The consequences of undersizing outlet boxes can include damaged wiring, overheating, and even electrical fires. But you can’t just oversize boxes “to be on the safe side” because that wastes space and money. A 10 × 10 × 4 box for three 12 AWG conductors would prevent an undersizing problem, but it would also be ridiculously big. So how small is too small, and what size is adequate?
NEC Requirements for Sizing Outlet Boxes
Under- and oversizing outlet boxes can wreak havoc on your installations.