Ques. What are the disadvantages of the unlined conduit?
Ans. The Underwriters require the use of double braided conductors instead of single braided which are allowed for lined conduits.
Ques. Where may unlined conduits be used?
Ans. In buildings where the conduit is not liable to corrosive action.
Flexible Conduit Wiring.—Flexible conduits are used to advantage in many cases where rigid conduits would not be desirable. It is especially adapted to completed buildings where it is desired to install the wiring by "fishing" without greatly disturbing the walls, floors, or ceilings.
Figs. 890 and 891.—Greenfield flexible steel conduit; [fig. 890] single strip type; [fig. 891] double strip type. The former ([fig. 890]) is formed with a single strip of galvanized steel, interlocked and gasketed in such a manner as to be suitable for concrete construction. The double strip type ([fig. 891]) is constructed of a concave and convex steel strip, spirally wound upon each other in such a manner as to interlock their concave surfaces. Thus the convex surfaces of the two strips form respectively the outer and inner surfaces of the conduit. This construction insures a smooth interior surface, thus reducing the possibility of friction in the drawing in of conductors. A gasket is provided between the inner and outer strips rendering the conduit moisture proof. This form of flexible conduit is especially adapted to use where the wiring is installed after completion of building, because it is very flexible.
Ques. How is a flexible conduit installed by "fishing"?