Junction boxes are an essential part of the electrical wiring systems for homes and buildings alike. These metal or plastic boxes house and safely protect a structure's electrical connections. These electrical casings typically come in two sizes: three-wire boxes measuring 2 inches by 3 inches with a depth of 2.5 inches, and boxes with five or more wires that measure 2 inches by 3 inches and have a depth of 3.5 inches.

A small metal or plastic junction box may form part of an electrical conduit or thermoplastic- sheathed cable (TPS) wiring system in a building. If designed for surface mounting, it is used mostly in ceilings, under floors or concealed behind an access panel—particularly in domestic or commercial buildings.