#6 Cu maximum required grounding electrode conductor?

[bburns]

i recently read an engineer's response to a third party report about an installation that he had designed. in the report it was noted that the #2 Cu grounding electrode conductor installed on a service that was fed with 3 parallel 500 kcm per phase was to small according to table 250.66 in the NEC. the engineer responded by saying that according to 250.66(a) the grounding electrode conductor doesn't have to be larger than #6Cu if it connected to a ground rod.

after looking at the referenced sections in the code, i have concluded that when 250.66(A) refers to "that portion of the conductor that is the sole connection to the grounding electrode...", it means that the part of the conductor that is in contact with the rod does not have to be larger than #6. the GEC itself must still comply with table 250.66.

how do you guys interpret this?

 

[charlie b]

I agree that the words "that portion" tend to throw the text into a state of confusion. But I know that a single, continuous wire can be used as a GEC, and it can run from the service panel to the ground rod. In that case, it is clear to me that it need by no larger than #6 copper, regardless of the size of the service.

 

[A/A Fuel GTX]

I would say that if the ground rods are part of the grounding electrode system in series with other electrodes, it would have to be sized according to 250.66. If the ground rods were the only electrodes, then I would say #6 would be fine.

 

[infinity]

I agree, the maximum required is #6 CU to a ground rod.

 

[jtester]

Everyone agrees that the largest conductor required to a ground rod is #6, but what happens if the only electrodes are ground rods, and you have say a 500 KCMIL service, requiring a #1/0 cu?

These posts seem to suggest that 250.66(A), which allows a #6 to rods, superceeds Table 250.66, which requires a minimum of 1/0 as a GEC.

How can we explain #6 is adequate when #1/0 is required by the Table?

Jim T

 

[peter d]

Everyone agrees that the largest conductor required to a ground rod is #6, but what happens if the only electrodes are ground rods, and you have say a 500 KCMIL service, requiring a #1/0 cu?

The #6 is still as large as it has to be for a ground rod. It is acceptable for a 100 amp service, or a 6000 amp service.

You have to remember what the #6 is connected to. A piece of metal stuck in the dirt. The contact impedance between the soil and the rod will be very high, and unable to carry much curent. That is why the #6 is more than adequate.

 

[ryan_618]

How can we explain #6 is adequate when #1/0 is required by the Table?
Jim T

In my opinion, the 6 AWG is adaquete because it will never carry fault current. Water pipes and building steel, on the other hand, might have to carry a fault if there is a fualt on the water line or building steel.

Again, this is just my opinion.

 

[jtester]

I don't disagree that we can't run only #6, or that it won't be safe. My concern is that we don't comply with Table 250.66. It almost seems that the NEC left out some words like, "...shall not be required to be larger than #6 if an electrode requiring a GEC complying with Table 250.66 is installed."

I guess my problem is that, in my mind, I can't find something that exempts me from Table 250.66, even though the wording of 250.66(A) only requires that portion of the GEC to be #6.

It seems that one could argue that 250.66 with the Table included requires you to install an electrode that accepts a full sized GEC.

Jim T

 

[LarryFine]

Everyone agrees that the largest conductor required to a ground rod is #6, but what happens if the only electrodes are ground rods, and you have say a 500 KCMIL service, requiring a #1/0 cu?

We're doing a restaurant that has a 200a 1-ph and a 125a 3-ph switch on an open-Delta system, each feeding a panel, and all that's required is #6.

There is no metal water piping, as the plumbing was completely redone in PEX, and the gas line is considered bonded by the EGC in the supplying circuit.

The building is 70- to 80-yrs-old, all brick, wood floors. There is no structural metal at all; the driven rods are the only electrodes. All that's required is a #6.

 

[iwire]

I guess my problem is that, in my mind, I can't find something that exempts me from Table 250.66,

250.66 Size of Alternating-Current Grounding Electrode Conductor.

The size of the grounding electrode conductor of a grounded or ungrounded ac system shall not be less than given in Table 250.66, except as permitted in 250.66(A) through (C).

(A) Connections to Rod, Pipe, or Plate Electrodes. Where the grounding electrode conductor is connected to rod, pipe, or plate electrodes as permitted in 250.52(A)(5) or 250.52(A)(6), that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire.

(B) Connections to Concrete-Encased Electrodes. Where the grounding electrode conductor is connected to a concrete-encased electrode as permitted in 250.52(A)(3), that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than 4 AWG copper wire.

