UL Listed - Safety Certified: Made from non-flammable, self-extinguishing materials. The patented real flange allows the extenders to fit snugly against every style box to assure no gaps are present between the existing box and your newly installed extender. This feature assures that they meet all electrical codes. Standard Interior Boxes Only: Designed for PVC, metal, or fiberglass electrical boxes; compatible with new work and old work; Not compatible with exterior waterproof boxes, which are larger and require watertight connectors, special outlets, and covers. May not fit traditional boxes.