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

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George Stolz

George Stolz

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Windsor, CO NEC: 2017
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Hospital Master Electrician
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?
 
Is your primary focus code and exam prep? The why is can be great for a better understanding of theory and electrical design, but for many practical purposes it is only necessary to know the answer, ie:

Why do we use 1.732 for three phase calculations?
Because the teacher said so.
 
The IEEC is giving you the reduction!

Someone else mention today that 50-60 amp is usally the average on 100 Amp service!

Engineered applications all have a safety factor, I won't give it much grease.
 
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jumper said:
Is your primary focus code and exam prep? The why is can be great for a better understanding of theory and electrical design, but for many practical purposes it is only necessary to know the answer, ie:

Why do we use 1.732 for three phase calculations?
Because the teacher said so.
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I think it would be an added bonus to know and explain why. It won't make it or break it. :)
 
George Stolz said:
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?
Click to expand...
I asked this question often when I first started as an apprentice and never got a completely satisfying answer.
 
Jumper, Thanks. :cool:

So, while we're waiting for the heavy hitters to swing, you think there is a code section to prohibit two services to exist on the same building and share the same ground rod, offering the potential to overload the #6? :)
 
ActionDave said:
I asked this question often when I first started as an apprentice and never got a completely satisfying answer.
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I don't know if you're aware (I had forgotten until this afternoon's research session) but Bryan Holland recently submitted a proposal to delete Table 250.66 altogether and require a #4 GEC minimum (IIRC) for all services, since that was a logical minimum - provided the example of a building with solely a CEE.

That would have been a biggie if they'd accepted it!
 
Suppose it was a condos that were all attached. Can I install 2 rods at one condo unit and use them for all the units?
 
The water pipe GEC sizing is left over from municipal systems where the water pipes were all part of a common metal underground water pipe system. When you have that, the water pipe electrode is the only one that could be counted on to be a grounded conductor and a fault clearing path in the event of an open neutral between the utility transformer and the service equipment. In fact, open neutrals could very easily go completely undetected when you had a common metal water piping system as the grounding electrode. With the use of non-metallic water mains or metal mains with bell and spigot connections, the water pipe system is not really bonded together like it was in the past and the water pipe doesn't really need a large GEC.


On the same line, why does the interior metal water piping system require a bonding conductor sized based on the service conductors and T250.66? This bond should really be based on T250.122 and the size of the largest branch circuit or feeder OCPD in the building. It would be very unlikely that the unprotected service conductors would ever energize the interior metal water piping system
 
don_resqcapt19 said:
On the same line, why does the interior metal water piping system require a bonding conductor sized based on the service conductors and T250.66? This bond should really be based on T250.122 and the size of the largest branch circuit or feeder OCPD in the building. It would be very unlikely that the unprotected service conductors would ever energize the interior metal water piping system
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This has always made me scratch my head and think what a waste of material...
 
George I think you are over thinking this.

Look at the purpose of the grounding electrode and the grounding electrode conductor. It is not installed to let the neighbor?s open neutral current to return on but instead it is installed for those reasons outlined in 250.4

In my opinion the question should be will the electrode and the electrode conductor carry the current imposed for one of the four reasons outlined in 250.4 and not for the purpose you have outlined.
 
jwelectric said:
George I think you are over thinking this.

Look at the purpose of the grounding electrode and the grounding electrode conductor. It is not installed to let the neighbor?s open neutral current to return on but instead it is installed for those reasons outlined in 250.4

In my opinion the question should be will the electrode and the electrode conductor carry the current imposed for one of the four reasons outlined in 250.4 and not for the purpose you have outlined.
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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)?
 
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