I had to install a new wire for my water heater (higher amperage/bigger wire), and was drilling through the wooden top plate to snake the wire down the wall to the pass-through. Everything seemed standard. Put the wire in and heard what sounded like wire scraping something (possibly metallic). Pulled it out, and the cable sheath had little scratches on it; not gouges, or cuts. The hole was at least 3x larger than the wire and was clean.
Now this house in Florida where metal studs are commonplace. Elsewhere in the house there have been metal bottom plates which transition to wood as we exit the exterior wall zone so I suspect under this wooden top plate there was a metal top plate – probably a few mm of metal.
I know that with metal studs anti-short bushings are used, but I am curious how this kind of situation is handled in old-work scenarios such as this where putting in an anti-short bushing isn't possible. Short length of conduit? Don't worry about it? Pulled the wire in and out a few times – more scratches but no cuts or gouges.
Best Answer
The simplest thing I could think of would be to use MC cable, the armor is pretty rugged. If you don't want to re-do the entire run, you could transition from NM to MC in a metal box mounted over the hole you made. Bring the MC in the back of the box, screw the box onto the top plate face up, and enter one of the sides with NM and a "button" NM clamp.
In my opinion MC with a right angle connector on top of the water heater makes a nicer install anyway.