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Posted By: Cindy #6 G-rod, #4 Ufer - 01/09/02 01:54 AM
Please tell me if I understand this right. if I do a service change and can't get to a ufer then i use 2 ground rods for a typical 200amp service. 250-66 says the gec can be a #6 cu if it is the sole connection. maybe i dont get the meaning of the sole connection bit, but was told that it might have to be a #4 cu. if thats true, could someone give me a case where i would have to use the #4. here's 250-66 [Linked Image] thanks, C

250-66.(a) Connections to Made Electrodes. Where the grounding electrode conductor is connected to made electrodes.. that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 6 copper..
Posted By: George Corron Re: #6 G-rod, #4 Ufer - 01/09/02 03:52 AM
Cindy,
No one can reference the code for more than a #6 to a ground rod. It is all the earth can possibly take for fault (Circular Mil Area/42.25 = 5 second fault rating for a conductor) and is sufficient for equalizing potential which is its intended purpose in our service side system anyway. Unless you are creating a system for the supply side, you never need larger than a #6 to a ground rod and your code quotation is correct. [Linked Image]
Posted By: Cindy Re: #6 G-rod, #4 Ufer - 01/09/02 04:38 AM
george could you explain about the Circular Mil Area/42.25 = 5 second fault rating for a conductor?
Posted By: sparky66wv Re: #6 G-rod, #4 Ufer - 01/09/02 05:17 AM
Then why have I been using #4 Bare Solid Cu all this time?!?!? [Linked Image]

Yes, George, please elaborate more on the theory...
Posted By: George Corron Re: #6 G-rod, #4 Ufer - 01/09/02 11:28 AM
Sorry guys, sometimes I assume everyone has a Soares to read.
The fault level (ampacity)of a grounding conductor is calculated for a 5 second bolted fault. That is what 250-122 (99 code) is showing. You can do this calc yourself fairly easy by going to Chap 9, Table 8, look up the circular mil area, and divide that by 42.25. That means a #6 with a cma of 26240 divided by 42.25 has a 5 second fault rating of 621 amps. Any fault is CODE designed to be cleared within 5 seconds, after that, either your ground has gone away (burned through) or your Overcurrent Protective Device has opened. That is assuming you've installed your grounds properly, of course.
Do you think the earth can 'dissipate' or return to XO a fault level greater than that ?
A 120 volt circuit fault, feeding into a ground rod, let's say is 20 ohms would deliver a fault level of 120/20=6 amps. It doesn't get much higher than that EVER for a ground rod.
I also realize I used another term up there that someone may want 'splained. Most electricians have had a short circuit happen fairly close to them. You know that an arcing wire tends to 'bounce and jump' therefore not staying in one place too long. The real fault bounces up and down so much as to be incalculable. The code, and most electrical calculations, are geared to the fault as if it were 'bolted' to the ground. It just makes it easier and yields the highest figure, giving you your margine of safety.
Too much, not enough? sorry for the length.

One tiny more thing, the 5 second calc is because it is the maximum current a conductor can withstand for 5 seconds without permanent damage.

[This message has been edited by George Corron (edited 01-09-2002).]
Posted By: sparky66wv Re: #6 G-rod, #4 Ufer - 01/09/02 03:00 PM
I'm gonna hafta aquire one of them Soare's Books...
Posted By: George Corron Re: #6 G-rod, #4 Ufer - 01/09/02 03:38 PM
anyone that is interested in Soares book, one source is construction bookstore
http://www.constructionbook.com/xq/ASP/qx/default2.htm
Posted By: Elzappr Re: #6 G-rod, #4 Ufer - 01/09/02 04:33 PM
Sparky66wv: #4 solid is for unprotected grounding conductors. If protected, then #6 is ok.
George, what happens during a lightning surge? Your explanation works for a ground fault, tho the ground electrode and conductor has just a small parallel current compared to the equipment ground current during a ground fault.
The real issue of grounding electrode sizing comes in when dealing with lightning, doesn't it? In the IEEE green book it calls for all sorts of multiple ground rods in order to keep the earthing resistance low enough to keep people from getting killed. The more ground rods you install, the higher the current you can dissipate from a lightning surge, so the grounding electrode conductor rightfully should be increased in size to keep from limiting that dissipation capacity.
Posted By: George Corron Re: #6 G-rod, #4 Ufer - 01/09/02 05:34 PM
YO,
The 'lightning' dissipation capability of the low voltage electrical system has been greatly exaggerated.

Why is a transformer grounded, to limit voltage to ground of course. Where is a transformer grounded ? At the transformer.

