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I built a wood shop a few years ago supplied with 100 amp box and service. I am not an electrician, brought a sunset book and went to wiring. Did not install grounding rod, I thought elec. meter was and is grounded. Lighting came in the lines one evening and burnt a switch and screwed up the outdoor motion light.
Installed 3 foot of grounding rod, we have very rocky soil in NE – Okla.
Question I read/heard code is 8 ft of grounding rod???, If I can only get 3 feet or less in the ground should I install say 2 more rods of 3 feet each and tie the grounding wire to all three rod. Any special instruction. I tell folks I know nothing about electricity . So please don’t assume I know much about the subject. But being somewhat inquisitive by nature is the purpose of 8 ft. in the ground due to surface area of the rod, contact with moist earth or what.
Thanks from a woodworker
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Dale,
The ground rod is just part of a system. The neutral conductor of the Power Company drop is indeed grounded, BUT both the ground rod and other things (like grounding any water and gas pipes in the building) also need to be wired to the panel for safety's sake. Keep in mind that the drop neutral is not always at zero voltage (potential). Having another path to ground is really important.
I suggest you get a book or two on advanced wiring and read up; it best to understand the why, not just the how, of properly wiring a service (and your overall shop, too). The Sunset book is probably not intended for someone installing a service. There's been some dangerously bad advice offered on bulletin boards (yes, even this one), so I wouldn't rely on bulletin boards as your only source of info.
You're onto something with your idea of using several ground rods. But don't cut 'em; use 8 footers and drive 'em in at an angle, or dig a trench as deep as you can, drive 'em in as far as possible, bend the exposed part over into the trench, and bury. Or just dig a trench as deep as possible and bury 'em horizontally. There's a powder you can sprinkle into the trench to increase the soil conductivity and give you a good ground. Get it from an electrical wholesaler like Graybar.
The key is to get the resistance (between the ground wire and the earth) down below a certain level...25 ohms. Generally people don't measure that; they do what's worked in the area before. Tthe building dept/inspector usually knows.
You're right again in thinking that a moist place is best.
You've been lucky so far, but hey, why tempt fate? I'm not saying that you can't do this right yourself, but the consequences of getting it wrong could be having your shop burned up, or worse (can you say "cardiac arrest"?)
Good luck.
*Dale:Cliff provides excellent and practical insight that you should act upon. Try like crazy to get one or several 8-foot rods into the ground - horizontal can also work (if you can dig a trench). The key is low-resistance path to earth.Another note to make you feel a bit better about the damage you received - even the best grounding system is no guarantee of total immunity from lightning damage. Unfortunately, lighting bolts are accompanied by a very large electromagnetic pulse (EMP). The EMP can damage equipment that isn't even plugged into an outlet because any wire (like speaker wires, power cord, etc.) is an antenna for the EMP. And, given the right, unfortunate combination of factors, your stuff can still get zapped.Complying with Cliff's recommendations will certainly reduce your risk - so go for it.TPB
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If the sub-panel is grounded to the main panel via a grounding conductor (the bare wire in the cable going to the barn) then adding grounding rods is unneccesary per the NEC. How many wires make up the feed going to the 100 amp panel? Also know that the grounding conductor is usually not counted in the markings on the sheath of the wire. I am unsure of the methodology for labelling the jackets, but they seem to only ever count the insulated conductors. You will have either three or four wires (at 220 volt) going to the panel. Of the 4 wire, there may be one wire uninsulated. For example, 10-2 romex does indeed have a grounding conductor.
I understand the NEC re-wrote this exact section this year to clarify when the ground rod is needed.
-Rob
*Dale,Everyone has offered you excellent advice so far.I just want to say that I had the comedic pleasure of watching an electricians very new helper try to pound in an 8'foot ground rod in about three inches away from a garage wall. He got down to 4' feet and hit the footing just like I said he would...these kids. Just my two cents.RM
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FWIW, and for _comparative_information_only_ I am posting the text of US Army TM5-6115-323-14, the technical manual for 3kw gensets, in reference to grounding. THIS IS NOT FOR RESIDENTIAL USE, and IS NOT APPROVED IN THE NEC. These are small portable generators, usually trailer mounted. It is interesting in their use of the steel, alum, or copper ground plates. Do not try this at home....
(1) Install one of the following items as a grounding device:
(a) Drive a ground rod to a depth of at least 8 feet.
(b) Drive a ground pipe, 3/4 inch copper or steel, to a depth of at least 8 feet. An existing underground pipe may be used in an emergency.
(c) Bury a 1/4 inch thick iron or steel plate approximately 18 inch x 18 inch size, with ground cable attached, to a depth of at least 4 feet.
(d) bury a 1/16 inch aluminum or copper plate approximately 18 inch x 18 inch size, with ground cable attached, to a depth of at least 4 feet.
(e) Position a 1/4 inch thick iron or steel plate, or 1/16 inch aluminum or copper plates, approximately 18 inch x 18 inch size, on the hard ground or bedrock beneath the trailer stand or roll the wheel of a trailer or truck until it comes to rest on top of the grounding plate.
(f) Saturate the area around the grounding device with water to increase conductivity.
........
agcc
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Actually - those are methods recommended by the NEC. Most localities want to see the groundrods though. In this manner they can asses the connection between the grounding conductor and the grounding device.
-Rob
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I built a wood shop a few years ago supplied with 100 amp box and service. I am not an electrician, brought a sunset book and went to wiring. Did not install grounding rod, I thought elec. meter was and is grounded. Lighting came in the lines one evening and burnt a switch and screwed up the outdoor motion light.
Installed 3 foot of grounding rod, we have very rocky soil in NE - Okla.
Question I read/heard code is 8 ft of grounding rod???, If I can only get 3 feet or less in the ground should I install say 2 more rods of 3 feet each and tie the grounding wire to all three rod. Any special instruction. I tell folks I know nothing about electricity . So please don’t assume I know much about the subject. But being somewhat inquisitive by nature is the purpose of 8 ft. in the ground due to surface area of the rod, contact with moist earth or what.
Thanks from a woodworker