Three-piece knockout seals are used to seal up holes punched in panels or knockouts from outlet boxes. They are held in place by a screw inserted through the seal into a threaded strap on the opposite side of the opening. Available in sizes from 1/2 through 6 in. Three-piece knockout seals are embossed to centralize knockout. Installs from the inside or outside of any box without rearranging wires. One screw secures unit. Size is stamped into seal.