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Old 03-08-2012, 10:29 AM   #1
snturner
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Default Sub-panel and grounding

I may have made a big mistake. I installed a new 100 amp sub-panel in my detached garage. I have already ran the conduit and cable from the house to the garage. It's buried and has several 90 degree turns in it. The box is in place and all the switches and receptacles are wired. Sheetrock has been hung. The sub-panel has not been connected to the box in the house yet. The wire is just hanging there right now. The wire is two hot and one neutral.

Question is: Do I have to have a ground wire running from the house box to the box in the detached garage? Can I run a copper wire from the sub-panel in the detached garage to a grounding rod just outside the garage? Do I need to do that?
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Old 03-08-2012, 11:02 AM   #2
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Default Re: Sub-panel and grounding

When I did mine I was told to ground the box in the garage to a grounding rod, box in house has its own and so does the one in the garage.
That's here, your area may be different.
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Old 03-08-2012, 11:28 AM   #3
Falcon67
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Default Re: Sub-panel and grounding

Current code requires 4 wires to the sub and the neutral bus at the sub to be held insulated from the panel. However, for many years three wires to a detached building with a ground at the building was standard procedure. You see it in farm buildings a lot. My old shop was two hots, neutral with an 8' rod at the shop. The power came right off the meter can, not out of the house panel. The install was blessed by a master. YMMV. If you are coming out of the house panel on a breaker, I'm thinking you're missing a wire.
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Last edited by Falcon67; 03-08-2012 at 11:31 AM.
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Old 03-08-2012, 11:29 AM   #4
Charles (in GA)
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Default Re: Sub-panel and grounding

Current code, and I think the past couple of code cycles, has required FOUR wires between a main panel and a sub panel. Two hots, a neutral, and a ground. At the sub panel, the neutral and ground must be kept separated and the neutral has to be insulated from the panel's housing.

Conduit rules allow no more than 360 of bends between pull points, that would be four 90 bends. When you say "several 90 degree turns in it" it sounds as if you have exceeded the 360 rule, which makes it very difficult to pull new wire in the conduit.

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Old 03-08-2012, 01:53 PM   #5
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Default Re: Sub-panel and grounding

Yep.. as you are hearing you need an equipment ground wire that runs from main panel to sub-panel. Also at the detached garage you need to run a ground wire from the sub-panel to two 8' ground rods driven into the earth at least 6' apart for each other.
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Old 03-08-2012, 02:00 PM   #6
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Default Re: Sub-panel and grounding

What size conduit and wire did you use? You mentioned cable, was it a 3 conductor sheathed cable or 3 individual wires? If it's a cable then you aren't allowed to pull a separate wire for the ground. If you have three individual wires then you can pull the forth wire for your ground. Try using a shop vac to suck through a string with a small piece of cloth tied to it to use to get a pull rope through to pull a ground wire.

Last edited by pattenp; 03-08-2012 at 02:06 PM.
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Old 03-08-2012, 04:53 PM   #7
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Default Re: Sub-panel and grounding

One of the posts talks about having both an equipment ground from the main panel to the subpanel and ground rods at the subpanel. I may be mistaken, but I thought that grounding at the main panel and grounding at the subpanel being connected was to be avoided. The reason is that if the grounding potential at the main panel and the subpanel are different, the interconnected grounding wire could become a current carrying conductor. Electrical gurus, please set me straight.
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Old 03-08-2012, 06:15 PM   #8
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Default Re: Sub-panel and grounding

Quote:
Originally Posted by 6768rogues View Post
One of the posts talks about having both an equipment ground from the main panel to the subpanel and ground rods at the subpanel. I may be mistaken, but I thought that grounding at the main panel and grounding at the subpanel being connected was to be avoided. The reason is that if the grounding potential at the main panel and the subpanel are different, the interconnected grounding wire could become a current carrying conductor. Electrical gurus, please set me straight.
It's exactly like Pattenp posted:

Quote:
Originally Posted by pattenp View Post
Yep.. as you are hearing you need an equipment ground wire that runs from main panel to sub-panel. Also at the detached garage you need to run a ground wire from the sub-panel to two 8' ground rods driven into the earth at least 6' apart for each other.
If anyone tells you otherwise, it's a pretty good indicator they aren't an electrician. So take it with a grain of salt....
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Old 03-08-2012, 07:16 PM   #9
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Default Re: Sub-panel and grounding

Pull wires out, add another one, fix it, no way around.
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Old 03-08-2012, 09:11 PM   #10
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Default Re: Sub-panel and grounding

