Ground wire outbuilding requirements?
Question:I've read NEC 250.32 and 250.122. It appears to me that, to be safe, I should be running a ground wire from my main service panel to the new 100A subpanel in a detached garage building. The inspector I called said I did not need this 4th wire if there was no water pipe/telephone/cable in the building...(which could be a ground path to the original building)...implying that if I ever did want to put any of those things in the building, I needed a ground wire.
Since I might add those things someday, I want to go ahead & take the extra step and put a ground wire into the 2 inch underground plastic conduit before rough inspection. It appears that using 4 Ga aluminum wire, protected by a 90 A breaker, will require 8 Ga copper wire (or 6 ga aluminum) as the ground...connected from the bonded ground in the main panel to the equipment ground in the subpanel , where the Ufer ground to the building is also connected (but NOT bonded to the neutral). Do I have it all correct? Thanks...
Answers:
I don't know where you are so I can't comment on your local codes. You should be using table 310.16 for ampacity for the feeder. I would guess you will use a XHHW type wire. You should use a #2 aluminum for the feeder. You can us a 2-2-4 aluminum cable assembly such as URD. You can protect that with a 100A breaker. Or a 90 if that is what you have. You are correct in the #8 copper ground. Do not bond the ground and neutral in the sub panel. Any other questions, email me.
I would not use aluminum cant stand the stuff but I would still include a ground wire as you never know what you will add at a future date
You better read that article 250-32 again and this time place attention to the exception that says: A grounding electrode shall not be required where only a single branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the conducting carrying parts of the equipment. For the purpose of this section a multibranch circuit shall be consider a sinlge branch circuit.
So that tells you that what it is not required is the grounding electrode since the total system already has one at the main building. Adding any other wire as "ground" back to the source might provide faulty circuit breaker trips. By the way, the only way that you size a ground wire is using Table 250-66.
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