For convenience in comparing the conductivities of wires, Table IV is given. As an illustration, it is seen from the table that instead of a single No. 2 wire we might use four No. 6 wires; two No. 5; four No. 8; etc. Of course, nothing smaller than No. 14 can be used for interior wiring. The conductivity is directly proportional to the total cross-section of all the conductors in multiple, and the total resistance is inversely proportional to the total crosssection. 16. Circuits of several wires in multiple are sometimes run where a large drop in voltage is not objectionable, but where a single wire small enough to produce that drop will not carry the current safely. Two or more small wires will safely carry more current than one large wire of equivalent cross-section, because two small wires have a greater surface area from which the heat can escape than has one wire of twice the cross-section. For instance, suppose it is desired to run wires in molding to secure a drop of 4 volts with 65 amperes over a distance of 100 feet. Calculating the required size of wire by means of Table II, we see that No. 5 will give the required drop. But No. 5 rubber-covered wire will safely carry only 54 amperes, while 65 amperes are to be transmitted. By using two No. 8 wires, which are equivalent in cross-section to one No. 5, we can safely carry the current with the specified drop. If the current were still greater, we could use one No. 8 and two No. 10 wires with about the same results. However, such arrangements to secure a drop are only used in emergencies or under special conditions, and are usually only temporary expedi ents. 17. Calculation of Wires in Multiple.-If a number of wires are connected in multiple, where R is the combined resistance of the wires and r1,,,,, etc. their separate resistances. 27 To apply this formula, suppose that a wireman at a distance from a supply house has on hand a large amount of No. 12 wire, but no larger wire, and that he desires to run mains to carry a current of 40 amperes 150 feet with 3 volts loss. How many No. 12 wires should be connected in multiple to secure this result? Resistance of line = volts drop 3 = 4 = .075 ohm. The total length of line 150 X 2 = 300, or .3 thousand feet. The resistance of the line per 1,000 feet = .075 .3 = .25 ohm. Now, 1,000 feet of No. 12 wire has a resistance of about 1.586 ohms. All the wires in multiple are to be No. 12; hence, r, r,r,= 1.586. = Let r = the number of No. 12 wires required; then in the formula given above, we have R = .25, and since r1, r1, ra Let us take another example. In an old building, wired with too much drop, it is desired to reenforce the mains so as to reduce the drop to 2 volts. A circuit of No. 8 wire carrying 20 amperes a distance of 200 feet is to be reenforced. What size of wire should be used? Total resistance of the line should be = .1 ohm. Since this line, whose resistance is to be .1 ohm, is 400 feet long both ways, then the resistance per 1,000 feet must be .1 x 1000 400 = .25 ohm. The resistance of a No. 8 wire per 1,000 feet = .627 ohm. Let r1 = .627 and r, = the resistance per 1,000 feet of the wire required. Then we have, by substituting in the formula .25 627 ra ra .25 .627 1 1 R = 2.41; hence, r, = .415. A No. 6 wire of which the resistance is .394 ohm per 1,000 feet most nearly meets this requirement. WIRING IN DAMP PLACES. The 18. Where wiring is done in damp places, special precautions must be taken and special rules observed. following Underwriters' rules apply to this work: Wires In damp places, such as breweries, sugar houses, packing houses, stables, dye houses, paper or pulp mills, or buildings especially liable to moisture, or acid, or other fumes liable to injure the wires or their insulation, except where used for pendants: a. Must have an approved rubber-insulating covering. b. Must be rigidly supported on non-combustible, non-absorptive insulators that separate the wire at least 1 inch from the surface wired over, and wires must be kept apart at least 24 inches for voltages up to 300 and 4 inches for higher voltages. Rigid supporting requires under ordinary conditions, where wiring over flat surfaces, supports at least every 4 feet. If the wires are liable to be disturbed, the distance I between supports should be shortened. In buildings of mill C. Must have no joints or splices. Sockets. a. In rooms where inflammable gases may exist, the incandescent lamp and socket must be enclosed in a vapor-tight globe and supported on a pipe hanger, wired with approved rubber-covered wire soldered directly to the circuit. b. In damp or wet places or over specially inflammable stuff, waterproof sockets must be used. When waterproof sockets are used, they should be hung by separate stranded rubber-covered wires, not smaller than No. 14 B. & S., which should preferably be twisted together when the drop is over 3 feet. These wires should be soldered direct to the circuit wires, but supported independently of them. 19. Fig. 5 shows a waterproof globe for use where inflammable gases may exist. In wiring damp cellars, it is especially desirable to have the lamps. divided among several small circuits, so that the blowing of a fuse will not put out many lamps. In such work, rosettes should never be used, but the drop wires should be soldered direct to, but preferably not supported by, the line wires, and the joints should be thoroughly wrapped with insulating tape. The cut-outs should be placed outside the cellars, in a dry place if possible, otherwise they should be placed in waterproof boxes. 20. Rubber-Covered Wire.-In all kinds of work, except open work in dry places, rubber-covered wire must be used. The Underwriters require it to comply with the following specifications: FIG. 5. |