Does anyone know what the NEC says on this for a 200 amp residential system. My local code defers to the NEC without any variations, but I don’t own a copy of the NEC.
I drove one ground rod yesterday, and with our incredibly rocky soil, it took over an hour with the sledge to get 8′. I thought I was supposed to have two rods, 8′ apart, but everyone else seems to be doing just one. As hard as it was to drive the one, if I don’t have to do the second, I’d rather not.
-Rich
Replies
My understanding (my copy of code's too old to count) is that you've got to have multiple grounds, but one can be a metal water pipe or a Ufer ground.
The NEC requires that a single driven electrode be installed. If the electrode has a resistance to earth of less than 25 ohms you are good to go. In the event the driven electrode has a resistance to earth greater than 25 ohms you are required to drive a second electrode and connect it along with the first driven electrode to the electrical service with an unspliced conductor not smaller than #8 copper conductor if physical protection is provided or a #6 copper conductor if no physical protection is provided. The resistance measurement to earth requires a fall of potenital resistance bridge. Most of the time the cost to measure the driven electrode's resistance to earth exceeds the time and materitals cost to install the second electrode. So it is rarely done at a dwelling. I would just drive the second rod and be done with it. Of course in all of these matters the AHJ has the final say. Roger
Thanks! That was exactly the answer I needed. I don't have the tools/skills to measure resistance to earth, but I can operate a sledgehammer. I'll drive the other one (my arms are getting tired just thinking about it), and be done with it.-Rich
It may be worth your while to search the archives about ground rods as it seems I recall someone posting about being surprised at the ease of driving a ground rod using a rotary hammerdrill.
half of good living is staying out of bad situations
I know Hilti has an attachment for some/all of its hammer drills to drive ground rods. I'm not sure how you get up above 8' feet to drive the long ones, a ladder I guess.
Being too cheap to pay Hilti, Bosch, etc., big $, I made one out of a hammerdrill bit the tip had broken off of and a foot of 3/4" pipe. Braze the end of the pipe near the motor end and drive away. I also learned to put a slight bend (approx 3 deg.) in the end of the ground rod. If you are driving next to the foundation the footing may catch the rod. The rod should be started at about a 15 deg. angle as it will tend to make a curved path. The kick also helps it bypass rocks.
Have fun . . .
My situation is a remodel, so I don't know how far the water pipes are metal (probably the whole way to the main, but don't feel like digging to find out). The ufer is out as well, the foundation for the addition is long since poured and I didn't think of the ground until now.I'll just pound in the second rod, to be sure. It's good exercise. I will bond the plumbing system to the ground. I thought you weren't supposed to ground the gas lines. Or is it that you should bond them to the ground, but they can't count as your ground?-Rich
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Ever try an air hammer or rotary hammer to get the rod in? Might save you some sledge swinging.
"Let's get crack-a-lackin" --- Adam Carolla
Yeah, forget to mention that. I would love to use an air hammer, or a hammer drill, but I don't own one and can't justify the expense.-Rich
What about renting one? I had to rent a Bosch a few weeks ago, and it was $22 (I think) for the day. I would think better rental places would also have the attachment for pounding rods in the ground.One thing I found helpful if pounding a rod in with a sledge hammer is to have one person hold the rod steady with a pair of channel locks or vise grips to get the rod started. Otherwise, it seems to dance around after each strike. If you don't have someone around to hold it, you can probably use a piece of wood on an angle as a stake, with a hole for the rod.
"If you don't have someone around to hold it, you can probably use a piece of wood on an angle as a stake, with a hole for the rod."I was solo for the first one, so I drilled a hole in the end of a ~4' 2x4 and had it resting on the ladder while I hammered. A little pressure on the board from my leg held the rod in place well enough that I could use both hands on the sledge. The only problem I had was that the end mushroomed so much, I couldn't get the board off when I was done. I want to leave the mushroomed end on the rod so the inspector doesn't think I got the rod in 3 or 4 feet, got frustrated, and cut it off there. Obviously not a big problem, just cut the board, but it was a funny situation that I didn't anticipate.-Rich
Dig a starter hole with a shovel, and fill it with water several times the day and evening before.
-- J.S.
Maybe the following will help..... We have to put in pretty robust grounding systems in the industrial plants we design for the automation systems; both for code compliance and to reduce noise on the low voltage signals. The "Getting down to Earth" book was one reference a vendor suggested reading. If anything, it may prompt you into a good nights sleep before beating the snot out of the second rod......
http://www.metercenter.com/biddle-ug/GettingDownToEarth.pdf
"Or is it that you should bond them to the ground, but they can't count as your ground?"Exactly.
