In retrofit applications, where cutting into a wall or ceiling to add additional outlets or light fixtures isn't practical, a great option is to use a surface-mounted wiring system. Several different systems are available that use either metal or plastic raceways that extend from an existing wall outlet or ceiling box and allow you to run wires along the face of the wall or ceiling to one or more surface-mounted outlets or light fixtures. It can be a great way to bring better electrical service to an apartment, for example, where a renter may not have the ability to make major renovations. Generically, this type of wiring is often known as wire mold, although technically Wiremold is a proprietary brand from the Legrand company. Wiremold comes in either a plastic or metal form and varies in depth and thickness, depending on the installation.
Individual wire conductors are fed from box to box through the surface-mounted raceway that protects it, with special elbows used to make inside and outside corners, left- and right-hand turns. There are also union splices used to join individual straight sections for long runs.
Surface-mounted parts and raceway come in metal and plastic. At home centers, you can buy kits including lengths of straight raceways and fittings, or individual lengths of raceway and fittings, as needed. Manufacturer's instructions for installation are generally very easy to follow. Metal is more durable than the plastic, but plastic is easier to cut and work with. Both types are paintable.
So if you're in a pinch and need to extend a circuit to add a light fixture, switch or outlet and don't want to tear into the walls, surface-mounted wiring may be for you!