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Old 04-23-2011, 10:42 PM   #1
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Sub Panel Grounding


Hi Sparkys and All! This is my first ? on an electrical project. I've been asked to install a 100 amp electrical sub panel in a large garage/storage area. I live in Cincinnati, Ohio and the property is in Parrottsville Tenn. There are some things that I can't verify prior to my being there, (going there on 4-26-2011) I will be cut off from my computer after Monday ,so if anyone can respond before then, that would be great! I know the main service panel in 200 amp with an underground cable run through a 2" pipe to the building I'll be working on. I'm not sure if is 3 wire or 4, I understand if it 3 wire that I need to bond the neutral bus as if it were a main panel, is that correct? The building will have no water hookup, so do I need to add another ground rod to make up for no water line grounding? What size wire from the rod to the panel, #8 copper? If I add another ground rod, how far apart should they be? How do I wire them together, underground wire? Finally, how close to the foundation and how close to the service cable entry should the grounding rods be. I know the ground rods won't be easy to put in the ground, the property owner (my sister) says ground is VERRY rocky there. Any Suggestions or comments (except, hire an electrician) will be Greatly Appreciated! Thanks again, Bluwolf


Last edited by Bluwolf; 04-23-2011 at 10:46 PM.
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Old 04-23-2011, 11:21 PM   #2
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Sub Panel Grounding


is the panel in a separate building?

it must be 4 wire regardless. You no longer have the option.

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Old 04-24-2011, 06:47 AM   #3
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Sub Panel Grounding


The 2008 code removes the option of running three wires to a detached structure, but since this is an existing application in could remain under the exemption. But since there is 2'" pvc going out to panel, than I would run an ground wire. Keep grounds and neutral seperated at subpanel and intall 1 ground rod with #6 thwn.
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Old 04-24-2011, 09:18 AM   #4
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Sub Panel Grounding


I believe 2 ground rods 6 feet apart and bonded with #6 AWG is required.
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Old 04-24-2011, 11:08 AM   #5
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Sub Panel Grounding


This is covered in artrilce 250.
This falls under 250.32 since this panel is feed by feeders and not the exception for a single or multibranch circuit.
It than send us to 250.50 and 250.52.
In this system if the ground is added to the feeders and it now becomes part of the grounding electrode system.
Only one ground rod at the remote structure is required.
Two ground rods are required when there is no metal water pipe in contact with the earth for 10 feet or when the resistance of a single grounding electrode is not 25 ohms or less.
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Old 04-24-2011, 11:24 AM   #6
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Only one ground rod at the remote structure is required.
Two ground rods are required when there is no metal water pipe in contact with the earth for 10 feet or when the resistance of a single grounding electrode is not 25 ohms or less.
unless OP plans on running a ground resistance test, one typically plans on 2 ground rods.



due to the rock, you may have to use plates instead of rods. If you use rods, place them at least the length of the rod away from each other. Code requires at least 6 feet but you have better functional grounding further apart.
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Old 04-24-2011, 04:48 PM   #7
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Sub Panel Grounding


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unless OP plans on running a ground resistance test, one typically plans on 2 ground rods.



due to the rock, you may have to use plates instead of rods. If you use rods, place them at least the length of the rod away from each other. Code requires at least 6 feet but you have better functional grounding further apart.
THANKS to nap, rjniles and njmarine, very much!
The Garage/storage building is about 100 ft. away from the small cabin where the main 200 amp panel is located.
This is what I've got so far, If the feed from the main breaker is Not 4 wire, I should pull a ground wire ( #6 awg?) through the 2" conduit and keep the neutrals separated from ground bus, as it is a sub panel. Then I will install 2 ground spikes 8 ft. apart, connected by a #6 awg wire and connected to the sub panel by the same #6 awg. Right so far?
Questions? Does it matter how far the ground spikes are placed from the foundation? There is about a 4 feet wide path of fill dirt arround the foundation that they pound into fairly easy.
How far in the ground should the connecting ground wires be? Happy Easter!
Thanks much again!!! Bluwolf

Last edited by Bluwolf; 04-24-2011 at 04:51 PM. Reason: Additional greetings
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Old 04-24-2011, 05:08 PM   #8
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Sub Panel Grounding


rods can be anywhere within the area you suggest. I keep them close to the house so they have less chance of getting uncovered over time and if they do, you won't trip on them. They need to end up below grade. Using an "acorn" connector, just put the attachment an inch or so from the top.

the rest I have to look up to verify but I think it's good. Do you know what size wire is in place already? just want to be sure it is proper.
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Old 04-24-2011, 07:01 PM   #9
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Sub Panel Grounding


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rods can be anywhere within the area you suggest. I keep them close to the house so they have less chance of getting uncovered over time and if they do, you won't trip on them. They need to end up below grade. Using an "acorn" connector, just put the attachment an inch or so from the top.

the rest I have to look up to verify but I think it's good. Do you know what size wire is in place already? just want to be sure it is proper.
I just found out that the cable and conduit haven' been installed yet but I know the contractor who built the building and hired the electrician to run the wire and conduit. I can't contact him right now but I'm sure it will be up to code, he's a good guy and I've seen his work, very detail oriented.
So I take it the connecting wire from the grounds should be just slightly buried an inch or so?
Thanks again, bluwolf
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Old 04-24-2011, 07:04 PM   #10
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Sub Panel Grounding


I make the connection near the top of the rod. I then dig a trench and run the wires deeper. Just trying to avoid them from getting uncovered and becoming a trip hazard or being unsightly. They don't need to be deeper legally but I do it for the sake of being detail oriented.
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Old 04-24-2011, 08:56 PM   #11
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Sub Panel Grounding


Quote:
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I make the connection near the top of the rod. I then dig a trench and run the wires deeper. Just trying to avoid them from getting uncovered and becoming a trip hazard or being unsightly. They don't need to be deeper legally but I do it for the sake of being detail oriented.
Thanks much nap. I'm looking forward to this project more now, that I have things clarified.
Are you near Ely Mi? I'm familiar with the International Wolf Center located there. Sincerely, bluwolf
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Old 04-24-2011, 11:44 PM   #12
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Are you near Ely Mi?
Oh God no. That would make be a uper. I'm down as far south as you can get and about 30-40 miles from Lake Michigan. I can almost see the University of Notre Dame from my house (I could if there wasn't any trees), in South Bend Indiana.
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Old 04-25-2011, 05:29 AM   #13
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Sub Panel Grounding


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Oh God no. That would make be a uper. I'm down as far south as you can get and about 30-40 miles from Lake Michigan. I can almost see the University of Notre Dame from my house (I could if there wasn't any trees), in South Bend Indiana.
Oops! I had my abbreviations mixed up. Ely is in Mn.

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Old 04-25-2011, 12:16 PM   #14
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Oops! I had my abbreviations mixed up. Ely is in Mn.
that would probably be worse than Ely Michigan.
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Old 04-25-2011, 05:09 PM   #15
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Sub Panel Grounding


Another quick question. I plan on installing the 100 amp sub panel as close to the main as possible. That would be 36" from the garage door, at eye level. No windows near it. Does that sound right?

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