ECN Forum Forums General Topics of the Trade General Discussion Area #6 G-rod, #4 Ufer

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.

Guys,
i've read your posts 3 times over, and i'm still .
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?

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.

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

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?

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).]

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?

Cindy,
that is the way I read it

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

You got it, Cindy!