2 x 4 LED panels and troffers fit into 2-ft. x 4-ft. drop ceiling grids as part of new lighting installations or as replacements for fluorescent troffers of the same size. They use significantly less energy than fluorescent troffers and have built-in LED arrays so there are no light bulbs to install or replace. The 2 x 4 size is commonly used to light large classrooms, hallways, office spaces, department stores or similar retail settings, and wide areas with few walls. Sometimes called flat panels, these slim fixtures are typically recessed into the ceiling so that the majority of the fixture is concealed for a clean look that doesn't draw attention to the fixture. The height of the panel must be less than the height above the ceiling for a proper fit within a recess.