Open wiring was the earliest form. It is the lowest in first cost and is, if properly installed, safe. The insulated conductors are supported along the surfaces of the walls and ceilings on porcelain knobs or cleats. In many factory buildings, particularly in those of the wooden mill type construction, much open wiring is used. For this service it is economical and quite satisfactory.
The knob and tube method provides a concealed installation, but is more expensive than the open type, and is applicable only in frame buildings. The wiring is placed while the structure is under construction. Where the conductors are carried along the sides of joists or other timbers, they are supported on porcelain knobs. Where they pass through studs or other wooden members the holes are bushed with porcelain tubes.
The principal tools used in both knob and tube and in open wiring are the screw driver, brace and bit, knife, saw, soldering iron, and blow torch.
PLAN No. 1190. MOLDING WIRING
In molding wiring conductors are held in a flat wooden or metal molding screwed to the wall or ceiling surface. Wooden molding, however, is not now commonly used, and in many cities its use is prohibited. The metal molding which is superseding the older nonmetallic molding is small, unobtrusive, and readily installed. It is used principally for extensions to existing wiring installations. By its use the cutting of holes and channels in walls and ceilings, which is necessary for the installation of concealed wiring, is eliminated.
PLAN No. 1191. STEEL CONDUIT WIRING
This is the most expensive, but the safest and best method. In fireproof buildings it is used exclusively, and in some communities wiring of other types is prohibited. Many frame buildings are now being wired with metal conduit, either of the rigid or the flexible type. Rigid conduit is merely wrought iron or steel pipe of the usual trade dimensions, which has been specially treated to prevent its corrosion and to render its interior smooth. Flexible steel conduit resembles metal hose in construction. Metal conduit is installed while the building is under construction, and subsequently rubber-insulated conductors are pulled into it. Usually the conduit is concealed within floors and walls, but it may be supported on surfaces. In concrete construction buildings the conduit is embedded in the concrete, being placed in the forms before the concrete is poured. At outlets where the conductors must be carried from the conduit system to feed lights and switches, steel or cast-iron outlet boxes are connected to the tubes. Pipe tools, such as dies, reamers, hack saws, drills, and bending “hickeys” are used in working conduit.
PLAN No. 1192. WIRING AS AN OCCUPATION
Wiring work is interesting and diversified. Some wiremen in the cities specialize on one type of work. For example, certain journeymen may ordinarily do nothing but conduit work day in and day out, while others may do all sorts of installing. Some men specialize on power wiring for motors, generators, switchboards, and similar equipment.
Usually a wireman must rely on his own experience and judgment for the detailed layout of the wiring installation. The architect’s plans generally indicate merely the locations for the different lights, switches, motors, and other devices. The wireman must then plan his circuits so that these devices may be served with the minimum expenditure of time and material.