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grounding detached structures


rickm
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i have a fun one. 200 amp service panel feed undergroud from a pole 150 feet away. 100 amp sub panel mounted on the feeder pole, and a detached shop feed from the 100 amp sub panel. the only grounding currently is a ufer at the service panel"i dont trust ufers". The way i read nec i need a ground rod at the service, a rod at the pole for the sub, and a rod at the shop sub for the building. and do not need to pull a ground between these three points

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If I understand correctly the sub panel is fed from the main panel (by wires run back from the main panel), and is mounted on the service pole where the underground feed to the main panel starts (?). Then there's another sub panel (in a detached shop) being fed from the first sub panel, right? Sounds awkward. I'd be concerned about voltage drop at the detached building.

As far as grounding (rod, ufer, etc.), I'd say you probably need it at all three. I think you will need a ground (fourth green wire / equipment ground) between the main and the first sub panel, but it's iffy. The first sub isn't in the same building (with the main panel) or a detached building, so I'm not sure how that plays in the NEC (Jim?). The sub in the detached building doesn't have to have the fourth wire IF there are no other metallic pathways between that building and the one the main panel is located in (other wires for phone, TV cable, etc., or metal pipes / fencing / rebar in concrete sidewalks, etc.). The safest thing to do is wire all of the sub panels as if they don't meet the exception (4 wires, separated grounds / neutrals, etc.), but if there's no sign of a metallic pathway on a detached building you can sound a little overzealous recommending it. If I were actually wiring it that's exactly what I would do.

Brian G.

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Rick,

I'm unable to understand exactly what goes where in your description. Try it again but dumb it down for me. Explain it as you would to a child.

In the meantime, I agree with what Brian said.

Particular points to remember:

* Each separate structure will need a grounding connection to earth (driven rod, Ufer, water pipe, etc.), unless there's only one circuit run to the structure.

* You'll need to isolate the grounding and neutral conductors at separate buildings unless if there's a continuous metal path between them or unless if there's a 4-wire feed already in place.

To answer Brian's question about how a panel on a pole fits into the NEC -- I don't know. However, I've heard people argue that a pole or pedestal qualifies as a "structure" (NEC definition of structure: That which is built or constructed.) Re-read the section with that in mind and all should be revealed.

I hope this helps.

- Jim Katen, Oregon

Edit for grammar

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Originally posted by Jim Katen

* You'll need to isolate the grounding and neutral conductors at separate buildings unless there's a continuous metal path between them or unless there's a 4-wire feed already in place.

I'm sure you meant to say that you must isolate the grounds and neutrals if there IS a continuous metallic pathway between the buildings.

To answer Brian's question about how a panel on a pole fits into the NEC -- I don't know.

WHAT? Unacceptable sir, I want immediate and definitive answers at all times (and at no cost)! [:D][:-dev3][:D]

However, I've heard people argue that a pole or pedestal qualifies as a "structure" (NEC definition of structure: That which is built or constructed.) Re-read the section with that in mind and all should be revealed.

The pole is a structure. I like it. After all, it was at the very least "erected" (stay out of this Chad) for the purpose of supporting a weight load.

Brian G.

Are Trees Structures Under Development Then? [:-mischie

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