Quote:
Originally Posted by pattenp View Post
Yep.. as you are hearing you need an equipment ground wire that runs from main panel to sub-panel. Also at the detached garage you need to run a ground wire from the sub-panel to two 8' ground rods driven into the earth at least 6' apart for each other.
X2 - glad that was added. Or a UFER, if available.
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Old 03-08-2012, 09:13 PM   #11
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Default Re: Sub-panel and grounding

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Pull wires out, add another one, fix it, no way around.
Simple & to the point answer. There is no other way, BTW w/ 4- 90 degree bends in a conduit run it can still be a difficult pull.
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Old 03-08-2012, 09:14 PM   #12
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Default Re: Sub-panel and grounding

You may be getting confused with the requirement that the neutral and ground are not to be bonded at the sub-panel vs at the main panel the neutral and ground are bonded.

Quote:
Originally Posted by 6768rogues View Post
One of the posts talks about having both an equipment ground from the main panel to the subpanel and ground rods at the subpanel. I may be mistaken, but I thought that grounding at the main panel and grounding at the subpanel being connected was to be avoided. The reason is that if the grounding potential at the main panel and the subpanel are different, the interconnected grounding wire could become a current carrying conductor. Electrical gurus, please set me straight.
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Old 03-14-2012, 08:30 PM   #13
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Default Re: Sub-panel and grounding

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Originally Posted by Falcon67 View Post
X2 - glad that was added. Or a UFER, if available.
UFER?

I'll be doing this myself to my garage; I've already run the four individual #6 wires through the conduit, in preparation for the subpanel, so this is good information.

What is the standard way to run the ground-wires out through the wall to the grounding-rods? More small-diameter PVC conduit from the subpanel box, through the outer wall to carry the THHN wire, and then just have a short run of the wire out in the open to the clamp on the ground-rod? The wire in my basement from the box to the grounding-rod in the hole where my water-meter sits is just uninsulated stranded copper with no special protection, but then again, it doesn't see any weather.
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Old 03-14-2012, 09:45 PM   #14
Charles (in GA)
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Default Re: Sub-panel and grounding

Use a solid #6 copper for the ground, and you don't need to give it any protection beyond not installing it where it can get damaged. Drill a tiny hole in the wall out thru the wall and down to the ground rod. Anything smaller than #6 or not solid, and you have to have it protected with conduit, etc.

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Old 03-15-2012, 10:09 AM   #15
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Default Re: Sub-panel and grounding

Quote:
Originally Posted by Charles (in GA) View Post
Use a solid #6 copper for the ground, and you don't need to give it any protection beyond not installing it where it can get damaged. Drill a tiny hole in the wall out thru the wall and down to the ground rod. Anything smaller than #6 or not solid, and you have to have it protected with conduit, etc.

Charles
Odd, the last two permits I pulled that involved new panels with groudn rods I used a #4 stranded bare copper and each of them run outside through a small hole and then exposed for 6" or so before going underground and to the rods.

Passed inspections fine. Is that solid wire thing a preference or a requirement?
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Old 03-15-2012, 10:30 AM   #16
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Default Re: Sub-panel and grounding

I bet the solid wire corrodes more slowly and is probably cheaper. Vote preference.
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Old 03-15-2012, 11:45 AM   #17
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Default Re: Sub-panel and grounding

Code only specifies that the ground conductor is to be #6 or larger to not be in conduit and not subject to physical damage. Can be solid or stranded.
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Old 03-29-2012, 09:55 AM   #18
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Default Re: Sub-panel and grounding

Quote:
Originally Posted by pattenp View Post
Yep.. as you are hearing you need an equipment ground wire that runs from main panel to sub-panel. Also at the detached garage you need to run a ground wire from the sub-panel to two 8' ground rods driven into the earth at least 6' apart for each other.
so do you run 2 grounding wires out of the box, one to each grounding rod; or do you run 1 grounding wire out of the box and split it outside to go to the 2 rods? I've never seen 2 grounding rods being used. is this new? what is the point?
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Old 03-29-2012, 10:05 AM   #19
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Default Re: Sub-panel and grounding

You use one continuous wire running from the panel to the first rod and then on to second rod. The two rod suggestion is because you need to reach a certain level of resistance to earth of 25 ohms or less. If you can meet that resistance level using one rod then only one rod is needed. To test the fact that one rod meets this requirement is a pain so it’s easier to just add the supplemental rod as allow by code. 2011 NEC 250.53
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Old 03-29-2012, 02:23 PM   #20
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Default Re: Sub-panel and grounding

"New" since 2008 if I recall right. Another reason why a UFER is common practice and an easier deal if done during construction.
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