![]() If the boxes are metal, the metal shims make electrical contact for the ground. Use one, two or more as necessary to get the device tight to the box and at the correct position relative to the finished wall. If the box is set back into the wall, snap off the ears at the score marks and put them on the screws. The ears on the ends of the tabs of switches and receptacles have holes to allow them to be used as shims through which the screw passes. I can also imagine a jig that would hold a sample of the finish wall material against the edge of the stud and have an edge to which the electrical box would be pulled to position the edge flush with inside of the finished wall. I can imagine an electrical supervisor lightly stapling a piece of drywall or other material of the correct thickness on the stud at each box location, and instructing the installers that they are to have the outside edge of the box lined up with that and leave it in place for removal at the pre-inspection check, or for the official inspection, or even for the drywallers to remove. When installing boxes on studs without drywall in place it seems to me that laying a 1.5 inch wide piece of material the same thickness as the intended finish wall against the face of the stud would allow correct positioning of the boxes. When the set back is small enough that an extension ring doesn't fit, or when an extension ring reduces the gap but doesn't quite make things snug, shims on the mounting screws are the way to go. Whether the set back is the maximum 1/4 inch or something minimal like 1/16 inch this support is important. Then forces transfer from the device, through the shim, into the box. When the drywall support is insufficient shims should be placed on the device mounting screws so that the device is held snug. Ideally the drywall is trimmed close to the box so that the ears of the device yoke rest on it. Face plates can break when they're called upon to resist those forces. Proper support from behind is important especially for devices like receptacles which see pushing-into-wall forces during plug insertion. 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. 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.). However, if the existing boxes were well-placed originally it may not be necessary to do any additional work to bring them to the new drywall surface. If you're installing new boxes or otherwise need to adjust existing boxes, by all means set them in the right position.
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