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I just finished up installing two new subpanels, which both feed branch circuits back into the exterior wall. The original subpanel had an NM clamp mounted with the clamp screws outside the box (conventional way). Unfortunately, that meant the hole in the wall had to be significantly larger, and since I have stucco walls, it also meant an easy way for water to get inside my walls because it would be nearly impossible to seal the hole. Additionally, it made it difficult to run new wires into the subpanel (and even if I could, I couldn't access the clamp to tighten it down over the new cables.

I've seen this question on NM cable clamp orientation, but for a subpanel, where I am not worried about the fill ratio and where access to the clamping screws is required, does it make sense to mount the NM clamps with the clamping screws on the inside?

I know that (according to the linked post), it's not necessarily "per manufacturer instructions," but putting the clamp on backward seems to be a better solution for this application.

Must an NM clamp be installed with the clamp on the outside for exterior wall applications?

It seems to be that the normal orientation makes sense when putting metal boxes on interior walls where the NM wire is simply running freely into the box and the box is recessed into the wall.

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The main thinking of inspectors in my state is making sure that that NM cable entering any metal enclosure is properly bushed to prevent the bare metal from cutting into the conductor insulation. Manufacturers make plastic bushing just for this purpose. It snaps into a code size hole and has "Fingers which hold the cable in place. For reference try Googling "plastic romex connectors" and see what pops up. I know Arlington makes these thing as well many other manufacturers.

  • I have seen these. They are great for thin wall boxes, but in outdoor boxes, which typically have threaded "knockouts," they don't fit. – Hari Ganti Jun 2 '17 at 18:54

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