Grounding Electrode Conductor size.

[a7]

Can anyone give me an example when 250.66(A)" Connection to Rod, Pipe, or Plate Electrodes" does apply? How is this meant to be?
The grounding electrode conductor size when used for underground water pipe should be sized according to 250.66, it's as clear as vodka.
Also, is 250.53(E) same as 250.66(A)?
Thanks.

 

[don_resqcapt19]

Re: Grounding Electrode Conductor size.

That section is not talking about a water pipe, it is talking about a 3/4" or larger pipe driven in the ground like a ground rod for use as an electrode. Look at 250.52(A)(5)(a).
Don

 

[iwire]

Re: Grounding Electrode Conductor size.

Originally posted by a7:
Can anyone give me an example when 250.66(A)" Connection to Rod, Pipe, or Plate Electrodes" does apply? How is this meant to be?
The grounding electrode conductor size when used for underground water pipe should be sized according to 250.66, it's as clear as vodka.

Piece of cake.

Table 250.66 provides the minimum size GEC to be used with a metal underground water pipe.

Now in 250.66(A) Connections to Rod, Pipe, or Plate Electrodes the 'pipe' in that section is a pipe used as a ground rod.

250.66(A) has nothing to do with a metal underground water pipe

The 'pipe' in 250.66(A) fits this description.

250.52(5) Rod and Pipe Electrodes. Rod and pipe electrodes shall not be less than 2.5 m (8 ft) in length and shall consist of the following materials.

(a)Electrodes of pipe or conduit shall not be smaller than metric designator 21 (trade size 3/4) and, where of iron or steel, shall have the outer surface galvanized or otherwise metal-coated for corrosion protection.

(b)Electrodes of rods of iron or steel shall be at least 15.87 mm (5/8 in.) in diameter. Stainless steel rods less than 16 mm (5/8 in.) in diameter, nonferrous rods, or their equivalent shall be listed and shall not be less than 13 mm (1/2 in.) in diameter.

So if you drive a 3/4" conduit into the ground to be used a grounding electrode you can forget the size in Table 250.66 and use a 6 AWG CU or 4 AWG AL GEC as allowed by 250.66(A).

 

[a7]

Re: Grounding Electrode Conductor size.

Example: If I have 1200A service, the grounding electrode conductor size is required to be 3/0 copper connecting to water main, correct?
What about I don't have water main pipe in the structure or building and I decide to use that mentioned 250.66(A) as main grounding electrode, for example rod or combination of rod and plate electrode. Do you want to say thatin this case the requirement of the size of grouding electrode conductor goes from 3/0 to just 6AWG?

 

[iwire]

Re: Grounding Electrode Conductor size.

Originally posted by a7:
Do you want to say thatin this case the requirement of the size of grouding electrode conductor goes from 3/0 to just 6AWG?

That is correct, even if the ground rod is the only electrode you are only required to run 6 AWG to the rod no matter what size the service is.

 

[a7]

Re: Grounding Electrode Conductor size.

Thanks. Yeah, I wasn't sure, that's why I asked. It seems to be almost unbelievable. How is that possible that there is such a difference in grounding electrode conductor size when connecting to different electrodes? in this lets say 1200A service example? Am I still missing something? In case of ground fault with 1200A service, I can imagine that 6AWG copper can perhaps evaporate.

 

[iwire]

Re: Grounding Electrode Conductor size.

Originally posted by a7:
In case of ground fault with 1200A service, I can imagine that 6AWG copper can perhaps evaporate.

It might if it was connected to a metal underground water pipe that was bonded multiple times.

But that is not going to happen with a ground rod.

A ground rod driven into the earth can not pass that much current.

If the rod had 25 ohms of resistance a 277 volt ground fault would only result in 11 amps of current flow on the ground rod.

 

[a7]

Re: Grounding Electrode Conductor size.

Yes, but I think that applies to supplemental rods or pipes. What about that this should be main grounding system and should be able to provide efficient ground path with low impedance. If there is a severe ground fault and I use #6AWG copper, the initial burst current passing through the wire can be way above 1200A, lets say 2000A. That's enoug current to make the trouble. Better yet, evaporate that conductor.

 

[petersonra]

Re: Grounding Electrode Conductor size.

keep in mind the gec should not be ever seeing any fault current. fault current is seen on the egc, if there is a fault.

the gec only conencts your electrical system to earth.

 

[iwire]

Re: Grounding Electrode Conductor size.

Originally posted by a7:
If there is a severe ground fault and I use #6AWG copper, the initial burst current passing through the wire can be way above 1200A, lets say 2000A. That's enoug current to make the trouble. Better yet, evaporate that conductor.

No that can not happen with a any rod that is simply drive into the dirt.

The earth is simply not a good conductor.

Remember that the fault current to the rod is not trying to get to the earth, it is trying to get back to the source.

In order to do that the fault current must travel down the rod into the earth and find a path through the earth back to a utility company grounding electrode then back to the XO of the transformer.

That will never be a low impedance path.

By the way, strictly speaking the grounding electrode system has nothing to do with ground faults that occur on the load side of the service disconnect.

The fault path for ground faults on the load side of the service disconnect is through the Main bonding jumper and back the the source through the utility's neutral conductor.

 

[ryan_618]

Re: Grounding Electrode Conductor size.

Why would you have a 1200 amp ground fault to an electrode? There is no way your electrode has a low enough impedance for that to ever happen.

If your service is 277/480, you would have to have a bolted 277 volt fault to an electrode with an impedance of 0.23 Ohms in order to get a 1200 amp ground fault! For 2,000 amps to occur, as in your example, your electrode would have to have an impedance of 0.13 Ohms!

Also, I don't think the short time withstand rating of the #6 would be reached either. When you consider that a 6 AWG equipment ground is good for 200 circuit, that means that the anticipated fault current should be above 1,000 amps in order to reach the instananeous trip level of the 200 amp device.

[ June 15, 2005, 05:28 PM: Message edited by: ryan_618 ]

 

[a7]

Re: Grounding Electrode Conductor size.

Yes, I know. I was just thinking extreme "what would happen if"

 

[hurk27]

Re: Grounding Electrode Conductor size.

a7
The "what ifs" can not ever happen this is what the others is trying to tell ya. The simple reason the NEC does not require a sole GEC to a driven rod or pipe electrode, is simply the impedance between the rod and Earth will never be low enough to ever allow the current to exceed the capacity of the #6. This is simple ohm's law.

Think about it, if the resistance of the rod to Earth is 25 ohm's and the service is a 240/120 volts then the highest voltage to Earth is 120 volts which would only apply a current of 4.8 amps to the rod. This would not open a 5 amp fuse. A lightning strike would exceed this by many amps but the current imposed on the #6 would not be of a long enough duration that would likely lead to the conductor being damaged. Even if there was a primary to secondary fault at the transformer the current available from it is much lower then the secondary and would most likely open the primary fuse way before damaging the GEC.

[ June 16, 2005, 01:46 AM: Message edited by: hurk27 ]

 

[LarryFine]

Re: Grounding Electrode Conductor size.

Think of the grounding electrode as a means to bond the earth to your system's neutral, not the other way around. The idea is to minimize the difference in potential between any metallic enclosures and the earth, aka voltage gradients.