PLAN No. 1189. LIGHT AND POWER WIRING
Installation of light and power wiring is more skilled work. On these circuits the electric pressure is always relatively high—110 volts or above—and there is the ever present danger to be guarded against of fire caused by short-circuiting. Not only may improperly installed wiring involve great fire hazard, but it may involve life hazard also. To minimize these dangers, codes of rules indicating the proper and safe methods of installing wiring and electrical equipment have been formulated. The National Board of Fire Underwriters has prepared and revises periodically such a compilation of rules, called the “National Electrical Code.” This, or some modification of it, is in force in practically all communities. These rules specify the conditions under which wiring of certain types may be used and indicate rigid requirements for the protection of circuits and the installation of electrical machinery.
Methods of wiring.—The several methods of wiring include the following: (a) open wiring; (b) knob and tube wiring; (c) rigid conduit wiring; (d) flexible conduit wiring; and (e) molding wiring.
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.