Grounding conductor of service feeds

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Richard Pra

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What would be the proper way to size the grounding conductor for parallel service feeds from the secondary side of a 277/480v transformer
going to a 3000A switch board. Conduit is run in 8 parallel PVC feeds with 4 500mcm conductors in each conduit.
 
Re: Grounding conductor of service feeds

Table 250.66

This table is based on the size of the conductors supplying the service. Your supply conductors are larger than 1100 kcmil, so you reference the table and see that you would be required to install no larger than a 3/0 copper conductor as your GEC. There are a lot of people who think that is too small and will say larger is good. As far as the NEC is concerned, 3/0 copper is compliant.

There is no math calculations involved with figuring the size of the GEC, just the table reference - using a straight edge of course. ;)
 
Re: Grounding conductor of service feeds

I politely disagree, if this is a "grounded" conductor from a service transformer, it should be sized according to load. In addition 250.24(C)(1) requires it to be 12-1/2% of the total pahse conductor size. In this case 8 x 500 x 12.5% I believe will be another 500MCM conductor.

[ November 15, 2005, 03:48 PM: Message edited by: augie47 ]
 
Re: Grounding conductor of service feeds

I politely disagree with your polite disagreement. The question talked about having four 500 MCM conductors in each conduit. One of those would be the neutral (grounded) conductor. The question is therefore asking about the Grounding Electrode Conductor. Pierre has the right answer.
 
Re: Grounding conductor of service feeds

opps...red-faced... thank you! also sorry Pierre,,,,
leads me to another question, however, why would you have a grounding electrode conductor in the service conduit, and depending on what the grounding electrode is, say a ground rod, what about 250.66 (A). Does the #6 max rule apply? what if you are paralleling conductors?
 
Re: Grounding conductor of service feeds

The question and the answers reinforces what I stress at seminars I have presented. You must use the correct terminology as defined in Article 100, especially when dealing with the requirements concerning grounding. Good catch Charlie
 
Re: Grounding conductor of service feeds

The question was worded properly in the first place.
Good point about using correct terminology John.
Another point is to read the question carefully before answering.
Been down that road :(
 
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