Ground rod at an outbuilding or not?

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Old 01-04-09, 06:04 PM
B
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Ground rod at an outbuilding or not?

This is likely to be a controversial subject, but I want to ask the opinion of the various members of this forum, and especially the moderators, on whether a ground rod should be driven at a sub-panel located at an outbuilding. I have an opinion and will try to explain it, but I find paragraphs in the code which could support either side.

I have long held the opinion that the system is to be grounded at the point of service entrance and from that point on all ground fault current should be routed in a low impedance path back to the point of bonding between neutral and the grounding electrode system. (NEC 250.20(B) and 250.24) To me this means a feeder to a sub-panel must have an accompanying grounding conductor (sized in accordance with table 250.66) and it is to be isolated from any neutral connections. If that ground wire is there and properly sized, it certainly is a much lower impedance path than anything that would depend on the earth. NEC 250.6 instructs us to avoid arrangements that open the possibility of objectionable ground currents. It seems to me, if you choose to drive a ground rod at a sub-panel, the parallel earth path opens the possibility of objectionable ground loops for both AC and DC currents. Also, NEC 250.4(A)(5) tells us the earth shall not be considered an effective ground fault current path. To me all of this adds up to an argument for not installing a ground rod (or rods) at a sub-panel.

Now the other side --- When I read NEC 250.32(D) I think I see an argument that says a ground rod should be installed to support a separate building or structure on the same premises. To be truthful, I find the language a little bit confusing, but it does seem to say that a grounding electrode system is appropriate at least under some circumstances. I have seen opinions expressed by people I consider qualified electricians that say a ground rod should be installed.

So which way is right? Or possibly are both ways correct?
 
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Old 01-04-09, 06:25 PM
J
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Code requires a grounding rod at any outbuilding, but some inspectors don't.

Note that grounding rods have absolutely nothing to do with fault current. Fault current never flows to a grounding rod. Considerations of whether or not you should use a grounding wire (an EGC, or equipment grounding conductor) between panels are independent of whether or not you need a grounding rod and a grounding wire (a GEC, or grounding electrode conductor) between the panel and the earth.

To understand grounding, it is essential to understand the different purposes of the two different grounding systems (the EGC and the GEC).
 
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