Ufer Grounding - Sizing Conductor

[short circuit faulted]

I've never used concrete encased (Ufer) grouding. Is the grounding conductor for a service sized in accordance with NEC Table 250.66 using Ufer method? Service is 6 sets 4" with 4 - 500MCM (Ok, KCMIL)
Thanks in advance for your help.

Steve

 

[don_resqcapt19]

Re: Ufer Grounding - Sizing Conductor

Steve,
Take a look at 250.66(B).
Don

 

[short circuit faulted]

Re: Ufer Grounding - Sizing Conductor

Don,
Thanks, 250.52(A)(3)referenced in that section, helped clear it up for me.

Have a good one!

Steve

 

[ryan_618]

Re: Ufer Grounding - Sizing Conductor

Steve: The 4 AWG conductor mentioned in 250.52 is describing the electrode itself, not the grounding electrode conductor. You could have 20' of 4 AWG (or larger) for the electrode and satisfy the requirement for the electrode itself. Once you have installed that, you now have an electrode and must install a grounding electrode conductor to attach to it. That conductor is sized based on Section 250.66, more specifically, 250.66(B), as Don mentioned.

 

[infinity]

Re: Ufer Grounding - Sizing Conductor

The 4 AWG conductor mentioned in 250.52 is describing the electrode itself, not the grounding electrode conductor. You could have 20' of 4 AWG (or larger) for the electrode and satisfy the requirement for the electrode itself. Once you have installed that, you now have an electrode and must install a grounding electrode conductor to attach to it. That conductor is sized based on Section 250.66, more specifically, 250.66(B), as Don mentioned

Are you saying that if the GEC is connected to 20' of rebar it is required to be sized by 250.66?

 

[pierre]

Re: Ufer Grounding - Sizing Conductor

I will take the liberty of speaking for Ryan - If I am wrong, I am sure he will let us know

Ryan is very detail oriented, I believe he is pointing out the difference between .52 (electrodes) and .66 (size of grounding electrode conductors ).

It is so very important to remember to read the words that are there, and to remember which Section number, Part, Article, Chapter (always going backwards when referencing) in the NEC.

This will help to keep one from getting "lost" in the NEC.

"Are you saying that if the GEC is connected to 20' of rebar it is required to be sized by 250.66?"

Yes Infinity, 250.66(B) to be specific as in Don's and Ryan's posts.

[ June 27, 2005, 08:39 PM: Message edited by: pierre ]

 

[ryan_618]

Re: Ufer Grounding - Sizing Conductor

Ditto Pierre's post. I am pretty comfortable when Pierre speaks for me. We seem to see things pretty well eye-to-eye

 

[infinity]

Re: Ufer Grounding - Sizing Conductor

So according to 250.66(B) the GEC to the concrete encased electrode, for the service in the original post, would be a #4 copper conductor.

[ June 28, 2005, 05:19 AM: Message edited by: infinity ]

 

[charlie]

Re: Ufer Grounding - Sizing Conductor

The way I understand the text. If you decide you wish to install 50 ft of 500 kcmil Cu. in the footers for the electrode, you may do that as long as you don't use anything smaller than #4 Cu. for the electrode. After the electrode is installed, the GEC must be taken to the service equipment panelboard and that conductor is not required to be larger than a #4 Cu. Therefore, if you have a 100 ampere service you are required to run a minimum #8 Cu. to that grounding electrode.

To keep it simple, most people will install a #4 Cu. grounding electrode and continue the #4 Cu. into the service equipment panelboard.

 

[infinity]

Re: Ufer Grounding - Sizing Conductor

Thanks Charlie and Ryan. This is exactly how I've always interpreted 250.66(B). Just wanted to be sure that I had it right.

 

[bphgravity]

Re: Ufer Grounding - Sizing Conductor

It really bothers me that what is considered the best electrode in terms of total resistance only requires a #4 conductor yet a water pipe or a metal frame could still require as high as 3/0. This makes no sense.

 

[petersonra]

Re: Ufer Grounding - Sizing Conductor

Originally posted by bphgravity:
It really bothers me that what is considered the best electrode in terms of total resistance only requires a #4 conductor yet a water pipe or a metal frame could still require as high as 3/0. This makes no sense.

My personal opinion is that the size of the incoming service wires is really not something that should affect the GEC size. I suspect a lot of this kind of thing is rooted in practices that go back far enough no one knows why they did them, but no one wants to change them due to the fear that there might well have been a good reason why it was done that way.

 

[bphgravity]

Re: Ufer Grounding - Sizing Conductor

Originally posted by petersonra:
I suspect a lot of this kind of thing is rooted in practices that go back far enough no one knows why they did them, but no one wants to change them due to the fear that there might well have been a good reason why it was done that way.

This is a great statement. Most of Article 250 probably falls into this category.

 

[charlie]

Re: Ufer Grounding - Sizing Conductor

When talking about water piping and the metal frame of a building, either or both may end up carrying the unbalanced load of a grounded conductor or the entire anticipated fault current under fault conditions. Consider the failure of the service neutral, what now is carrying the above current? I am assuming buildings that are close together and metal underground water piping for all the close buildings. How many times have we talked about the return current going through the neighbor's home?

Also consider a failed piece of conduit that is supported by bar joists. A fault occurs and finds a path back through the building steel to the service equipment and on to the serving transformer. Why indeed is the GEC required to be sized in accordance with 250.66 and its table?

 

[wirenut1980]

Re: Ufer Grounding - Sizing Conductor

My personal opinion is that the size of the incoming service wires is really not something that should affect the GEC size. I suspect a lot of this kind of thing is rooted in practices that go back far enough no one knows why they did them , but no one wants to change them due to the fear that there might well have been a good reason why it was done that way.

Charlie still knows!