Ground Rod in Ledge Rock

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  #1  
Old 11-02-06, 06:32 AM
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Ground Rod in Ledge Rock

OK all, here's one.

I have a deteched barn sitting on a short (30") concrete footing/wall on top of ledge rock (there's about 18" of soil before you hit solid limestone). I've scanned the Q/A forum and found inconsistant opinions regarding whether I NEED a ground rod for the sub-panel. But, if I do:

Question: What are the requirements regarding ground rods in this situation?? I can only drive a ground rod in the earth anywhere near the barn about 18" before it is stoped by solid rock, let alone sink it beneath grade. What to do for grounding??? Horizontal?

Background regarding needed a ground rod: The barn will be supplied by 10/3 with ground wire from main panel in house (house is about 150' away). Some said, "detached" buildings REQUIRE ground rod. Some said, with ground wire from main panel, do NOT use ground rod. Some said, don't need one, but it couldn't hurt.
 
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  #2  
Old 11-02-06, 07:22 AM
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Need to know how your using the 10/3 G as a feeder to the barn. Is it just a 20 amp multi-wire circuit running lights and receptacles or is it a 120/220 volt feeder to a subpanel with branch circuit breakers.

Roger
 
  #3  
Old 11-02-06, 08:32 AM
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The 10/3 comes off the main via 30 double pole brk. It feeds a small sub-panel in the barn (no main brk). The sub-panel is connected by two hots (red, black), a neutral (white) and a seperated ground (bare copper). The sub-panel will have four circuit brks for lights (15A) and a couple outlets (20A). The current plan is for 120V, but I guess I could run 240V with the proper brk.
 
  #4  
Old 11-02-06, 09:09 AM
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Ok, you do need GES at the barn. If by chance you would have connected rebar in the concrete footing around the barn this would be great if you could access that rebar and get a clamp on it to connect your ground wire from the sub-panel.

If not then code does allow you to lay 8 foot ground rods in a trench 30" deep or more. You say only 18" is your best depth. So I would just install two or 3 ground rods in a trench as deep as I could get it. Catch each ground rod with the grounding conductor from the ground bar in the subpanel. Use acorn ground rod clamps.

You can also build a ground ring using appropriate #2 bare copper wire that encircles the barn 30" deep but this could be rather expensive.

You can also bury a metal plate/plates 1/4" thick and at least 2 feet square minimum.....

Your problem of getting deep enough isnt all that unusual, sometimes you have to go a ways from the barn to find an area that you can get the rod or electrode deep enough.

Ground rods are primarily for protection of equipment and property from lightning strikes and other high voltage events.

In the real world an inspector needs to prove 25 ohms or less on the GES before he can approve it. So if you cant get deep enough then your only choice is to get as deep as possible.

You could change the double pole breaker to 20 amps and feed the barn 20 amps on a 120 volt multiwire circuit. Using a simple double pole switch at the barn for a disconnect. This would provide you 2... 120 volt branch circuits at the barn which sounds like that is plenty. In this case you are not required to have ground rods or a GES.

Roger
 

Last edited by Roger; 11-02-06 at 10:20 AM.
  #5  
Old 11-02-06, 09:38 AM
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A subpanel in a detached building always requires a grounding electrode; however it does not have to be a rod. There are different electrodes allowed such as undeground water pipes, ground plates, ground rings, UFER concrete-encased-rebar ground. All electrodes should be buried a minimum of 30" down though, so you might have trouble with any method given only 18" of soil.

I think you'll have to get a ruling from the inspector as to what he will allow. There may be a local code that addresses grounding in rocky areas if that is common in your area.
 
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