Art. 250-66

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Ben Trueblood

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I need some clarification on sizing my water pipe electrode when my Utility brings the service directly into my main disconnect switch and I don't have any service entrance conductors to size to. I have a 1200 Amp main disconnect that is modular and bolts together directly to my tenent metering. I read note 1 below the table and the way I read it you have to size your electrode according to the sum of the parallel conductors in the set. Is note 2 saying that you have to size the electrode according to the conductors you would need to carry the load "if" you did have them? Is my scenario listed above why note 2 was created?

Thanks
 
In 310.16 I find the parallel copper conductors for 1200 amps would be 1500 kcm each. Forget de-rating as you don't have the conductors anyway. This 3000 kcm would be over the 1100 listed in 250.66. So the GEC would be 3/0 copper or 250 kcm aluminum. Per note 2. Lets see what the others say.
 
Note 2 is correct.......3/0 copper GEC

I may have gotten there different, I would use 4 sets of parallel 350s to get my 1200 amp service conductors and 350x4=1400. 250.66 ends at 3/0 cu @ 1100 kcmil or over.
 
Yes you need to use note 2 of Table 250-66 but I would install #6 cu for the grounding electrode conductor

In premises wiring we have to feed the capacity of a circuit, in utility they serve the calculated load not the service disconnect capacity. So secondary service is usually served smaller than what we would feed with but it can be enlarged later without regard to the GEC by the utility, so note 2 should be used.

But 250-66(A) is also safe and applicable, "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." In other words regardless the size of your service disconnect the GEC never has to be larger than 6 cu
 
I also agree with the answer of 3/0

Ben because a member is not a senior member does not necessarily mean he doesn't know as much as a senior member. The status means that they have less post ot less time at the forum and that's all. There are many competent people that are not senior members.

It is easy to think because the member is new that they are inexperienced-- not always true.
 
tryinghard said:
Yes you need to use note 2 of Table 250-66 but I would install #6 cu for the grounding electrode conductor
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Then you would be in violation unless you were using rod, pipe or plate electrodes, #6 is not acceptable for all electrodes.

In other words regardless the size of your service disconnect the GEC never has to be larger than 6 cu
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Not true in all cases!
 
barbeer said:
Then you would be in violation unless you were using rod, pipe or plate electrodes, #6 is not acceptable for all electrodes.



Not true in all cases!
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Glad that you pointed that out.
You beat me to it.:)

steve
 
tryinghard said:
Yes you need to use note 2 of Table 250-66 but I would install #6 cu for the grounding electrode conductor

But 250-66(A) is also safe and applicable, "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." In other words regardless the size of your service disconnect the GEC never has to be larger than 6 cu
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You are reading that out of context. 250.66A is relevant for rods, plate or pipe electrodes as referenced by 250.52 (A)(5) or (A)(6). Ground rods, etc is hat this is about not water pipe or steel etc.
 
barbeer said:
Then you would be in violation unless you were using rod, pipe or plate electrodes, #6 is not acceptable for all electrodes. Not true in all cases!
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Your right! Around here in California it's a rarity to have metal underground water pipe as the electrode rather it's just bonded.

If a water pipe is used as the electrode it requires a supplemental electrode (250-53(D)(2)) but the other electrodes do not require a supplemental. I believe the NEC recognizes non-qualified individuals can destroy the water pipe electrode unknowingly.

The purpose of the electrode is to provide a route to earth, 250-4(A)(1) “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”.

I personally don't want this stuff trying to route through my water pipe! Get you and your family out of the shower during a lightning storm!
 
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tryinghard said:
Your right! Around here in California it's a rarity to have metal underground water pipe as the electrode rather it's just bonded.

If a water pipe is used as the electrode it requires a supplemental electrode (250-53(D)(2)) but the other electrodes do not require a supplemental. I believe the NEC recognizes non-qualified individuals can destroy the water pipe electrode unknowingly.

The purpose of the electrode is to provide a route to earth, 250-4(A)(1) ?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?.

I personally don't want this stuff trying to route through my water pipe! Get you and your family out of the shower during a lightning storm!
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Maybe I'm misunderstanding what you are saying here, but if you have a metal underground water pipe in contact with the earth for 10 or more feet, it qualifies as a grounding electrode and 250.50 requires you to use it.

Also, if you do not have an underground water pipe that qualifies as a grounding electrode, but you do have a metal interior piping system that you are "just bonding", 250.104(A)(1) sends you to Table 250.66, so the size of the conductor will be the same whether you have a grounding electrode or are just bonding the metal water piping system.
 
Thanks for the response. Thanks for the clarification on member's status. I only questioned the 3000 KCMil that was referenced in the first response. I am using Aluminum for my GEC so I figured 4/0 because of the sum of the KCMil calc for Al.
 
Ben Trueblood said:
Thanks for the response. Thanks for the clarification on member's status. I only questioned the 3000 KCMil that was referenced in the first response. I am using Aluminum for my GEC so I figured 4/0 because of the sum of the KCMil calc for Al.
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The kcm for Aluminum would be even larger than the 3000 for copper. To arrive at the 3000 kcm I used 310.16 at 75C. It looked like 2 1500kcm would be it. I suppose it could be correct to use the 90C column too. That would make it 2 1000kcm. So that would still be over 1100 in 250.66. That would then be 250 kcm in aluminum for the GEC.

I sure don't want to sound like a know it all, because that would be a long way from the truth for me, but I started my apprenticeship in 1968. I hope that's senior enough for you. ;)

I'm always trying to learn too. And this is a good place to do that.
 
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eprice said:
Maybe I'm misunderstanding what you are saying here, but if you have a metal underground water pipe in contact with the earth for 10 or more feet, it qualifies as a grounding electrode and 250.50 requires you to use it...
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Correct but the criteria of 250-52(A)(1) is most often not met here in California, so the water pipe is most often not used as the electrode that I have seen. In my opinion I think a water pipe is a poor choice to route lightning, line surges, or higher-voltage lines through to earth. I think the ufer is best but not always available so rods are fine and any more routing should be through a designed lightning rod system due to its purpose.

Think about the quantity of installations that receive grounded service systems without water lines; not all structures served have a water line available. I know the NEC has placed the preference with the water pipe as an electrode and I feel it's a bad choice in light of why it's there or needed.
 
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