I have a metal box and conduit outlet in my kitchen where a bar cabinet is going to be installed. I assume it would be way too much effort to rip out plaster and move the box to a new location, so I would rather just have the outlet inside of the bar for lighting purposes. Would I just need a metal extender for the box to extend the outlet into the back of cabinet? I assume non-metallic plastic extender is no good.
The National Electrical Code does not allow devices to sit back inside combustible material.
The pertinent article attention to the second paragraph:
314.20 Flush-Mounted Installations. Installations within or behind a surface of concrete, tile, gypsum, plaster, or other noncombustible material, including boxes employing a flush-type cover or faceplate, shall be made so that the front edge of the box, plaster ring, extension ring, or listed extender will not be set back of the finished surface more than 6 mm (1∕4 in.).
Installations within a surface of wood or other combustible surface material, boxes, plaster rings, extension rings, or listed extenders shall extend to the finished surface or project therefrom.
They make cheap box extensions that are basically a piece of stamped metal with a slit in one side that can be installed without de-terminating the device. Just remove the receptacle, slip the conductors through the slit in the side and reinstall it flush with the surface.
Why not just cut a hole in the cabinet? One advantage to doing it this way is the plugs end up farther back, therefore taking up less space inside the cabinet.