I'm in the process of installing a transformer that needs bonding for 3 different ground conductors: one from the line side, one from the load side, and the main building ground. Butthe transformer only has 2 bonding lugs available. I'm really confused about how to handle this. My initial thought was to just add an extra lug, but now I'm not so sure if that's the right way to go about it. Can anyone tell me the proper way to bond all 3 of these ground conductors to the transformer?
Transformer question
- Thread starter nos4ratuu
- Start date
Don't add an extra lug!! The most common and code-compliant method is to use a double-barrel lug instead. This lets you land two conductors, so you can put the line-side and load-side EGCs in one and the building ground in the other. Another option is an exothermic weld, which creates a super low-resistance bond, but requires specialized tools. A third option but a lot less common is using a split bolt connector to tie two conductors together before landing them in the lug. Also make sure to check NEC Article 250 for specific grounding requirements!!
General_Valeria
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- Aug 31, 2025
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Quick hack that still plays it safe... put in a grounding/bonding terminal bar inside the transformer enclosure (on a solid metal part, not over vents), then land all three conductors there. That gives you one neat point that meets the NEC without adding random lugs on the case.
Attached below are some samples of items we used for pad mounts when I was a linesman. The double barrel, crimp-on, one-hole lugs were the standard go-to if space allowed. (4th photo from left.)
PG clamps are typically used outdoors with messenger wires, but feeding one conductor all the way through (ending at the bus) and dead-ending the other works well in transformers. I think the others are self-explanatory. If the factory bonding ribbon is proper size, the PG clamp can bite that ribbon, leaving all the lug spaces available.
Another option is to put crimp-on single hole lugs on each conductor, plus on a new jumper conductor. Bolt them together with a #10 or larger screw and a toothed lock washer. The jumper goes to the bus. For flat lugs, a pin wheel arrangement was used. For offset lugs, they usually can be stacked.
Hope This Helps,
Paul
PG clamps are typically used outdoors with messenger wires, but feeding one conductor all the way through (ending at the bus) and dead-ending the other works well in transformers. I think the others are self-explanatory. If the factory bonding ribbon is proper size, the PG clamp can bite that ribbon, leaving all the lug spaces available.
Another option is to put crimp-on single hole lugs on each conductor, plus on a new jumper conductor. Bolt them together with a #10 or larger screw and a toothed lock washer. The jumper goes to the bus. For flat lugs, a pin wheel arrangement was used. For offset lugs, they usually can be stacked.
Hope This Helps,
Paul