We have no equipment to capture or dissipate, that is the function of the arrestors at the transformer on the pole. While it is true that some houses have arrestors, most do not, nor is it required. The rare overvoltage that comes from the neutral into the house will dissipate into the ground rod (water/sprinkler pipe, ufer..etc.)and a #6 is way more than sufficient.

The grounding electrode at the structure does not come into play at all for an overvoltage that would come across the phase conductors, that's how people get computers burned up that don't have their surge protectors on them. In other words, by that time the utility equipment has failed. If it did it very often, you would not protect it with the $20.00 protectors picked up at Wal-mart.

The real issue of grounding may be lightning, but not at the structure. As for calculation, impossible, since it isn't even designed for it ie: no required surge arresting equipment, no 25 ohm requirement only that if it doesn't meet 25 ohms you drive another rod, not make it 25 ohms or less, the utility side is a different story. So calculation based on the hypothetical strike that is not supposed to get through anyway would be a 'crap' shoot at best.
Posted By: Elzappr Re: #6 G-rod, #4 Ufer - 01/10/02 02:01 AM
Ok Jorje, consider this. Why do we have Table 250.66? Is the grounding electrode conductor size increase, per phase conductor size, only relative to ground fault calculations? If so, why do we have minimum requirements for #2 to ground rings, and #4 to Ufer grounds? If there is no concern about the electrodes' having varying capacities for dissipating surges, then there should just be one size for all grounding electrode conductors to whatever type of electrode we choose (not taking into consideration corrosion resistance or need for physical protection).
Another way to look at this issue: 250.122 deals with minimum EGC's per O.C. protection rating. Why don't we just use THAT Table to figure our grounding electrode conductor size? Why do we go to larger sizes when dealing with GEC's?
For example, 250.66 at 2/0-3/0 calls for #4GEC.
Looking in 310.16, 3/0 equates to 200Amps. Jumping to 250.122, 200A O.C. requires a #6 EGC. Why the difference? Why are the GEC's consistently larger than the EGC's, if all that we are really concerned about is the amount of ground fault before clearing? Perhaps it is a complex mix of both concerns, ground faults & surges, I don't know.
Mind you, I'm not trying to one-up you, I'm just out to learn..like everyone else here, I suppose.
Maybe the GEC's are sized larger because we don't have a definite size for our O.C. protection device at the pole or vault, but we DO at the service (as well as the downstream O.C. devices). But that still doesn't explain away the code minimums which change relative to what type of grounding electrode we use (250.52).
Posted By: George Corron Re: #6 G-rod, #4 Ufer - 01/10/02 04:00 AM
The average ohmic reading to ground on a ground rod (8') is around 40 ohms in loamy soil. The average ohmic reading to ground for a water pipe is around 3 ohms. These do tie to the infinite buss feeding from the utility, and the increase in the size of the conductor is reflected because of the possibilty of fault delivery capability. The possible fault from an infinite buss being....well, infinite, is why we have 2 completely separate tables with very different sizes.

That is also why we have to bond the service pipe with grounding bushings, locknuts are not good enough because of the "infinite" possibility. The AIC on the branch circuit side is quite a bit lower, and has a definite 'topping' out point, unlike the utility.

The only fuse on the utility transformer is often (usually in this area) a piece of wire which must burn clear ( or fuse if you will) and bunches of damage can occur to your system before this happens, the quicker we can make it happen with a conductor that will take the punishment the better for our local fire departments.
Posted By: sparky Re: #6 G-rod, #4 Ufer - 01/10/02 12:31 PM
Guys,
i've read your posts 3 times over, and i'm still [Linked Image] .
If I may impose my basic understandings for your elaboration;
the K.I.S.S. method if you will..

Electricity works a circle in a fault, no circle no fault. A GE is usually a dead end.

The earth , as pertains to lightning, is part of said circle, the GE's job 'scrip.

Some elude the 'voltage gradient' to GFI operation, but i've seen them in shed's ( single branch, no GE) 400' away with no ill effects. I've also installed them per fire marshals in tenaments ( K&T) with little to no viable GE left ( rotted, abused..)to speak of....

Unfuzz me please....speaka de ingleesh? [Linked Image]
Posted By: George Corron Re: #6 G-rod, #4 Ufer - 01/10/02 01:30 PM
Sparky,
My fault. The answer to Cindys question is: There is no time you need to have more than a # 6 conductor to a ground rod. My fault for going into an explanation that is way to lengthy for a quickie forum answer.

Lemme try one more time to amend my error.