(C) Connections to Ground Rings. Where the grounding electrode conductor is connected to a ground ring as permitted in 250.52(A)(4), that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than the conductor used for the ground ring.

 

[LarryFine]

I guess it would make more sense if " . . . that portion of the conductor that is the sole connection to the grounding electrode . . . " was rephrased as " . . . that portion of the conductor that is the conductor to only the grounding electrode . . . "

 

[sandsnow]

Ok somebody explain this please. Without the Code. I understand what it requires. I just want to know why>>>>>

If a single ground rod (25ohms) is acceptable as an electrode and

#6 is the largest GEC needed, then

Why wouldn't #6 be the largest you ever need??

 

[iwire]

I will give you my guess.

In the case of a metal underground water pipe it is entirely possible that a service conductor fault to ground could subject the GEC running to the water pipe (and the water pipe itself) to the full available fault current.

This is because it is very likely that the metal water line is bonded at other peoples services.

So we could have a direct metallic path back to the source where a ground rods fault path is only through the earth.

 

[Pierre C Belarge]

The code does say 25 ohms to ground for ground rods, but that is rarely acheived. In our area the ground rods are usually over 100 ohms, sometime over 200 ohms. The current that will be conducted is very low.


Try to visualize the water bottle that sits on the office water dispensor... a narrow neck with a large container full of water.
One turns the bottle over so water flows out of the bottle. There are gallons of water, but the neck of the bottle is so narrow only a little water flows out at one time/

This is the same principle as the large conductor installed to a ground rod. Only so much current can flow so we will be wasting copper if we install a larger conductor.

 

[j.orlett]

Technical reports indicate that a no.6 copper will handle as much current as ground rod can dissipate electrons into the earth for 5 seconds. A larger conductor will choke a larger current flow. the testing assumes a breaker will trip within 5 seconds if the system is functioning properly.

 

[ryan_618]

Technical reports indicate that a no.6 copper will handle as much current as ground rod can dissipate electrons into the earth for 5 seconds. A larger conductor will choke a larger current flow. the testing assumes a breaker will trip within 5 seconds if the system is functioning properly.

Why would there be 50 amps of current on the grounding system?

How would the ground rod trip the breaker unless it was less than about 1 or 2 ohms?

 

[charlie b]

I think the point is that if lightning strikes a building, all the metal stuff that is bonded will be energized, and all the related EGCs will be carrying current back to the main panel. At that point, the current will not be driven to cross over from the G bus to the N bus. Rather, the current will travel via the GEC to the ground rod, and will dissipate into planet Earth. So if there is a limit to the amount of current that can dissipate via a ground rod, then there is no reason to use a GEC that can carry more current.

I do not know if this really is the answer. But it makes sense to me. (There's a danger signal for you! :wink: :lol: )

 

[LarryFine]

Why would there be 50 amps of current on the grounding system?

How would the ground rod trip the breaker unless it was less than about 1 or 2 ohms?

Good point. It may be that #6 is specified as much for resistance to damage as for ampacity.

 

[sandsnow]

I think the point is that if lightning strikes a building, all the metal stuff that is bonded will be energized, and all the related EGCs will be carrying current back to the main panel. At that point, the current will not be driven to cross over from the G bus to the N bus. Rather, the current will travel via the GEC to the ground rod, and will dissipate into planet Earth. So if there is a limit to the amount of current that can dissipate via a ground rod, then there is no reason to use a GEC that can carry more current.

I do not know if this really is the answer. But it makes sense to me. (There's a danger signal for you! :wink: :lol: )

Which leads back to my original question. If one ground rod is good enough to dissapate that current, why would the Code minimum GEC for any electrode have to be any larger than #6??

Since all new buildings have CEC's then we can use the same question for the minmum #4 there.

So maybe I'm asking the un-answerable question!
The reason I ask is that I want to be able to explain this in a class I'm going to teach. I always like to be prepared for any question that I can think up.

There's always the answer "Because we've always done it that way!!" OR "People have forgot why that was ever put in the Code!!!"

I'd rather not use those.

 

[infinity]

Since all new buildings have CEC's then we can use the same question for the minmum #4 there.

A maximum of #4 is all that is required to a CEE. However, it can be smaller. A 100 amp service for example would require only a #8 to the CEE.