EGC as GEC (for xfrmr in MCC)

[amatsu42]

Good Morning Everyone,

EGC as GEC seems to be a hot topic that has been discussed in great depth already on this forum; unfortunately, I have not seen a definitive answer to my specific question so I thought I would spark a continued discussion now.

We use the same vendor for compressors on all of our sites and provide the same single line for each installation. The compressor MCC has a 5kVA 480/120V step down transformer to provide control power and 120VAC auxiliary power. Dozens of other sites have received inspector approvals, but this new site we're building is receiving push-back because we do not detail a connection back to a local grounding electrode (and didn't size a GEC).

I haven't personally inspected the other sites to know if the contractor knew to tie the transformer to a grounding electrode, but since no GEC was sized or called out in our drawing I would be surprised if they did. My question to everyone is am I correct in my current review that my transformer, a 5kVA 120V single phase secondary, is a separately derived system that requires a direct connection to a grounding electrode? I see that there is an exception for <1kVA transformers that is commonly used for control transformers, but I can't use that to justify no GEC for a 5kVA.

My second question is I thought that my EGC served dual purpose as an EGC connection, but after reviewing all of the previous topics I am not confident that this is a code compliant installation. The EGC run in the feeder for the control transformer is sized per table 250.66. Reading NEC 2014 250.121 makes me think that this isn't proper:

250.121 Use of Equipment Grounding Conductors. An
equipment grounding conductor shall not be used as a
grounding electrode conductor.
[I]Exception: A wire-type equipment grounding conductor installed[/I]
[I]in compliance with 250.6(A) and the applicable requirements[/I]
[I]for both the equipment grounding conductor[/I]
[I]and the grounding electrode conductor in Parts II, III, and[/I]
[I]VI of this article shall be permitted to serve as both an[/I]
[I]equipment grounding conductor and a grounding electrode[/I]
[I]conductor.[/I]

Chiefly, we aren't using Cadweld or permanent compression connections for the EGCs.

Tl;dr: Previous installations used the EGC as a GEC (sized per 250.66) but this isn't approved on a new site.

Final question/takeaway: Is there a summary of what exactly I would need to do to my EGC to use it as a GEC or is it just cheaper/easier to add a new GEC connection to a nearby grounding electrode?

 

[__dan]

If the building steel or another listed electrode is nearby, likely it is far cheaper and easier to run a new GEC to the nearby electrode than it is to run a EGC compliant with the far more restrictive requirements for a GEC.

 

[amatsu42]

... likely it is far cheaper and easier to run a new GEC to the nearby electrode...

That's what I was thinking as well. Previously, I thought it just resulted in an upsized EGC which isn't too costly, but upon this deeper review it looks like there's a lot more involved with a code compliant EGC/GEC combo :weeping:.

Additional question: we run a ground loop in the hazardous location and back to the electrical compound. The MCC enclosure is tied to this ground loop as an added precaution. If the MCC enclosure has a GEC to the ground loop, would attaching the transformer GEC to the enclosure meet this GEC requirement? Or does the SDS GEC need to run directly to the ground loop/ground rod?

 

[__dan]

That's what I was thinking as well. Previously, I thought it just resulted in an upsized EGC which isn't too costly, but upon this deeper review it looks like there's a lot more involved with a code compliant EGC/GEC combo :weeping:.

Additional question: we run a ground loop in the hazardous location and back to the electrical compound. The MCC enclosure is tied to this ground loop as an added precaution. If the MCC enclosure has a GEC to the ground loop, would attaching the transformer GEC to the enclosure meet this GEC requirement? Or does the SDS GEC need to run directly to the ground loop/ground rod?

First: The GEC and the EGC often take different paths. The GEC runs from point A to B, the ECG runs from point A to C. If they happen to take the same paths, the more restrictive requirements for the GEC would include bonding bushings with jumpers entering and exiting boxes, irreversible splices ...

Second: The GEC suitable for tapping is 3/0 copper or larger, or a listed copper busbar (from memory 2" wide x 1/4" thick). If the MCC has a copper common grounding busbar meeting the minimum size required for group GEC's and it has a GEC jumper to the electrode, you should be able to attach your GEC to that busbar. GEC to suitable building steel yes, to the enclosure, imo no (you would bond to it with a bonding bushing as you pass through entering and exiting, but it is not an electrode or a common group electrode busbar).

 

[amatsu42]

Thank you so much for the detailed response. I'll just tie the transformer to the ground ring directly with a GEC unless someone proposes a more cost effective solution. Definitely going to check some previous installations to see how they were installed--might have some rework to do.

Always learning something new with the code. Love the forum and appreciate the help.