The purpose of OUR (meaning low voltage, supply side electricians) designed intent of the Grounding Electrode System is to prevent any potential difference between electrical equipment and the earth upon which we are standing. No more than that. The circuit you speak of, in this case, is the equipment, your hand - through your body to your feet to anything metallic to earth (whether this path is through your water heater, insta-hots, or any plumbing/electrical connection) back into the electrical equipment. If they are not 'equalized' through a ground connection any inductive effects which you would have because it's AC, would be felt, and potentially deady.

Which is why there is never more than a #6 required to a ground rod. If you use larger than a #6, you create a choke coil, actually slowing down the process through capacitance. So why does the utility often use larger than a #6 ?

The UTILITY cannot afford low impedance/reluctance to earth on the line side or a near strike (remember NOTHING survives a direct strike) would be completely dissipated in one spot, destroying equipment. They 'spread' out the strike over several miles of line and arrestors and 'ease' the strike out. That is why you design 2/0 connections to ground rods at transformers, #2 to ground rods at poles, etc.. All this depends upon several things, length of circuit, voltage, # of transformers, # of reclosers, # of arrestors, etc.. It is complicated and # of recorded strikes, lineman callouts in storms, storm days, etc, go into your design and consideration. None of this goes into the low voltage design criteria.

Our design does allow, as it should, the occasional "drain" (if you will) of overvoltage, but only when the utiity system has failed or been overwhelmed, which does happen. My fault (or problem) for getting a bit 'ticked' at hearing that the design intent of our system is for lightning dissipation.

See why the ground rod (or whatever electrode you care to use) at a 2nd building fed from the same service is there ? Not to prevent, or dissipate lightning, but to eliminate potential difference between the earth upon which you are standing, and the electrical equipment you are using. The dissipation, removal (drain) of inductive influences is a secondary benefit, not a designed benefit. The earth is simply not a good enough conductor to be relied upon for this task is is is separated from the primary structure by any distance at all.

One example, and I'll quit. Installing a service at a convenience store that had burned down (arson). A utility transformer at the rear of the store was 'leaking'. My galvanometer told me we had 5 volts seeping into the ground from the xfrmr. I told the utlility, who really didn't care (they did a month later when the leaking feeder blew). Refrigeration equipment at the rear of the store was bonded to my service and sitting on fiberglass pads. I measured, on dry ground, a potential difference of the full 5 volts between the ground and the correctly grounded equipment. What could be the consequences of walking away ? How about an A/C service man servicing equipment after an thunderstorm, or snow, or rain ? Will he feel that 5 volts given that he is probably wet, kneeling beside the equipment ? you bet he would. Cure ? Drive a ground rod at the equipment (You can supplement your system anywhere you like, and that is why) and eliminate the potential difference.

There is no requirement to use arrestors on our equipment which would mean we design for lightning dissipation, and there ain't gonna be that requirement, because that portion of grounding has to be considered BEFORE it ever enters the structure, as it should be.

Ok, we've 'bout beat this puppy to death, thanks for listening, and I apologize to all for beginning a very detailed analysis in a forum when I should not have. Mostly because I've still only covered the bare tip of a VERY large iceberg, and one which is difficult enough to understand without half an explanation.
Posted By: Bill Addiss Re: #6 G-rod, #4 Ufer - 01/10/02 08:23 PM
Cindy,

Maybe this is what you mean;

In a situation like a typical 200A residential service chamge if you cannot get a #4 to the metallic waterpipe where it first enters the building, then it has been allowed to use #6 to 2 rods (placed a minimum of 6 feet apart) plus a # 4 to the water pipe at any convenient location.

Bill
Posted By: Cindy Re: #6 G-rod, #4 Ufer - 01/11/02 02:56 PM
what is the point of saying "the sole connection" in 250-50(a)2. & in 250-66 a,b,c?

is that to say if i use a 4awg to a water pipe then i dont have a sole connection to the GEsystem when i use a g-rod as a supp. ge, so then i have to use 4awg to supp. rods also?
Posted By: Elzappr Re: #6 G-rod, #4 Ufer - 01/11/02 04:24 PM
First off, thanks to George for the details. I don't think this forum needs to 'dumb down'. Keep up the effort, George, sometimes things are just complicated.
Now, Cindy, the "sole connection" wording has to do with the opposing scenario in which someone uses the same conductor to bond to more than one electrode, in which case the conductor has to be sized for whichever electrode requires the largest conductor.

