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NateHanson 05-06-2007 10:20 AM

Ground Rod Hell
 
I've finished the install for a sub-panel in my detached workshop. It's a 3-wire feeder from a 30A breaker on the main panel (to be 50A when I find one). The ground and neutral are bonded at the sub-panel, and I have sunk two ground rods about 8 feet apart near the sub-panel, connected to the neutral bus with #8 CU wire.

The only problem is, both ground rods appear to have hit ledge at about 6 feet. I'm in very rocky country, so it's not unexpected, I guess. I'm just wondering what my chances are of getting past an inspection this way?

I know resistance <25 mOhms on a single rod is enough, and you don't need a second. I don't know yet whether our inspector comes equipped and willing to check ground resistance, but I'll ask. However, does the 25mOhm rule still apply if I don't have a single 8' rod completely buried?

Has anyone run into this situation before?

Maybe I should have driven them in at a shallow angle, like 30 degrees of horizontal. But it's a little late for that unless I have to go buy two more rods and sink them again. It took hours of pounding with a 16# sledge to get these two rods in as far as they are, and there's no chance they're coming out.

I'll call the inspector tomorrow to discuss it with him, but I'd like to know as much information as I can before talking to him.

Thanks, Nate

Big Dave 05-06-2007 11:04 AM

I've run into the same situation in my neck of the woods. I'm not an electrician but in my area we are allowed to wire houses as long as they pass the inspection, which is no problem.

The last time I had this problem I had the electrical service ditch which is where I drove the rods. I couldn't get them down but the inspector allowed me to bend them over and bury them in the ditch.

This was allowed in my area but may not be allowed in yours so check with your building inspectors.

NateHanson 05-06-2007 11:11 AM

Ah, that's an idea I hadn't thought of. Bending them over. Thanks for the suggestion, I'll check with the inspector.

Any other ideas?

sootybuttercup 05-06-2007 12:59 PM

Are you perhaps allowed to use "plate" electrodes instead of rods in that area, where bedrock gets in the way? Doesn't help your present situation...plus you have to dig for those. Just curious. Never too old to learn.

SeanR 05-06-2007 02:08 PM

I kinda had the same problem, must have hit a hard pan. What I did was use a 'T' fence post setter. It's like a pipe that fits over the fence post or in this case a grounding rod. It is closed at the top with several inches of iron for a cap. You side it over the grounding rod and pound. There is no chance of missing the 'hit' and no chance of it slipping off, so you can hit it as hard as you can. Works wonders.

jwhite 05-06-2007 03:01 PM

You can drive the rods at up to a 45 deg angle, or lay the entire rod in a ditch 2 foot deep.

Btw you should only need one rod for the detached building as you are already grounded at the main building.

NateHanson 05-06-2007 07:46 PM

Quote:

Originally Posted by jwhite (Post 43882)
You can drive the rods at up to a 45 deg angle, or lay the entire rod in a ditch 2 foot deep.

Btw you should only need one rod for the detached building as you are already grounded at the main building.

Those are two good pieces of information I wish I had known a week ago. :huh::laughing:

gooch 05-06-2007 08:50 PM

you dont want your neutrals and grounds bonded together in a subpanel, you are going to want to float your neutrals in one bar so that they are not bonded to the panel, and put your grounds in another bar that is bonded to the panel.
-gooch

NateHanson 05-07-2007 08:02 AM

Quote:

Originally Posted by gooch (Post 43919)
you dont want your neutrals and grounds bonded together in a subpanel, you are going to want to float your neutrals in one bar so that they are not bonded to the panel, and put your grounds in another bar that is bonded to the panel.
-gooch

Can one of the electricians in the group respond to this post? What gooch wrote is not the understanding I've gotten. This sub is fed by two hots and a neutral from the main (actually a sub because my main breaker is outside). My understanding was that the ground and neutral should be bonded (with the green bonding screw supplied) in the sub panel box, and a grounding electrode conductor connected to the neutral or ground bus bars.

HouseHelper 05-07-2007 08:43 AM

It is permissible to run just three wires to a subpanel in a detached building IF there are no other metallic paths between the two buildings (i.e.: phone, cable, Cu water lines). The grounding bus and the grounded (neutral) bus will be bonded in this case.

However, in your case you may run into a problem with your inspector since your service to the detached building is from an existing subpanel, which I assume is correctly wired with 4 wires, and the neutral and grounding buses are isolated. This isolation should then be maintained. Had you run your feeder to the detached building from the service entrance, there there would be no problem.

jwhite 05-07-2007 07:53 PM

I just re-read this thread several times, and it looks like Nate's feed is from a main panel to a sub panel in a detached building with no other metal paths between the two structures.

You are good to go nate, bond the ground and neutral at the sub panel, and drive the rods. ;)

NateHanson 05-07-2007 09:12 PM

Well, I'm not sure I was thinking quite correctly about the way my (new) house is set up electrically.

We have a box on the outside of the house, where the underground electrical service arrives, with a 100A breaker in it, and nothing else. This is the main breaker for our house. What I was (I think perhaps incorrectly) calling the main panel in previous posts is a panel with main lugs, but no main breaker, and all the ordinary household breakers for each circuit. This panel is inside the house (about 50' away from the 100A main breaker). I'll have to look inside that panel again, but my recollection is that the 100A breaker and the circuit panel inside the house are connected by three conductors and a ground.

I'll need to verify that the neutral and ground busses are separate in this circuit panel, but I'm beginning to think they must be.

If that's the case, am I right in understanding that I would need to keep my neutral and ground separate all the way from the shed subpanel, through the house circuit panel (from which the shed sub is fed)?

I didn't pull a messenger line through when I pulled the wires through the conduit, so I guess I might be pulling one wire out (to pull a string through) and then pulling two wires back through.

For a 50A sub panel, how big of a ground wire should I pull (THWN)? I've pulled #6 for the hot and neutral conductors to keep voltage drop to a minimum (115' one-way distance on that circuit).

HouseHelper 05-07-2007 09:56 PM

The minimum size grounding conductor for the #6 ungrounded conductors is #10.

NateHanson 05-08-2007 12:00 PM

Well, I spoke to the Inspector today, and he seems to think I'm fine with a 3-wire feed. He also said as long as I have 8' of rod in the ground (between the two) then he'd pass it.

So he'll do final inspection tomorrow, and with any luck this project will be done.

Nate

RobertWilber 05-09-2007 10:59 AM

Bonded neutral and ground at subpanel is strictly no good.
You will have created a parallel grounded conductor [neutral], not have a separate grounding conductor.
Float the neutral or abandon the grounding conductor. A green or bare conductor shouldn't be carrying your neutral current [which it will be]
In any case, if you used metal conduit for the raceway, a "neutral" or grounded conductor bonded to the enclosure by using a service entrance rated panel as the subpanel and a bonded neutral bar will cause the neutral current to flow on the raceway as well.


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