Ques. Describe the unlined type of conduit.
Ans. It consists of an iron or steel pipe, similar in size, thickness, and in every other way to gas pipe, except that special precautions are taken to free it inside from scale or any irregularities; it is then coated inside with enamel, outside it is sometimes enameled and sometimes galvanized.
Ques. Describe the lined type of conduit.
Ans. It usually consists of a plain iron pipe lined with a tube of paper which has been treated with an asphaltic or similar compound; this paper tube is cemented or fastened to the inside of the iron pipe so that it forms practically an integral part of the same.
Ques. What are the advantages of unlined conduit?
Ans. It is cheaper, because having no lining a smaller size of conduit can be used for any given size of conductor; it is also cheaper to install, as it can be bent, threaded, and cut more readily than the lined conduit. Wires may be more easily inserted and withdrawn as the inside is smoother than that of the lined conduit.
NOTE.—Conduits for inside wiring which are subject to inspection, must have an inside diameter of not less than ⅝ inch. They must be continuous from outlet to outlet or to junction bores, and must properly enter and be secured to all fittings, and the entire system be mechanically secured in position. In case of service connections and main wires, this involves running each conduit continuously into a main cut out cabinet or gutter surrounding the panel board as the case may be. Conduits must first be installed without the conductors, and be equipped at every outlet with an approved outlet box or plate. Outlet plates must not be used where it is practicable to install outlet bores. The outlet box or plate must be so installed that it will be flush with the finished surface, and if this surface be broken, it shall be repaired so that it will not show any gaps or open spaces around the edge of the outlet box or plate. In buildings already constructed where the conditions are such that neither outlet box nor plate can be installed, these appliances may be omitted by special permission, providing the conduit ends are bushed and secured. It is suggested that outlet boxes and fittings having conductive coatings be used in order to secure better electrical contact at all points throughout the conduit system. Metal conduits where they enter junction boxes, and at all other outlets, etc., must be provided with approved bushings or fastening plates, fitted so as to protect wire from abrasion, except when such protection is obtained by the use of approved nipples, properly fitted in boxes or devices. Conduits must have the metal of the conduit permanently and effectually grounded. Conduits and gas pipes must be securely fastened in metal outlet boxes so as to secure good electrical connections. If conduit, couplings, outlet boxes or fittings having protective coating of insulating material, such as enamel, be used, such coating must be thoroughly removed from threads of both couplings and conduit and from surfaces of boxes and fittings where the conduit is secured in order to obtain requisite good connection. Where boxes used for centers of distribution do not afford good electrical connection, the conduits must be joined around them by suitable bond wires. Where sections of metal conduit are installed without being fastened to the metal structure of buildings or grounded metal piping, they must be bonded together and joined to a permanent and efficient ground connection. Junction boxes must always be installed in such a manner as to be accessible. All elbows or bends must be so made that the conduit or lining of same will not be injured. The radius of the curve of the inner edge of any elbow must not be less than three and one-half inches. Must have not more than the equivalent of four quarter bends from outlet to outlet, the bends at the outlets not being counted.
Fig. 889.—Conduit box showing arrangement for combination side outlet with open cover. Outlet or junction boxes are of two general types: 1, those which are made for a particular position and have a given number of outlets, and 2, those which have a variable number of outlets which are plugged with metal discs in such a manner that the latter can be knocked out by a slight blow of a hammer. The illustration shows a universal plugged steel conduit box, which can be used as a straight electric, or combination gas and electric, ceiling or side wall outlet, or for flush rotary or push button switches, or for flush receptacles. When rigid conduits are used, they are screwed to the outlets by means of lock nuts and washers. In the case of flexible conduits, the entering ends of the conduits are provided with clamp bushings which are secured to the outlet by means of lock nuts. All outlet boxes are fitted with covers of various designs, which permit their use for various types of construction such as ceiling and wall work in lath or plaster, fireproofing ceiling work, etc., while many designs of outlet plates and receptacle plates may be obtained from the supply houses to satisfy the requirement of any special case.