[This message has been edited by Elzappr (edited 01-11-2002).]
Posted By: Cindy Re: #6 G-rod, #4 Ufer - 01/12/02 07:22 AM
so Bill said , "In a situation like a typical 200A residential service chamge if you cannot get a #4 to the metallic waterpipe where it first enters the building, then it has been allowed to use #6 to 2 rods (placed a minimum of 6 feet apart) plus a # 4 to the water pipe at any convenient location"

elzappr... "someone uses the same conductor to bond to more than one electrode, in which case the conductor has to be sized for whichever electrode requires the largest conductor."

so using one gec, going to a g-rod first, then to a water pipe would need a #4 if one length of wire? this would be a case where you have to use a 4awg to the g-rod, since it's on its way to the water pipe? is that right?
Posted By: sparky Re: #6 G-rod, #4 Ufer - 01/12/02 10:55 AM
Cindy,
that is the way I read it [Linked Image]
Posted By: Bill Addiss Re: #6 G-rod, #4 Ufer - 01/12/02 01:51 PM
Cindy,

It may be worth noting here that all NEC-permissable scenarios may not be acceptable where you are. For instance, where I am the local utility wants separate #6s to each rod when 2 are used.

Bill
Posted By: Elzappr Re: #6 G-rod, #4 Ufer - 01/12/02 04:03 PM
You got it, Cindy!
Posted By: Anonymous Re: #6 G-rod, #4 Ufer - 01/17/02 07:55 AM
What is the difference between resistance and impedance? Where do we have to be concerned with impedance? What is reluctance?
Posted By: George Corron Re: #6 G-rod, #4 Ufer - 01/17/02 11:39 AM
Impedance is the total opposition to the flow of current in an AC circuit. A DC circuit is only opposed by the resistance of a circuit (wire and and other device in the ckt) an AC circuit is opposed by the resistance of the wire, coupled with the capacitive reactance (two wires, carrying current = capacitance) and the inductive reactance of the ckt (such as the pipe the wires are in, bx skin, bldg. steel, etc.). you need to be concerned with it in all ckts, and design that way. Reluctance is alas an OLD electronics term I probably should not have used, the proper more up to date term is reactance. Sorry, geezerism strikes on occasion [Linked Image]
Posted By: Bill Addiss Re: #6 G-rod, #4 Ufer - 01/17/02 01:54 PM
This (Impedence/Resistance) sounds like a good topic for the Theory area. Anyone care to start sonething there?

Bill
Posted By: dana1028 Re: #6 G-rod, #4 Ufer - 01/17/02 09:07 PM
Along this same thread - Creighton Schwann wrote a good article for EC Mag. "Things Are Not What They Seem"...discusses grounded, bonded, grounding conductors - their function and sizing them properly. This article is available online at ecmag.com in the back issues section of the web sites. It is pretty well written and helps clarify these 3 conductors.
Posted By: Anonymous Re: #6 G-rod, #4 Ufer - 01/18/02 02:18 AM
Thanks for the link, Dana.

Make that:

Things Are Not Always What They Seem

W. Creighton Schwan

and you'll have a little less trouble searching for it than I did. [Linked Image]
Posted By: George Rueff Re: #6 G-rod, #4 Ufer - 01/18/02 07:05 PM
Whats a ufer?
Posted By: dana1028 Re: #6 G-rod, #4 Ufer - 01/18/02 09:26 PM
George I can't believe a master electrician doesn't know what a ufer is!!

It's rebar used as a grounding electrode - 20' long, run along the lower part of your foundation footing (at least 2" above ground level)....don't have my notes with me but I believe the guy who actually tested and started this system was named Ufer -thus the name ufer for this system.
Posted By: dana1028 Re: #6 G-rod, #4 Ufer - 01/18/02 09:39 PM
nothing like the internet! "during WWII, a retired vice president of UL, Herbert G. Ufer developed it for the US Army.....a cheap earthing system that consistently outperforms typical ground rod installation....developed it for grounding bomb storage sheds.....rebar is in the concrete foundation which is more conductive than all but the best soil...tests confirmed stability...many of his findings are detailed in IEEE Transactions paper #63-1505....so ....the way to install a Ufer is use a piece of 20' rebar, bend it up on the end (the above ground connection), tie wrap the encased portion of the bar to the other rebar grid-work and you have a Ufer that is far superior to most other grounding systems.
Posted By: sparky Re: #6 G-rod, #4 Ufer - 01/18/02 11:22 PM
Hat's off to ol' Herb, a Ufer beats a municipal H20 line amyday!
http://www.scott-inc.com/html/ufer.htm

[Linked Image]
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