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less than those through water and solid bodies, except those which are very poor conductors of heat, such as wood. For similar reasons the temperature rise of a bare wire in air is usually greater than that of the same wire covered with insulating material. The effect of the latter is to increase the surface from which heat is radiated and carried away by convection. In most cases a considerable increase in the temperature of a bare wire is not objectionable except so far as it represents loss of energy. The real limitation to the heating of electrical conductors is the point at which their insulation is likely to be injured.
The following are standard tables, giving the maximum currentcarrying capacity of different sizes of insulated copper conductors:
Table No. 1 is based upon Kennelly's experiments, and is intended to allow a rise in temperature of 75° F. for twice the current specified, thus giving an ample factor of safety. The normal current would only raise the temperature 18^° F., since the heating effect is proportional to the square of the current. The National Electrical Code permits a current density 20 to 25 per cent greater than the foregoing, the figures being given in Table 2. This would give a temperature elevation of 27° to 30° F., and still allows a considerable increase (about 60 per cent) in current above the rated value without injurious effects. This applies to rubber-covered wires, which should never be heated above 150° F., and should have a normal working temperature considerably below this limit, in order to have a margin for safety. Table 3 permits a still greater current density, and is used for wires with "weather proof" insulation, which is not so susceptible as rubber to injury by heat.
BIBLIOGRAPHY OF ELECTRICAL TRANSMISSION AND DISTRIBUTION,
Including Overhead And Underground Conductors And Interior Wiring.
Abbott, A. V., Electric Transmission of Energy, N.Y., 1895. Badt, F. B., Incandescent Wiring Handbook, Chicago, 1894. Bell, Louis, Electric Power Transmission, N.Y., 1897. Davis, C. M., Standard Tables for Electric IViremen, N.Y., 1896. Hering, Carl, Universal Wiring Computer, N.Y., 1894. Kapp, G., Electric Transmission of Energy, London, 1894. Kilgour, Swan, And Biggs, Electrical Distribution, Its Theory and Practice, London, 1893.
Noll, A., How to Wire Buildings, N.Y., 1893.
Raphael, F. C, Localization of Faults in Electric Light Mains, N.Y. and London, 1897.
Robb, R., Electric Wiring, N.Y. and London, 1896.
Russell, S. A., Electric Light Cables and the Distribution of Electricity, London, 1892.
Watson, A. E., Handbook of Wiring Tables, N.Y., 1892.
Weiller Et Vivarez, Lignes el Transmissions Electriques, Paris. 1892. CHAPTER II.
SERIES SYSTEMS OF ELECTRICAL DISTRIBUTION.
The various systems of electrical transmission and distribution are classified in the following table. They are especially selected with reference to their use in electric lighting; but they include those employed for power transmission and other electrical purposes, the same principles and methods being generally applicable.
SYSTEMS OF ELECTRICAL DISTRIBUTION.
Constant Current. Voltage usually varied. Direct Current.
1. Series arc lighting.
Usually operated at about 10 amperes and 50 volts per lamp.
2. Series incandescent lighting.
About 10 amperes and 10 to 30 volts per lamp (about 3 candlepower per volt).
3. Series incandescent lighting ("Municipal systems").
Three to 3.5 amperes and 20 to 50 volts per lamp (1 volt per candlepower).
4. Series-parallel incandescent lighting.
Similar to No. 2, but single lamps replaced by groups in parallel.
5. Direct current converter systems for incandescent or arc lighting.
Motor-dynamos in series, lamps supplied by secondary circuits.
6. 7, 8, 9, and 10. Alternating current systems corresponding to Nos. 1,
2, 3, 4, and 5.
Constant Potential. Current varies with number of lamps. Direct Current.
11. Two-wire incandescent and arc lighting (about 110 or 220 volts).
12. Three-wire incandescent and arc lighting (about 220 or 440 volts).
13. Five-wire incandescent and arc lighting (about 440 volts).
14. Two-wire with motor converters in parallel (primary 1,000 to 5,000
Single Phase Alternating Current.
15. Low tension incandescent and arc lighting without transformers.
This corresponds to No. 11. Other alternating current systems similar to Nos. 12, 13, and 14 have not been introduced.
16. High-tension incandescent and arc lighting with transformers.
Primary circuit 1,000 to 5,000 volts, two- and three-wire secondary circuits at about 50, 100, or 200 volts.
17. Very high tension systems with step-up and step-down transformers.
Long distance transmission circuit 5,000 to 25,000 volts.
Polyphase Alternating Current.
18. Two-phase system.
19. Three-phase system.
20. Monocyclic system.
For the sake of completeness, the above table includes almost every possible system of electrical distribution, but many of them are unimportant or entirely obsolete at the present time. The systems which-are now more or less generally used are Nos. 1,11, 12, 13, 14, 16, 17, 18, 19, and 20. The last three are primarily intended to operate motors, but are also employed in many cases for electric lighting.
SERIES SYSTEMS OF DISTRIBUTION.
The simplest arrangement of lamps or other devices to be supplied with electrical energy is a series system in which the current from the + terminal of the dynamo, D, passes first through
one lamp, L, and then through another, and so on, finally returning to the — terminal of the dynamo, as shown in Fig. 1. In such cases the current is usually constant, hence the expression constant current is practically synonymous with series in electrical distribution. The term high tension also applies, since the voltage usually employed is high, being equal to the sum of the pressures consumed in all of the lamps on the circuit. For example, sixty lamps are commonly placed upon a single arc-lighting circuit; and since each lamp (open arc) requires about fifty volts, it follows that the total pressure approximates 3,000 volts. The problem of designing or studying series circuits is not difficult, the path of the current being usually simple, and the current constant throughout the circuit. This last statement is only true, however, if the leakage of current is insignificant, which is generally the case in electric light and power distribution.
Distribution of Potential on Series Systems. —The potential on a series system falls throughout the circuit in direct proportion to the resistance. That is, E=IR, the difference of potential E in volts between any two points being equal to the product of the current / in amperes and the resistance R in ohms included between them. This simple fact completely covers any possible problem that can arise in connection with a series system, provided a direct current is used, and is easily applied in almost any
X~* X X X
X ; X X X X X
Fig. 2. Distribution of Potential on Series System.
case. In Fig. 2 an arc-lighting system is represented, D being the dynamo and L, L, L, the lamps, connected in series. The total difference of potential generated by the dynamo is assumed to be 1,000 volts, measured between the two brushes marked + and —. This potential falls as the current traverses the circuit, fifty volts being consumed by each of the twenty lamps. This is made up of forty-five volts actually used in the lamp itself, and a drop of five volts on the conductor between two lamps. That is, the drop on the line wire is usually about 10 per cent of the total E.M.F. The relative potential of the various points on the circuit is easily