding electrode conductor which is sized according to table 250.66 note#1...these taps are allowed to be sized according to the SEC feeding the enclosure...250.64(D)


#92695 - 04/04/05 03:46 PM Re: Grounding 2 service panels  
markp  Offline
Joined: Apr 2004
Posts: 60
Kent, WA, USA
Isn't 250.64(D) a permissive statement and not a requirement? Without 250.64(D), I'd think that this would not be allowed because this creates a parallel neutral and causes "objectionable currents" to flow on the common portion of the GEC (violates 250.6(A)). Yes you can run GEC taps to each panel, but I don't see where it is required.

Can't the service neutral that interconnects the two panels be considered a GEC (based on 250.64(C) exception and if it is sized large enough) and suffice for bonding (based on 250.142)?

If it was me, I'd run a GEC to your rods and a GEC to the water pipe each from the 200A panel. Do watch the size requirements as specified in 250.66 and be careful if you used parallel service entrance conductors (and you have to pick the largest ones, not just based on the size feeding one panel). For my 400A service, I used 400 KCMil copper SEC's and this required a 1/0 copper to the water pipe. If parallel 2/0 copper wires were used in my mast instead, I could have used #2 copper for the GEC.

Kent, WA

#92696 - 04/04/05 06:26 PM Re: Grounding 2 service panels  
Active 1  Offline
Joined: Feb 2003
Posts: 687
Grayslake IL, USA
We do the "400 amp service" also with 2 200 amp panels and a 320 amp meter base.

I seen some run 1 #4 to both panels for the water pipe ground. I have also been told you need to run 2 #4 (1 to each panel) or 1 1/0 to both panels. They said you had to add the conductor area together (2 3/0).

When I try the math I get
2 3/0=.2790x2=.4580
.4580 is just under to a 350 kmil (.4656)
So I would figure a #2 water pipe ground to both panels.

When they call ask for a #1 I am guessing they are figuring it:
400 amps=500 kmil = 1/0 ground
But that does not seem the corect way to figure it if you have 2 3/0.

250-66 Notes: 1. the equivalent size of the largest sum of the areas of the corisponding conductor set.

So my question is what is the corect way to figure a single ground with 1 service and 2 main panels and no parallels?


#92697 - 04/04/05 08:42 PM Re: Grounding 2 service panels  
shortcircuit  Offline
Joined: Jun 2004
Posts: 608
Tom... to find the common GEC size according to table 250.66 note#1, you add the equivalent area for multiple sets of SEC and base the size of the common electrode conductor to this figure. I don't agree with redsy's post which describes one #4 to electrode with a tap off the #4 to the 2nd enclosure.

What type of building are you feeding with parallel 3/0 copper conductors?

markp, yes, this tap method is permitted by 250.64(D), but not a required method for grounding AC premisis wiring systems.

It is also an accepted practice to run seperate GEC to the electrode from multiple service disconnecting means installed according to 230.40 ex#2, sized according to the service entrance conductors feeding each disconnect...


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