Transformer Grounding

[brian john]

There is an electrical contractor I work for that had the following question.

He is completing an upgrade of the electrical distribution system in a 12 story high rise. They are adding transformers on every floor. He wanted to utilize the equipment ground bus that is an integral component in an existing bus duct as the grounding electrode for the X/O-Neutral-Grounded conductor for the new transformers. The bus duct equipment grounding conductor meets the size requirements for the electrode conductor and terminates on the ground bus where he would terminate the new riser grounding electrode if required.

I told him I thought this equipment ground conductor in the bus duct did not meet the intention of the NEC. The AHJ and the electrical engineer tell him different. In which case he is OK to do the installation as he wants to. From a technical standpoint I think they are correct, but what about the NEC?

 

[dereckbc]

Re: Transformer Grounding

How is the building constructed? Is it steel framed or poured concrete?

I have ran into this before multiple times in several different cities. If it was poured concrete I used a riser cable approach. Or if steel framed you are required to use the BS IMO.

However I agree with the theory the AHJ and engineer are applying in this application of using the ground bus. A riser would just be duplicate.

[ May 21, 2004, 04:50 PM: Message edited by: dereckbc ]

 

[brian john]

Re: Transformer Grounding

This is a concrete building, 12 stories, standard office building nothing special about the tenant use.

 

[dereckbc]

Re: Transformer Grounding

Brian IMO you are required to go to the nearest GE. In that situation the riser cable is the best approach. Do not have code book handy, but there is allowance for a riser with common taps.

However, I feel the ground bus you outlined would be more than adequate, just just does not meet the description of a GEC per code. I know Don and I have discussed this approach, and I believe Don has asked for this method to be allowed. Maybe he can elaborate.

 

[don_resqcapt19]

Re: Transformer Grounding

250.30(A)(2)(b) would in my opinion permit this if the EGC is sized in compliance with 250.66. I made a proposal for the primary EGC to always be premitted for this application, no matter what its size, but that proposal was rejected.
Don

 

[dereckbc]

Re: Transformer Grounding

Don this relates to what I stated earlier about using a common GEC. Can you inter-change an EGC and GEC per code? I was called on it in Houston, and since that time have used a riser for a common GEC.

Any other inspectors out there willing to express his/her oppinion on this app? RYAN?

 

[don_resqcapt19]

Re: Transformer Grounding

Dereck,
I don't see anything in the code that prohibits using one conductor for both purposes as long as it is of the proper size. The code does not directly address this issue and neither did my proposal. My proposal called for the use of a combination conductor, but only sized per 250.122 based on the primary feeder OCPD.
Don

 

[dereckbc]

Re: Transformer Grounding

Originally posted by don_resqcapt19:
My proposal called for the use of a combination conductor, but only sized per 250.122 based on the primary feeder OCPD.
Don

So why not re-submit sized on 250.66 or in the event of a step-up 250.122 whichever is larger.

 

[ryan_618]

Re: Transformer Grounding

Originally posted by dereckbc:


Any other inspectors out there willing to express his/her oppinion on this app? RYAN?

Hi Dereck, sorry about the delayed response. I have seen the primary EGC used as the GEC before, and I had no problem with it, although I did scratch my head for a minute when I saw it.

It was for a 75KVA, 480Pri, 208Sec. 100 OCPD for the primary, 8 AWG EGC conductor required. I couldn't figure out why they ran a #2 for the EGC, especiall given the price of copper these days...I mean, that really is a h*ll of a size increase. Once I saw what they were doing and realized that the #2 was the required sized GEC, I had no problem with it. I must say though, I had my calculator, code book, scratch pad of paper and everything else trying to figure it out!!!!

 

[don_resqcapt19]

Re: Transformer Grounding

Dereck,

So why not re-submit sized on 250.66 or in the event of a step-up 250.122 whichever is larger.

Because where the transformer and primary feeder OCPD are in the same building, I see no electrical reason to require a grounding electrode conductor sized per 250.66. The only fault current that this conductor should ever have to clear is a primary side fault and it is already sized for that. If there is local water piping or building steel in the area of the transformer that is likely to become energized, then those items should be bonded with a conductor based on the transformer secondary conductors and 250.66.
Don