Questions from Art 250

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KTK

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Hi! I;m new to "On-Line Forums". I don't know exactly how they work? I have some questions as I've been going thru my NEC changes course on-line. Is this where I ask them??? KTK
 
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KTK

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Thanks for your reply, Dennis!

Q. Is the words "Separately Derived Systems" another way to say / mean "Transformers"?

Q. What does the phrase "Likely to become energized" mean? In what scenarios?

Q. What is the difference between tables 250-66 and 250-102-c-1? They both use the same #'s = 66 is for the GEC (which is used to ground the Neutral, right?) and 102 is for the "Grounded Conductor" (which I was taught means the "Neutral") and the bonding jumpers...

Q. I don't see in Art 250-52 "Metal Structural Frame" (Building Steel) classified as a "Grounding Electrode" to ground the Neutral. Aren't Metal Water pipes, Ground Rods, and "Building Steel" commonly used to ground the Neutral?

Thanks for your time!

Kirk T.
 
Dennis Alwon

Dennis Alwon

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KTK said:
Thanks for your reply, Dennis!

Q. Is the words "Separately Derived Systems" another way to say / mean "Transformers"?

Most all transformers are separately derived system (S D S)

Q. What does the phrase "Likely to become energized" mean? In what scenarios?

That is very subjective. A piece of conduit must be grounded because it is likely to become energized. Some inspectors may say that if a non metallic cable is running across a metal beam then it is likely to be energized. Mostly it would apply to any thing that has power run to it. A furnace is likely to be energized because it is fed with a 120v cir.

Q. What is the difference between tables 250-66 and 250-102-c-1? They both use the same #'s = 66 is for the GEC (which is used to ground the Neutral, right?) and 102 is for the "Grounded Conductor" (which I was taught means the "Neutral") and the bonding jumpers...

At one time there was only one table for bonding and for the grounding electrode conductor. The grounded conductor is the neutral.. You could have a neutral sized smaller than the feeder or service conductor but it cannot be smaller than T-250-102. The only difference in the table is when you are dealing with very large conductor.

Q. I don't see in Art 250-52 "Metal Structural Frame" (Building Steel) classified as a "Grounding Electrode" to ground the Neutral. Aren't Metal Water pipes, Ground Rods, and "Building Steel" commonly used to ground the Neutral?

Not all building steel & water pipes are connected to the earth so they aren't used as a grounding electrode conductor however they still need to be bonded because they could be energized (likely to be energized). This is where 250.102 comes in.


Thanks for your time!

Kirk T.
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