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.

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.

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.

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...

shortcircuit

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

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.

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

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)

shortcircuit

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.

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?

Tom

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...

shortcircuit

It's always a good idea to talk with the AHJ.

Dave

Like I was saying earlier, I like to hit the meter section first, and lugs from there to each main, bond on the way. This way, the grounding won't have to be messed with if there ever needs to be a panel change. The meter section is as perminant as it can be, sized at 400/320A, panels get changed.... Who knows you might be the guy doing the changing.

Any way for examples sake, I have a portion of the handbook on my web-space.
http://www.markhellerelectric.com/25066.pdf

E57, Thanks. A piture is worth 1,000 words.

Short circuit,

Some larger homes around here have 2 200 amp panels. Each one has a seperate set of 3/0 feeding it from 1 meter base. They would not be paralell because they go to a different 200 amp panel.

Ok, I see I was using the wrong column for conductor area above. Let me try again.

2 3/0=167,800 x 2 = 335,600 circulal mils
335,600 is closest to 350 Kmil
250-66 says 350 Kmil CU = #2 CU ground

Did I get it right?

Tom

Tom...that is the formula that I use

What do you feed the meter base with...wire and pipe?

shortcircuit

shortcircuit,
I'm not sure why you disagree with my method.
It is the same as you described in your initial response, ecxcept that the 2nd panel is also 200-amp. Therefore, the tap is #4 copper just like the GEC.

e57,
For some reason, our local PoCo won't permit GEC connections in the meter socket.

Redsy...what you are telling me then is, that you run a #4 copper GEC for a 400 amp service?

shortcircuit

Redsy, here a picture for you... http://www.markhellerelectric.com/25024.pdf

Many linemen see grounds as a hazard, it seems. They seem to think that if the ground comes loose it will fall on the live busses of the meter, and KaBoom. (It could happen in a main the same way) Some PoCo's don't see the meter as "accessable" because they lock it, ours are just a wire, or lollipop seal.

The way I see it, if there is a bonding or ground failure in a set up where the GEC is in one panel, the other panel and the meter base is lost. Another reason I like "all-in-ones".

Anyway, what shortcircuit's getting at is, with a #4 GEC you're limited to 3/0 SEC's making it a 225A service if it were a single family dwelling. For a 320A continous, youd need 350MCM SEC's, with a #2 GEC. And another size up for 400A even.

Short Circuit,

We use RMC with bonding bushings between the panel(s) and meter. We run the ground rod wire to the meter.

Tom

RMC Tom...

1)What size service entrance conductors do you feed the 320 meter can with on the line side?

2)What size and type conduit are SEC installed in on the line side of the 320 meter can?

3)What size bonding jumper do you install from the RMC nipples on the line side of the service disconnecting means?

shortcircuit

shorty,
I have only done a couple of these and now that I think about it, they had wellwater, so the only GE was (2) rods, which, although only #6 is required I ran #4.
However, in my area, I have seen several installations like the one described, and never saw more than a # 4 GEC.

BTW,
I'm not sure that this is a "true" 400-amp service, although Note 1 to Table 250.66 would still seem to require an upsized GEC .

These are residential set-ups that I'm talking about. They allways seem to be underground fed from the utility.

The utility feeds the meter base with ... I don't remember.

From the meter it's 2" RMC with 3/0 to each panel.

From the water to the panels we pipe it in EMT (some places want ridgid). We do it continous from water pipe to panel to bushing, to next panel to other panel bushing.

Outside pedistal we use the same size wire for the bushings, pedistal, and G rods. I know we only need a #6 for the G rods.

What sizes. I thought I remember one where they wanted 1/0. #2 sounds about right.

TOM

On one multi disconnect service I attached my Grounding Electrode Conductor (EGC) to the neutral conductor of the service mast on the load side of the drip loop splice and the outside wiremen from the utility about had a fit. I insisted that the engineer be called to the sight and he had no problem with it.

When I first started in the craft the POCO in that area was a Rural Electrification Administration (REA) coop. Apparently REA had a rigid rule that the load side of the drip loop splice was the only acceptable place to attach the GEC believing that would provide better lightning protection to the electrical service. To this day if I'm doing a rural overhead service were the only grounding electrodes will be driven rods I make the connection there.
--
Tom Horne

Tom,

Never seen it done like that but it makes sence. To bad doing things a bit different can cause a lot of hassel.

Tom

Can someone tele me what the letters POCO stands for? These leters are not familiar to me.

George

The local POwer COmpany, or utility supplier