rly protected when there is between it and the a fuse which will be melted by a current which -r than the maximum safe carrying capacity" onductor. rings us to the subject of carrying capacity of The term "safe carrying capacity," or, as we nmonly say, "carrying capacity," of a wire is uite commonly misunderstood. It has, howery definite meaning. The safe carrying capawire is the maximum current in amperes which wed to carry by the underwriters. The safe capacity for any ordinary size of wire is given de, in the table which is printed above. This s the maximum currents that the various sizes will carry without unduly overheating. The ve been determined by calculation and expeWe have stated that a wire is unduly overhen it becomes hot enough to injure the material with which it is covered. Of course, of insulation will stand a higher temperature jury than another, and again, the temperaich a given current will heat a certain sized lepend upon the original temperature of the :he opportunity which it has for cooling. We owever, have a different table of carrying for every kind of wire and construction, and given above is one which leaves a margin of any ordinary construction. Upon examining table, the reader will find that apparently o direct relation between the size of a wire current which it is allowed to carry. It may be well, therefore, to take a little space to show why this is so; as such an explanation will show the importance and necessity of a table of maximum safe-carrying capacities. We have seen that any conductor offers a resistance to the flow of a current of electricity in a manner analogous to that in which friction opposes the flow of water in a pipe. We have also seen that it is necessary to expend work to overcome resistance, and that this work represents a loss of energy, the same as when work is done to overcome friction. This lost energy, like the energy lost in overcoming friction, is transferred into heat, so that the electrical energy lost in a wire has the effect of raising the temperature of the wire. The calculation of the loss of energy in a wire carrying an electrical current is a much simpler matter than the calculation of the loss from friction in a water pipe. The loss per foot in a given wire is proportional to the square of the current, i. e., the loss in power or the heat generated will be four times as great with a current of two amperes as with a current of one ampere. Again, with a given current the loss is inversely proportional to the sectional area or "cross section" of the wire; so that, if we have two wires, one one-half of an inch in diameter and another one-fourth of an inch in diameter, the sectional area of the first wire will be four times as great as that of the second, and if the two wires carry the same current, the loss in the larger wire will be only one-fourth of that in the thinner one. It would seem at first thought, therefore, that the larger wire could be allowed to carry four times the current of the smaller one without becoming any hotter. The prob- lem is not, however, quite as simple as that. take the example of the two wires, the first one-half inch and the second one-fourth inch in diameter. Suppose that each wire carries a current of one ampere; then the loss in the larger wire will be but one-fourth that in the smaller wire, as the larger wire. has four times the area, and therefore one-fourth the resistance of the smaller. If now we increase the current in the large wire to four amperes we shall increase the loss, or, what is the same thing, we shall increase the amount of heat generated not four but sixteen times, or in the ratio of the square of the currents. The result will be, therefore, that the heat generated will be four times that generated by one ampere in the small wire. As the larger wire has four times the mass of metal in the smaller one, it might still seem as if the temperature of the two wires would be the same; but the temperature of the wire is determined, not only by the amount of heat generated, but by the amount of heat which the wire loses in a given time. When the amount of heat generated in a given time is equal to that lost in the same time, the wire comes to a fixed temperature, and the faster the heat can be conducted or radiated from the surface of the wire the lower will be that temperature. (This is the reason that we have two tables of carrying capacity, one for wire suspended freely in the air and another where the wires are enclosed so that the heat does not get a chance to radiate into the cooler air.) The amount of heat generated in our larger wire is four times that generated in the smaller one, but the amount of heat lost by radiation is proportional to the surface exposed, and the larger wire has not four, but only two times the exposed surface of the wire of onehalf the diameter. The result is that the large wire will get hotter with four amperes than the small wire will with one ampere. It follows, therefore, that if one wire has twice the sectional area of another, it will have something less than twice the safe-carrying capacity. Upon consulting the table, we will see that while a No. o wire has almost exactly twice the area of a No. 3 wire, the No. 3 wire is allowed to carry 75 amperes, while the No. o wire is allowed to carry only 125 amperes. The greater the difference in the sizes, the more conspicuously is this shown in the table. While a No. Io wire may carry 25 amperes, a No. o wire (with ten times the sectional area) is allowed to carry only five times that current. The calculation of the relative currents allowed for different sizes of wires involves considerable figuring, and a table of "capacities" is a most useful thing, and no wire used in electrical construction should ever be allowed to carry a greater current than that allowed in the table. The table of capacities shows what current a wire may carry with safety, but the wireman must not make the mistake of thinking that the table is any guide to the size of wire to be selected for any particular circuit. The wire must not be smaller than that given in the table, but in many cases the wire must be larger in order to give the proper pressure at the lamps. The temperature of a wire is determined by its size, the current which it carries and rroundings, it is independent of the length. The ount of energy lost in a wire is proportional to , so that in calculating a wire from an engistandpoint, the length of the circuit must be ed. The proper way to select the size of wire for any particular case is to calculate the size. 1 give the maximum loss consistent with good nd then consult the table of capacities." If lated size is larger than that allowed by the he current to be carried, use it; if it is smaller size allowed by the code, then take the size the table. le of safe-carrying capacities does not apply sed in the construction of dynamos or motors wires used in rheostats. The rules which we dy considered demand that the installation of rheostats, etc., shall be so made that they me hot without causing a hazard. |