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#99973 - 09/29/06 08:38 AM Multiple service disconnects & neutral  
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
400A service feeding two 200A panels-

NEC 250.58 requires all grounds to share the same grounding electrode.

NEC 250.24(B) states that the neutral must be bonded to ground in each service disconnect. (edit: typo here, I meant .24 not .22)

Am I reading this right? Doesn't that create a potential for neutral current to flow through the ground wire(s)?


[This message has been edited by SteveFehr (edited 09-29-2006).]


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#99974 - 09/29/06 09:20 AM Re: Multiple service disconnects & neutral  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Steve, 250.58 is requiring all bonding to be common to the Grounding Electrode System.

250.24(B) is requiring all Grounded Conductors (neutrals) to be bonded to their service disconnect.

This in itself does not create parallel current flow if the raceways or cables are nonmetallic. Note: an EGC is not present at service equipment

If the raceways or cables are metallic, they will in fact be in parallel with the Grounded Conductor (neutral) and will have current flowing on them.

If a connection is made to the GES in the way of individual conductors from each Panel or Disconnect enclosure, there will also be a parallel neutral path created on these conductors but it is allowed.

I don't think it should be, but it is at service equipment.

Roger


#99975 - 09/29/06 09:54 AM Re: Multiple service disconnects & neutral  
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
If the electrodes are common, then the electrodes themselves offer an alternate path. Is this configuration what the code requires?

[Linked Image]

[This message has been edited by SteveFehr (edited 09-29-2006).]


#99976 - 09/29/06 10:17 AM Re: Multiple service disconnects & neutral  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Steve, what you are showing as EGC's are actually GEC's, and yes, this configuration puts them in parallel with the Grounded Conductors (neutrals).

This is what I said here.
Quote
If a connection is made to the GES in the way of individual conductors from each Panel or Disconnect enclosure, there will also be a parallel neutral path created on these conductors but it is allowed.


Now, even though this is allowed it is not necessary.

What you are not showing in your graphic is the "Bonding Jumpers" (can be the factory supplied green screw through the bar into the enclosure)

With the Neutrals bonded in both enclosures, you would only need a GEC from one of the enclosures or the meter.

The grounding only needs to be done once.

Roger


#99977 - 09/29/06 11:01 AM Re: Multiple service disconnects & neutral  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
Yes, you are reading this correct for services and that is why service disconnects for the building are required to be grouped. Grouping will limit the parallel paths. A similar path is created when the meter and the service disconnect both bond the nuetral normally right next to each other. This is why bonding metal conduits in these locations are so critical.

This also occurs in residential applications where multiple services are fed from a single transformer and there are metal pipelines between the properties. Thus the requirement for the large GEC to the water pipes (to carry your neighbors imbalance when their neutral connection fails).

This is typically not the case if the building is large enough to have two separate services in different locations. These buildings even if fed from the same poco feeder, they are fed from different transformers. Any current that leaves a transformer secondary must come back to that transformers secondary. The "good" impedence paths are not through any distant transformers secondary bond (though there is some minor current flow through that high impedence path).

What it comes down to is the system is not perfect. A better method would be to require the poco to carry a ground to each building and no bonding taking place on our end. We are not likely going to see that happen.

If the distances between services cause the parallel current to be a concern try: Relocating service disconnects next to each other, or using a single main disconnect then feeding the two panels. This is why code requires the grouping and limiting the number of service disconnects. (OK, I am repeating myself. Oh, to have the clarity of thought that many of you express in writing.)

One thing about answering questions like this is you can come up with all these quirks, pluses and minuses of each option, and then you just need to go back to the code for it is following the best general practice for what we have.


#99978 - 09/29/06 01:12 PM Re: Multiple service disconnects & neutral  
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
I wrote EGC instead of GEC? Damn, either dyslexic there, or just not thinking. NFPA really needs to find an easier way to refer to all these grounding terms to keep us from getting tongue-tied...

This particular installation has the two 200A panels side-by-side, btw.

So, when 250.64(C) requires the GEC be a single continuous length with no splice, they only require it to be to ONE panel of a group, and the other panel(s) can be just connected to the bus bar of the first? None of the excepions appear to apply to a typical residential installation, though?

[This message has been edited by SteveFehr (edited 09-29-2006).]


#99979 - 09/29/06 02:58 PM Re: Multiple service disconnects & neutral  
markp  Offline
Member
Joined: Apr 2004
Posts: 60
Kent, WA, USA
Yep. As long as the neutral to each panel is sized per 250.66 of the whole service(typically 1/0 copper for a 400A service), you only need to run a GEC off of one of the panels. In fact, you could chose to run the GEC from any other place along the service neutral -- in the meter can, at the weatherhead, from a trough if you used one.


Mark
Kent, WA


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