This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
17. Conducting cables or wires for any system of electric house lighting are always arranged as a closed circuit; that is, one wire must be provided to convey the current to the lamps, and another one to allow it to flow back to the starting point. It is not, however, necessary to provide one set of wires for each lamp for the entire distance. The arrangement should be somewhat like the trunk and branches of a tree. Heavy wires carry the whole current for the house up to the first distributing point, and wires continually branch out, becoming smaller at each such division. The name lead (pronounced leed) is frequently given to the conductors in a house.
18. Electric-light conductors for inside wiring are of two kinds: solid and stranded. The heavy conductors, which carry the whole current to a distributing center, are called feeders, and are usually made of stranded wire for the sake of flexibility; that is, they are composed of a large number of interwoven wires of small diameter. Fig. 5 shows the construction of such a cable. The wires w project at the end, the insulation being all removed; a thin sheet of rubber r is first laid over, then a gutta-percha compound g, and over this a covering of braid b as a mechanical protection. For smaller conductors, such as mains or sub-mains, a solid wire may be used for the core, the insulation being of the same description. For connection to single lamps, a double flexible conductor is used, such as A or B, Fig. 6. The core of each is formed of many fine wires w, insulated with rubber and braided. These two sets of conductors are then twisted together, and may be used to suspend the lamp, or they may be laid side by side and braided together.
19. When it is necessary to make joints between wires, it is important to remember that the work cannot be too well done. Under no circumstances should a joint be left unsoldered. When connecting a branch line to the main, the insulation is cut away as shown in Fig. 7; the cut should not be made straight down towards the wire with the edge of the knife, forming a sharp shoulder on the insulation, as the knife is very likely to make a nick in the wire, and subsequent bending might produce a crack at this point. Such a fault would increase the resistance locally, and cause heating, and, possibly, fire risk. In the illustration, the branch wire b, after being carefully bared of insulation and scraped clean with sandpaper, is shown wrapped over the main m, similarly exposed. This operation should be done with a pair of pliers of convenient size, and the turns of b should be close together. The joint should then be soldered, no acid being used, but resin only, as a flux, the reason being that it is impossible to clean off all the acid after the joint is finished, as some remains in the crevices and will eventually corrode the wire and break the electrical circuit. When the joint is cool, the wire is held firmly by the solder; all the exposed wire should then be covered by wrapping rubber insulating tape carefully over it, continuing across a short distance on the main insulation. When, however, the outer covering is braided, this should never extend into the insulation of the joint; as, when exposed to dampness, the moisture is likely to penetrate and cause trouble. It may here be remarked that it is not so easy to make a resin joint as one on which acid is used, which explains the disfavor in which the former is usually held by poor workers. Acid removes grease from the wire, such as a careless workman may have smeared on from his fingers, but when the wire is not handled after cleaning, resin will make a good joint. An alternative method is to tin both wires before wrapping, using acid as a flux; then wipe carefully, cleaning thoroughly, to remove all trace of acid, and wrap over, using pliers to bend the wire. The joint should then be completed, with resin as a flux. When two wires are to be connected together to form a continuous conductor, the Western Union joint, Fig. 8, is employed, the wires being twisted one over the other, soldered and taped.