centre is 15 feet from the main, while the true lamp centre is 23 feet. It is to preclude the error of geometrical centres that the branches and mains are laid down, or imagined, revolved. SUB-BRANCHES and TAPS may in general be considered as groups of lamps directly on the branch itself and thus included in the calculation for the branch. The above method is applicable to all systems of wiring, and is particularly valuable and economical in securing proper distribution of light on low voltage circuits having a small percentage of loss. By stringing the branches first, when possible, this method may be easily followed without the aid of a diagram, even in complex cases. With the "closet" system of wiring, diagrams and calculations as a rule will not be required. The "tree" system of wiring is to be avoided where possible, on account of the unequal distribution of light it entails. In many cases secondary centres of distribution may be substituted, and if carefully calculated the weight of wire in the latter case need not exceed that in the former. The voltmeter should always be connected with the centre of distribution and not with the feeder near the dynamo, unless it is desirable to have a steady light in a particular locality, when it should be connected with the line there. Owing to the exceedingly small current passing through a voltmeter, the resistance of even a very small wire in ordinary cases will not practically affect its readings. Where the line is very long, a No. 12 or 14 insulated iron wire may be used and the voltmeter at the dynamo set once for all by comparison with a standard voltmeter temporarily attached at the point which is to be maintained at a constant potential, WIRING FORMULE AND TABLES. Tables I-IV were calculated on the basis of 55 watt 16 candle-power lamps, While there are lamps taking somewhat less current than the above, it is advisable to allow some margin in wiring. The formula deduced for calculating the where A section in circular mils. W = watt rating of lamps. N: distance to centre of distribu tion, in feet. = number of lamps. a=percentage of drop. The AMPERES (C) being given, the size of wire in circular mils can be found from the following formula: 100; EXAMPLE. Volts, 50; amperes, feet to centre of distribution, 100; drop, 2%. 2150 x 100 x 100 = 215,000 circular mils. 2 × 50 By referring to table IX, page 30, we find this to be about No. 0000 B. & S. The HORSE-POWER (H. P.) and efficiency (eff.) of a motor being given, the size of the conducting wire in circular mils can be found from the following formula : A = 160,400,000 × H. P. × L (3) EXAMPLE. Horse-power, 10; volts,500; drop, 3%; feet to distributing point, 600; efficiency of motor, 75%. A = 160,400,000 × 10 × 600 3 × 500 × 500 × 75 = 17,109. Referring to table IX, page 30, we find this to be about No. 8 B. & S. The gauge may be directly taken from the tables below 500 volts by means of the following formula of lamp equivalent, (2): 1360 H.P. 2= ef. (4) EXAMPLE. Horse-power, 5; volts, 110; drop, 3%; feet to distributing point, 200; efficiency of motor, 75%. 1360 × 5 Q= lamp equivalent = 90. 75 Referring to table III, we find that No. 2 B. & S. wire corresponds to 90 lamps and 200 feet, the drop being 3%. If the distance had been 400 feet, we could still have referred to the 200-feet column, but with 2 × 90 or 180 lamps. Having the HORSE-POWER, and efficiency of a motor to go on a 500-volt circuit, to find the amperes with which to enter Table V, or to apply formula (2), we have the following: EXAMPLE. Horse-power, 7; efficiency. 80%. 7 × 149 C= = 13.9 amperes. 80 Table IV. is also a 220-VOLT table, and can be used as such for motor wiring by means of formula (4). |