I current have plastic wall boxes. I need to connect to a MC conduit, which connects to an metal handy box with an receptacle. (NM to receptacle is not appropriate in this case).

I also have another 2 gang plastic wall box which needs to be connect to a FMC conduit attached to a cooktop. Can I use a two gang extension ring with knockouts?

Is it up to connect to install a metal extension ring with knockouts over a plastic wall box, as long as I put in a metal grounding click with wire, and join that wire to the other electrical ground conductors?

  • i seriously doubt the holes are in the right place for that to work, metal extension boxes have holes in the corner, oh, I see BatPlasterson has foind an exception. Oct 22, 2018 at 16:46

1 Answer 1


There are extension rings that fit over a standard single gang ring

single gang box extender

Like any metal box, you'd have to bond the box to the EGC (green / bare wire) with a grounding screw or clip.

It will work just as shown in the photo for a single gang ring. I learned in a comment below, you can rotate the box 90* and the four holes in the back of the box will line up for a double gang ring.

  • 1
    That picture you have is actually for single or dual gang. If you notice the two holes to the right of the back opening, they correspond to the mounting holes of a dual gang box. (There's another pair hidden behind the left edge in the photo.) You just have to turn the extension box 90 degrees.
    – longneck
    Oct 22, 2018 at 13:01
  • Never noticed that, thanks @longneck! Will edit... Oct 22, 2018 at 13:03
  • Thank you! This has been my thinking, but I’ve not been able to find any forum posts or comments on doing it like this. Oct 22, 2018 at 13:13
  • You can also use a two gang masonry ring i.e a Garvin TBX-225 or equivalent Oct 22, 2018 at 22:51

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