1. The box needs to grounded! (mine was not) 2. Proper holes with proper connector, and bushing in the back. (The one I had was that cut-out, if one can call it that, with sharp expanded metal stucco lath with 2- 100A feeders going through it! Allowing the opprotunity to energize the lath, or the box.) 3. The Conductors should be in a cable, with the sheath extending at least 1/4" into the can, and the panel, both properly supported by a connector, and bushed. (mine was THHN) 4. Pentration of building sealed.
That's how it could be done.... The one I had also extended 3 1/2" in front of the flush main panel it was mounted below. So we couldn't....
To correct it and add more conduits we pulled this mess, chisty boxed it to the side, then brought the conduits (and some spares)into the panel inside the wall.
Mark Heller "Well - I oughta....." -Jackie Gleason