Thanks for clearing that up. It makes me a little nervous to think of current running through a metal pipe filled with a higly explosive gas. I heard someone once talking about all of the consumer protection safegaurds we have in place these days, and how propane/nat. gas would never pass if we weren't already used to it. If someone said "I have a new product that I want to pipe into your house under pressure. It is odorless, invisible, poisonous, and highly explosive." It wouldn't stand a chance.-Rich
The plain old electrical outlet would never make it. And think of the liability issues if you "invented" stairs.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
the way i drive rods is with a sledge hammer that has had it's handle replaced with a piece of pipe weled in . Slide the pipe "handle" over your ground rod , raise the hammer 6 0r 8 in then slide it down hard-- cant miss that way. once its too short for your handle to work- then you use your hammer the proper way and finish it off.
Gents,
This question is a tiny bit off subject but since we have a panel (no pun intended) of experts assembled here, I'll ask it.
I have been wiring as an avocation for decades but I've just done my first complete building (my neighbor's new office building) and my first meter base. I'm comfortable with all that I've done except the grounding.
Outside I have a Square D combination meter socket and circuit breaker in the same cabinet (wiring simplified page 79) and inside a Square D 200 AMP panel. The outside cabinet and the service panel are connected through a block wall with 2" rigid conduit which has grounding bushings on each end.
I have two grounding rods that are over 6 feet apart and a continuous #6 bare wiring running from the rods to where?. It seems that there are two possibilities.
Either the ground wire from the rods terminates on the grounding busbar of the service panel or it terminates on the neutral lug of the outdoor cabinet.
In either case, I assume that there is a ground wire from the grounding busbar on the service panel to the lug on one bushing and another wire from the other bushing to the nuetral of the outside cabinet.
So, I guess my question is where does the wire from the grounding rods terminate?
Thanks,
Dave
In my work, the first breaker after the meter is the service entrance and this is where the ground rods connect to the neutral conductor from the power company. Other places, I have heard, connect the ground rods to the meter base even if no breaker at that location.In your case, this combination meter and main breaker is the only place where ground and neutral conductors are connected together. From the service entrance (main) breaker to the 200 amp panel inside the building should be 4 conductors, hot hot neutral and ground. The ground connection bar in the inside 200 amp panel does not connect to the neutral connection bar. There are separate animals here.Frank DuVal You can never make something foolproof because fools are so ingenious.
Thanks Frank,
So I gues that in effect the panel is a sub-panel?
This must also mean that I don't bond the service panel to neutral?
Dave
Thanks Frank,So I gues that in effect the panel is a sub-panel?Yes, the inside 200 amp panel is a sub panelThis must also mean that I don't bond the service panel to neutral?The service panel is the outside meter & breaker combo panel, and the neutral is bonded to ground there. The inside panel is a sub panel, so no neutral to ground bond there.Frank DuVal You can never make something foolproof because fools are so ingenious.
I don't know all the product names, so I might be totally off base here, but it won't be the first time. The rigid conduit between the two boxes needs to connected in such a manner as to not lose the contact. For example a lock nut on each side of each box. Is this what a ground bushing does?
There is a locknut on each end of the conduit. The aluminum bushing screws onto the conduit after the locknut and includes a lug to which a wire can be attached.
Hey Rodger.
When measuring resistance to earth, what do they allow you to use as a reference (i.e. what is "earth")
Thanks.
"Let's get crack-a-lackin" --- Adam Carolla
One approach is to use the 120V line as reference -- see if applying 120V to the ground will blow a fuse or breaker of the appropriate size (about 2A, but I think it's most common to use a 5A to be super-sure).
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
>>"One approach is to use the 120V line as reference -- see if applying 120V to the ground will blow a fuse or breaker of the appropriate size (about 2A, but I think it's most common to use a 5A to be super-sure).
25 ohms will draw 4.8 amps at 120 volts to "ground." A fuse is a pretty inaccurate way to measure current but I suppose if a 5A blew, you could be confident that you've got a pretty good ground path.
That doesn't answer the question of what is "ground" when testing impedance of an 8' rod driven in the ground. Is it the neutral coming from the pole that has to measure 25 ohms to your rod in the ground (so your fuse test would pass)? If so, doesn't that assume that the neutral at the pole has an effective "ground" of less than 25 ohms to the actual earth.
