I have two single-gang metal boxes: one that's 3 1/2" deep, and another that's 2 3/4"-deep. The smaller box has the ability to adjust the depth of the ears more than half an inch. But the larger box can only adjust a small fraction of an inch. Is it a code requirement that larger boxes' ears cannot be as adjustable as smaller boxes? If not, are there any 3.5" single-gang metal boxes with widely-adjustable ears like the small one pictured below?
I ask because I need a box that can accommodate a receptacle (2 conductor equivalent), ground wires (1 conductor), and can feed another receptacle (2 x 2 = 4 conductors), for a total of 7 conductors. According to NEC 314.16(A) box size table for metallic boxes, I'll need a 3.5" box to do that. My jurisdiction adopts this section of the NEC without amendment.
But I'd been assuming that the easiest installation would be to screw the ears to the strip of lath above and below the cutout hole for the box. If I do that, then I'd need to move the box forward a half-inch or so, and those adjustable ears seem like an easy way to to do that, without the hassle of an extension ring.
Does a deep box with widely adjustable ears exist? If not, then what's another way to do this install? Should I find another way to anchor to the wall that doesn't require screwing into the lath? Should I use an extension ring? Something else?