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Dry-type XFMR GEC to TMGB in same room?

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    Dry-type XFMR GEC to TMGB in same room?

    This is a thread that was started by another individual under General Electrical that I inquired on. I wanted to bring this to the forefront to see if anyone else has any additional thoughts:

    My question was in relation to the GEC from a dry-type XFMR, and any allowance for it to be tied to a TMGB in the same room. Again, the TMGB is connected directly back to an MGB located in another room, which then serves as the point of connection for all building electrode connections to meet 250.30(A)(6); the question is with the TMGB connected to the MGB, why would this still not be a suitable point to tie the SDS GEC to given it being the the closest in proximity? What would the difference be by connecting to building steel which would serve as the grounding electrode, instead of the TMGB when both eventually connect back to the MGB?

    As referenced by jaggedben, I see that 250.64(C) may be the confirmation that says "NO", but it's hard for me to understand the difference between the two points of connection when both eventually tie back to the MGB. Is the building steel that much better of a point as compared to the TMGB? Is it perhaps means of code enforcement to avoid creating a whole series of ground bars that may or may not tie back to an electrode/MGB? Other reasons?


    Follow up to this question. Attached is a photo of the installation as currently performed. Further reading and interpretation of 250.64(C)(1) & (C)(2) makes it further unclear about the allowance of using the TMGB as a point to connect the XFMR GEC.

    The question is whether the TMGB to MGB connection falls under 250.64(C)(1) or (2):
    - (1) Splicing of the wire-type grounding electrode conductor shall be permitted only by irreversible compression-type connectors listed as grounding and bonding equipment or by the exothermic welding process. – Would bolting to the TMGB (XFMR GEC to TMGB and MGB to TMGB connection) meet this requirement?
    - (2) Sections of busbars shall be permitted to be connected together to form a grounding electrode conductor. – Would connecting via the same 120mm2 (4/0AWG) between TMGB and MGB be considered “connected together” since the MGB serves as a centrally located copper busbar that connects the grounding electrode conductor and individual taps per 250.30(A)(6)?

    Thoughts and questions are appreciated.

    Click image for larger version

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      If anyone is curious, it was decided/determined to move the GECs off the TMGB to meet 250.52 & 250.64(C). Installer will route and terminate each GEC to closest steel column, thus connecting directly to the grounding electrode system.


        So if I understand your question your structure has steel frame construction but your transformer GEC's terminate at a ground bar that has a common grounding electrode run back to the building grounding electrode system and not the nearest building steel?


        All responses based on the 2017 NEC unless otherwise noted


          I'll try and do my best to clarify the installation - two (2) XFMRs are located in the same telecommunications room as the telecommunications main ground bar (TMGB), each feeding power panels that feed to outlets in the room for telecommunications equipment. Rather then connect the GEC for each dry type transformer to building steel (grounding electrode per 250.52), the installation performed has instead routed the GECs to that TMGB. The TMGB is connected with a 4/0AWG conductor to a main ground bar (MGB) which is located in a separate room. The MGB serves as the centrally located copper busbar to connect other separately derived system grounding electrode conductors before connecting to the exterior ground loop of the building to meet 250.30(A)(6).

          The question has been if the TMGB can serve as a connection for the XMFR GECs instead of going to a grounding electrode directly if the MGB and TMGB are connected in this manner. The feedback in the other post referenced to 250.64(C) which requires grounding electrode conductors be run in one continuous length without splice or joint. I then thought that the two potential exceptions may fall under 250.64(C)(1) or (2), however I still question if either is applicable given that each ground bar seem to serve different purposes. Even with the connection between the TMGB and MGB performed in the manner described, my thought is that the XFMR GEC does have to connect to building steel (closest grounding electrode) and not to the TMGB.

          Hopefully that clarifies what is being asked. If others think differently, I'm willing to be enlightened. I've tried to research this for some time, and this was the conclusion/interpretation that I came to.