Still Not Getting 250.66 (A), (B) & (C)

[cadpoint]

Here try this the SEG is based on Circuit. The Ground for the EGC is based on the OCP.

 

[ActionDave]

What about 250.6?

250.58 is a bigger number.

 

[George Stolz]

250.58 is a bigger number.

Do you think 250.6 would override it?

 

[jwelectric]

Prey, why do you assume that the metal water pipe, structural steel, and listed electrodes would pass a greater current into the earth than the electrodes mentioned in 250.66(A)-(C)?

That I don't have the answer to but I do know what the purpose is;

250.4 General Requirements for Grounding and Bonding. The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.

Section 250.4 provides the performance requirements for grounding and bonding of electrical systems and equipment. Performance-based requirements provide an overall objective without stating the specifics for accomplishing that objective. The first paragraph of 250.4 indicates that the performance objectives stated in 250.4(A) for grounded systems and in 250.4(B) for ungrounded systems are accomplished by complying with the prescriptive requirements found in the rest of Article 250.

The requirements of 250.4 do not provide a specific rule for the sizing or connection of grounding conductors; rather, it states overall performance considerations for grounding conductors and applies to both grounded and ungrounded systems.


(A) Grounded Systems.

(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

 

[George Stolz]

Thanks, I have a code book too.

The question remains. I don't see the logic in two 250.66's.

 

[jumper]

George, ......Pray tell, not prey.

 

[George Stolz]

Curses, google! :lol:

 

[Strathead]

The water pipe bond is not the only illogical part of article 250. If I choose to put ground bushings on my conduits, I have to run a 250-122 sized ground conductor through it to satisfy code, but I don't even need to install a ground conductor at all, otherwise. I think that the entirety of 250 should be reengineered to provide safe and economical ground fault clearing, as it should. Including the requirement to run a ground in all raceways, under the assumption that time will deteriorate the current clearing capacity of a significant percentage of properly installed raceways, but eliminating ridiculous requirements like the 250-66 required bond to a water pipe and the above mentioned conduit bonds. A Steel locknut is enough connection to ground, but a #6 copper conductor isn't?

 

[iwire]

Prey, why do you assume that the metal water pipe, structural steel, and listed electrodes would pass a greater current into the earth than the electrodes mentioned in 250.66(A)-(C)?

My WAG as to the waterline is because it is possible that the metal water line could be common to many services GEC connections in the area a fault to the water line GEC at one service would be like a bolted fault, it would not be using the earth as a conductor.


Building steel? ........ I got nothing.

 

[iwire]

The water pipe bond is not the only illogical part of article 250. If I choose to put ground bushings on my conduits, I have to run a 250-122 sized ground conductor through it to satisfy code, but I don't even need to install a ground conductor at all, otherwise.

I am not following you here.

 

[jwelectric]

Thanks, I have a code book too.

The question remains. I don't see the logic in two 250.66's.

It has more to do with the type of electrode than the conductor installed to it. While I was at the Sothern Sectional in Chattanooga Tennessee I got to watch a short video clip from UL were a #6 was installed to a rod and current was applied to the point that the rod melted but the 6 didn?t.

The minds that size the grounding electrode conductors have the knowledge to figure the minimum size conductor that will carry enough current to destroy the electrode without destroying the conductor.

Anyone who has ever tried to repair a copper water line knows that a dry fitting can be heated till it glows red but if there is just a little water inside then it won?t get hot enough to melt the solder.

Is this what you are looking for?

 

[George Stolz]

Something like that, yes! Thank you!

 

[jwelectric]

Something like that, yes! Thank you!

George, I remember a few years back reading something about ground rods but I can?t remember where it was published. It could have been in an IAEI publication I just can?t remember.

I don?t remember the numbers but it was about ground rods and pipes. I do remember that the amount of current allowed to flow was a lot more when testing an 8 foot rod and a 10 foot rod.
I was truly surprised to see how little current that the 8 foot rod allowed.

I will try and remember where this article is at and if I do I will PM you with the information.

 

[jumper]

I found this article on grounding tests. I did not read it all, so I do not know if addresses your question or not.

Might help, might not.

http://www.nfpa.org/assets/files/PDF/Research/NEGRPFinalReport.pdf

 

[jwelectric]

George, I remember a few years back reading something about ground rods but I can?t remember where it was published.

After making a few phone calls the article was a ANSI/IEEE paper. To access this paper on the internet will cost $$$.

I was wrong about the 8 and 10 feet current flow, it was 8 and 25 feet that had the huge difference in resistance.

 

[don_resqcapt19]

There is a table on page 5 of this document that showes the change in resistance as you increase the length of the ground rod for various types of soils. I am sure it is not a detailed as the one Mike was talking about, but it gives a gereral idea.

 

[jwelectric]

Don, this looks like an insert from that paper

 

[K8MHZ]

I asked this question often when I first started as an apprentice and never got a completely satisfying answer.

Geez....

Almost our entire second year revolved around it.

1.73 applies to three phase because it is the square root of 3.

Since 1 is the square root of 1, we don't need to consider it. The square root part of single phase is still part of the equation, but since it's a wash it's usually ignored.

And thus, for a true two phase system, the multiplier would be 1.414 (square root of 2).

 

[yankj]

I'm in the process of building a class for grounding and bonding. I've reached 250.66 and am having a hard time making sense of why we're allowed to use a reduced GEC for these items.

I realize that a water pipe is commonly interconnected with other services, but I fail to see how that really matters in this section. When you get down to brass tacks, you can use any electrode to interconnect services and unbalanced neutral current would be just as inclined to travel down service A's #6 to a ground rod and return up service B's #6 and overload it.

Thoughts?

Table 250.66 is based on wire size for a service. Table 250.122 is based upon over current protection. We need 260.66; who wants to run a #4 grounding conductor for a 15 amp circuit?

I agree students need to understand why they are doing something and not just "because the instructor said so". When people understand why they are to do something, they can find alternate methods or better methods of doing something. For instance, knowing why we use 1.732 is important.

Keep up the good work! Someday your students will thank you. At least the good ones who care about what they're doing will.

 

[haskindm]

I'm in the process of building a class for grounding and bonding. I've reached 250.66 and am having a hard time making sense of why we're allowed to use a reduced GEC for these items.

I realize that a water pipe is commonly interconnected with other services, but I fail to see how that really matters in this section. When you get down to brass tacks, you can use any electrode to interconnect services and unbalanced neutral current would be just as inclined to travel down service A's #6 to a ground rod and return up service B's #6 and overload it.

Thoughts?

My understanding is that (A) Ground Rods, (B) Concrete encased electrodes, and (C) Ground Rings cannot and will not carry fault current. If you have a ground rod with 25 ohms resistance and energize it with 120-volts, you will read a 4.8-amp load. Is a #6 copper wire sufficient for that load? Of course! Other grounding electrodes such as building steel, water lines etc., may become energized by a service or branch circuit conductor. Since they are also connected to the "ground/neutral" bar in the panel and thus to the neutral conductor back to the "source" (transformer, generator, etc) they are part of the "effective ground-fault current path" (250.2) and so must be sized to carry fault current. The earth is not part of the "effective ground-fault current path" but may be part of the "ground-fault current path" (there is a difference - see 250.2) so there is no need to size the conductors to electrodes that are only connected to the earth to carry fault current. I believe the only reason these conductors are sized the way they are (IE a #6 CU to a ground rod) is so that they are large enough to be physically strong enough for the installation. They will usually not be called upon to carry current anywhere near their ampacity. For example a 277-volt fault to a 25-ohm ground rod would still only develop 11.08 amps.