5/8" Electrode Wire Size?

[dduffee260]

I see on alot of plans where they show to install up to a 3/0 copper wire to a 5/8"x 8ft copper ground rod. I believe that a ground rod or electrode such as this is only good for a #6. Some people say that it is all the rod will dissapate.
Is this correct?
I know there may be other stipulations such as 3/4"x 10ft and such but the most common around here is a 5/8"x 8ft. There are even some inspectors that want a larger wire such as a #4 for a 200 amp service going to a 5/8"x 8ft ground rod.
What do some of you other people know on this subject?

 

[jwelectric]

250.66 Size of Alternating-Current Grounding Electrode Conductor.
(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 (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.

 

[bphgravity]

It's a huge waste of materials and effort to install anything larger than #6 to a ground rod regardless of its length, size, or the size of the service you are grounding. I see this on prints a lot as well?

 

[tom baker]

I use 2 AWG BCS to my 8ft ground rods, but only for mechnical strength and this is for a radio mast grounding, for industrial applications. Also all the connections are CAD welded.

How would you even make a 3/0 AWG connection to a 5/8" ground rod?

 

[RUWired]

Tom, Ilsco makes them up to 500 mcm on a 5/8 rod, but are only UL approved for 250.Cat # GPL-7 .
Rick

 

[haskindm]

This is a good example of people in positions of authority that don't understand the purpose or limitations of a ground rod. If a ground rod has a resistance of 25-ohms and a 120-volt conductors is connected to it, how many amps wil be carried by the conductor? Answer 4.8. If the conductor is at 277-volts the amperage increases to 11.8. Why would a conductor larger than #6 be required? The fact that the NEC only requires a #6 conductor indicates that the code-panel understands that a grounding electrode will not be required to carry fault-current. A worst (or best) case scenario would be a ground rod with a resistance of 5-ohms at 277 volts. Total amperage 55 amps, which is still within the ampacity of a bare #6 conductor.
It would be nice if the ground rod would carry the full fault current of a 2000 amp service and trip the overcurrent device, but Ohm's law says it ain't gonna happen.

 

[bphgravity]

Ground rods aren't supposed to see fault current so your example doesn't really express the issue.

Ground rods are going to see surge current from high voltage cross over or more likely lightning. This can be several thousands of amperes. A ground rod has a very small surface area which limits its ability to inject surge currents into the surrounding earth. At best the withstand rating of #6 is all that is needed as this is approxiamtely the same withstand rating of a typical driven electrode.

 

[haskindm]

Bryan,
I guess I didn't make the reason for my example clear. Many people specify larger grounding electrode conductors for larger services. I agree that the reason for ground rods, etc. is for lightning and other high-voltage events, but a 2000 amp service is no more likely to be hit by lightning than a 100-amp service. The justification that is used for the larger conductor is the higher fault current needed to trip the overcurrent device. Since a grounding electrode will not and cannot carry fault current this argument is baseless.

 

[jtester]

quote from haskindm
"Since a grounding electrode will not and cannot carry fault current this argument is baseless."

Grounding electrodes see fault currents all the time, and good electrode systems will operate smaller breakers, although not anything near a 2000 amp device. Ground rods don't act as good fault clearing devices, but they see line to ground faults on a regular basis.

Concerning larger wire than #6 though, what happens when you have a 3/0 service with only 2 ground rods, say for a remote sign, etc.? How do you comply with 250.66 without running a #4 to the ground rod? Wouldn't #6 GEC's only violate 250.66?

Jim T

 

[Jljohnson]

quote from haskindm
"Since a grounding electrode will not and cannot carry fault current this argument is baseless."

Grounding electrodes see fault currents all the time, and good electrode systems will operate smaller breakers, although not anything near a 2000 amp device. Ground rods don't act as good fault clearing devices, but they see line to ground faults on a regular basis.

Concerning larger wire than #6 though, what happens when you have a 3/0 service with only 2 ground rods, say for a remote sign, etc.? How do you comply with 250.66 without running a #4 to the ground rod? Wouldn't #6 GEC's only violate 250.66?

Jim T

250.66.(A) says the conductor to a rod is not required to be larger than # 6 CU or # 4 Al. so , the way I read this, how we be in violation of 250.66 when that IS the section that tells us # 6 CU is the largest size required? As a sidenote, if you are working from an engineered drawing that has been through a municipality's plan review process and that plan shows a larger conductor, the inspector has every right to require the conductor to be installed as the drawings indicate. I agree that it is a waste of money but I have had this happen to me.

 

[jwelectric]

Grounding electrodes see fault currents all the time, and good electrode systems will operate smaller breakers, although not anything near a 2000 amp device. Ground rods don't act as good fault clearing devices, but they see line to ground faults on a regular basis.

Concerning larger wire than #6 though, what happens when you have a 3/0 service with only 2 ground rods, say for a remote sign, etc.? How do you comply with 250.66 without running a #4 to the ground rod? Wouldn't #6 GEC's only violate 250.66?
Jim T

I may be missing something here but the way I understand the code the Grounding Electrode System has nothing to do with opening an overcurrent device.

Why connect to earth in the first place?

2005 NEC

250.4(A)(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

If this is true what does open the overcurrent?

2005 NEC

250.4(A)(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

Notice the very last sentence of this subsection, ?The earth shall not be considered as an effective ground-fault current path?

 

[bphgravity]

Grounding electrodes see fault currents all the time, and good electrode systems will operate smaller breakers,

Jim T

If grounding systems are designed and used for the purpose of clearing premise faults, there is something horribly wrong with the designer and installation. Obviously, fault currents will use earth as a return path to the source but no in case can one assume or rely on the GES to clear faults.

A "good electrode system" is not based on its surge impedance, earth resistance or fault carrying capability.

 

[jtester]

Perhaps I mispoke. I am not suggesting that GEC's are designed today to operate OCPD's, but they still see fault currents because they create a parallel path with the utility neutral.

Because they are in parallel with a portion of the utility system, they see part of every line-ground fault that goes through the service disconnect. If you draw out the circuits you will find that they even see a portion of the ground fault current flowing from a neighbor's house when the fault is next door.

When I said a good electrode system, I was implying a system tied to substantial building steel, to metal water pipe, and to foundation steel, would have a sufficiently low impedance to trip a 50 amp or so breaker, not by design, but by the inherently low impedance of a "good GEC".

I completely agree that current GEC's are not generally useful for operating breakers.

Jim T

 

[busman]

A #6 is all that's required for a ground rod. Many will use a #4 anyway to get around the need to protect it from physical damage.

Mark

 

[haskindm]

Jim T,
The Grounding Electrodes that you mentioned (cooper waterline, building steel) may indeed see fault current and will play a role in operating an overcurrent device. In the event of a hot wire faulted to a copper water pipe or building steel the pipe or steel forms part of the low resistance path back to the source and will open the overcurrent device. The ground rod is NOT part of this low resistance path. It does form a path, but it is a high resistance path. It would be all but impossible for the earth to carry enough current at 120 or 277 volts to trip a 50-amp breaker. To reliably trip a 50-amp breaker at 120 volts the breaker would need to "see" at least 100-amps. This would require a resistance of 1.2-ohms. You are not likely to see a resistance this low no matter how many rods are installed.