"Let's get crack-a-lackin" --- Adam Carolla
Well, it partly depends on how many other folks are on your transformer. The powerline neutral/ground will be grounded at the pole and at every other service on transformer. You can measure the voltage between that and your new ground with a (relatively) high-impedance meter to assure that there's essentially zero ground potential on the powerline neutral/ground.Then, if you apply enough voltage to the ground in test to be "large" compared to the powerline neutral/ground offset, you're assured that the offset is being eliminated from the equation.Of course, in any case you'll be measuring the sum of your ground rod resistance and the powerline ground resistance, but that's where the other folks on the line comes into play.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
> 25 ohms will draw 4.8 amps at 120 volts to "ground." When I made my post I had it in my mind that the target was 50 ohms.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
you can use a Fall of Potential test.http://www.netaworld.org/files/ItemFileA184.pdfwhich is similar to what DanH is talking about, but you should use a qualified contractor who would have the training and equipment..........
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'Wer ist jetzt der Idiot?'
When using the "fall of potential" method, "earth" is the reference. The resistance bridge measures the ohmic value of the ground rod or "made" electrode to earth. The same meter can also measure the actual resistivity of the earth. Most places east of the Mississippi the earth's resistance is fairly low, like 5-10 ohms. Telco central office's with a large ground system will actually have dedicated pairs jumpered together running out for miles to remote electrodes to get far enough away from the sphere of influence from the C.O. ground system. When they make a periodic check of the C.O. they hook the resistance bridge up to dedicated pairs and take a reading.
Please, don't anyone intentionally hook 120VAC into a made electrode and try and use a current reading to determine the electrode's resistance to ground. First of it is just plain unsafe you or someone else could be electrocuted. Second the 60 HZ will provide a bogus reading due to stray and harmonic currents. The grounded utility system has an inherent resistance to earth so your reading won't be accurate. The resistance bridge uses a AC frequency that minimizes the effects of the 60HZ power distribution systems and the associated harmonics. Roger
A friend of mine used to sink ground rods for the TV transmitters on Mt. Wilson, some of them 45 - 60 ft. deep. His method was to make them out of type K copper water pipe with an NPT fitting on the top, and the bottom hammered nearly flat. Water from an ordinary hose would wash out the dirt and let them sink under their own weight. Rocks up to about grapefruit size would slow the rod down a while, but eventually enough dirt would wash out around them that they'd have room to get pushed aside. He got some prehistoric seashells out of some of the holes.
I did a 30 ft. one for my house in about 6-7 hours that way. I tried a second one, but hit some gravel at 6 1/2 ft. and couldn't get thru it. My place is a couple yards of adobe, then coarse sand the rest of the way.
-- J.S.
The code allows an array of different ground electrodes.
Amongs others the most common in residential are ufer (using rebar in the footers), metalic water pipe, and ground rods.
If you have a ufer connection then it must be used as part of the ground electrode system. Generally this is only on new construction.
If the water pipe is metallic and 10 ft or longer undergroun then it MUST be used a ground electrode.
However, a water pipe can not be the only ground electrode.
A rod rod is not required, but if used it must either be tested or a second ground electrode is used.
So where metalic water pipes are used the common system isthe water pipe and a ground rod.
Or if no metallic water pipe, then 2 ground rods.
Note, also that if if the underground water pipe is not metallic, but the house piping is metallic then the water pipe is BONDED to the ground electrode system anyway.
code says you can have as many as you want just not closer than five feet.i install two, one for house and one for cable tv and hvac unit, but still hook together. I got wrote up on having two by the electrical inspector. This is the dude that walmart refuse to hire.I had to appeal to the chief inspector. well junior dont like me no more.
I was under the impression that the point of attaching a ground wire to the water pipe system was to GROUND the pipe system. Similarly, the gas pipe system (if metal) also has to be grounded. And you still need to have the two ground connections.Which reminds me, I've noticed pipe grounding clamps of different materials. What is recommended for copper pipe? What about for black iron gas pipe?
If there is any substantial length of metal water pipe it must be BONDED to the grounding system, regardless. If the pipe continues underground the required distance it counts as a GROUNDING ELECTRODE, if not, not.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
The Code does have provisions for rocky soil and ground rods: per Article 250-52, "It shall be driven to a depth of not less than 8ft except that, where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or it shall be buried in a trench that is at least 2 1/2 feet deep."
I'm in Michigan, and the last couple of houses i've wired required two grounding rods.
out here in the west it's two 5/8" x 8' - six feet apart