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#68813 - 08/16/06 06:06 PM grounding electrode cond. and voltage drop
carjam Offline
Member

Registered: 05/06/05
Posts: 12
Loc: GrandBlanc, MI U.S.A.
I need some clarification/help on wire sizing.If you up your feeder wires for voltage drop you have to up the grounding electrode conductor as well. The code says that it goes by circular mills and thats where i get lost. Can someone clear this up for me or guide me to a previous post. For example if you have a 100 amp service (residential) you would use #4cu if it is 200ft away according to my math and 3% voltage drop it says i need to use a #1cu. i do not know how to size my ground wire though. Any help would be appreciated

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#68814 - 08/17/06 03:06 AM Re: grounding electrode cond. and voltage drop
macmikeman Offline
Member

Registered: 07/16/02
Posts: 718
Loc: Honolulu, Hawaii
Are we talking about the grounding electrode conductor, or equipment grounding conductors here.
The size of grounding electrode conductors is based on the size of the largest ungrounded service conductor. Equipment grounding conductors are sized based on the rating of the overcurrent protective device ahead of the conductors. In the case of upsizing branch circuits and feeders to allow for voltage drop, yes the equipment grounding conductor must also be increased by the same percentage. For grounding electrode conductors see table 250-66

[This message has been edited by macmikeman (edited 08-17-2006).]

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#68815 - 08/17/06 04:03 AM Re: grounding electrode cond. and voltage drop
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
The way I see it, if you're getting so much voltage drop on the conductors that you need to compensate with larger conductors, you'd run into the same problem during a high-current ground fault, and would need the extra size there to reduce resistance enough for an effective ground. So, same NEC formula would apply.

I'd use a #6 GEC in your case, per table 250.66.

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#68816 - 08/17/06 06:31 AM Re: grounding electrode cond. and voltage drop
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
I agree with Mike.

It appears as though there is some confusion over the function of a GEC verses an EGC, and Service Conductors verses Feeder as well.

A GEC will not need to be adjusted for VD, and I can't think of where it would be run with the Service Conductors, although I'm sure there is an exception to this somewhere.


Roger

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#68817 - 08/17/06 07:17 AM Re: grounding electrode cond. and voltage drop
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
Roger, while there does appear to be confusion between a GEC vice EGC, I have to disagree with your statement regarding not upsizing the GEC.

Table 250.66 "Ground Electrode Conductor for Alternating-Current Systems" does not give any exceptions for oversized conductors, nor state the size in terms of current, but specifically requires GECs based on conductor size.

Even when a "Grounded Conductor" is brought to the service equipment, Article 250.24(C)(1) still references back to table 250.66, which requires it as a % of the conductor size with a few exceptions that won't apply on a 100A service.

Now, I'd argue intent/letter of the law from an engineering perspective if the GEC was immediately sunk to ground 6' from the box and voltage drop not an issue, but the letter of the law appears very clear here.

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#68818 - 08/17/06 07:54 AM Re: grounding electrode cond. and voltage drop
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Steve, before we go any farther, do you think a GEC plays any part of opening an OCPD in a fault condition for services covered under the NEC?

Let's look at a Rod only installation, the GEC would only need to be a #6 CU even if we were talking about parallel (or more) 2000 kcmil's' 250.66(A)

Now,
 Quote:
Table 250.66 "Ground Electrode Conductor for Alternating-Current Systems" does not give any exceptions for oversized conductors, nor state the size in terms of current, but specifically requires GECs based on conductor size.
and 3/0 is the largest CU GEC required of any size conductor or parallel sets of conductors.

 Quote:
Even when a "Grounded Conductor" is brought to the service equipment, Article 250.24(C)(1) still references back to table 250.66, which requires it as a % of the conductor size with a few exceptions that won't apply on a 100A service.


That has nothing to do with a GEC, it is for clearing a fault and opening the OCPD, in otherwords the Grounded Counductor is an EGC (back to the source) in this instance, and is sized per table 250.66 since 250.122 could not be used due to this wording over the first column "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)", we (speaking for our installed componets) would not have an Automatic OCPD at the POCO's transformer.

Roger



[This message has been edited by Roger (edited 08-17-2006).]

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#68819 - 08/17/06 01:07 PM Re: grounding electrode cond. and voltage drop
carjam Offline
Member

Registered: 05/06/05
Posts: 12
Loc: GrandBlanc, MI U.S.A.
yea sorry for the confusion guys. I am part foolish somedays. I am talking about the equipment grounding conductor in reference to 250.122(b)

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