not insulated. Thus, to protect the insulation from too much  heat, the current through the conductor must be maintained  below a certain value. When electrical conductors are installed in locations where  the ambient temperature is relatively high, the heat generated  by external sources constitutes an appreciable part of the total  conductor heating. Allowance must be made for the influence  of external heating on the allowable conductor current, and  each case has its own specific limitations. The maximum  allowable operating temperature of insulated conductors  varies with the type of conductor insulation being used. Tables are available that list the safe current ratings for  various sizes and types of conductors covered with various  types of insulation. The chart in  Figure 4-76  shows the current  carrying capacity and resistance of copper wire continuous  duty wire in bundles at various temperature ratings.  Although silver is the best conductor, its cost limits its use to  special circuits where a substance with high conductivity is  needed. The two most generally used conductors are copper  and aluminum. Each has characteristics that make its use  advantageous under certain circumstances; also, each has  certain disadvantages. Several factors must be considered in selecting the size of  wire for transmitting and distributing electric power. One  factor is the allowable power loss (PR loss) in the line. This  loss represents electrical energy converted into heat. The  use of large conductors reduces the resistance and therefore  the PR loss. However, large conductors are more expensive  initially than small ones; they are heavier and require more  substantial supports. Copper has a higher conductivity; it is more ductile, can be  drawn out, has relatively high tensile strength, and can be easily  soldered. It is more expensive and heavier than aluminum. Although aluminum has only about 60 percent of the  conductivity of copper, it is used extensively. Its light weight  makes possible long spans, and its relatively large diameter  for a given conductivity reduces corona, the discharge of  electricity from the wire when it has a high potential. The  discharge is greater when smaller diameter wire is used than  when larger diameter wire is used. Some bus bars are made of  aluminum which has a greater radiating surface than copper  for the same conductance. The characteristics of copper and  aluminum are compared in  Figure 4-77 . A second factor is the permissible voltage drop (IR drop)  in the line. If the source maintains a constant voltage at  the input to the line, any variation in the load on the line  causes a variation in line current and a consequent variation  in the IR drop in the line. A wide variation in the IR drop  in the line causes poor voltage regulation at the load. The  obvious remedy is to reduce either current or resistance. A  reduction in load current lowers the amount of power being  transmitted, whereas a reduction in line resistance increases  the size and weight of conductors required. A compromise is  generally reached whereby the voltage variation at the load  is within tolerable limits and the weight of line conductors  is not excessive. The voltage drop in the main power cables from the aircraft  generation source or the battery to the bus should not exceed  2 percent of the regulated voltage when the generator is  carrying rated current or the battery is being discharged at a  5-minute rate. The 5-minute rate in this case means that the  battery should last a minimum of 5 minutes in an emergency,  with all battery operated equipment running.  Figure 4-78 shows the recommended maximum voltage drop in the load  circuits between the bus and the utilization equipment. The resistance of the current return path through the aircraft  A third factor is the current carrying ability of the conductor.  When current is drawn through the conductor, heat is  generated. The temperature of the wire rises until the  heat radiated, or otherwise dissipated, is equal to the heat  generated by the passage of current through the line. If the  conductor is insulated, the heat generated in the conductor is  not so readily removed as it would be if the conductor were  4-47 Voltage Drop in Aircraft Wire and Cable Factors Affecting Selection of Conductor Material Factors Affecting the Selection of Wire Size number of strands in the wire or cable. Figure 4-75.  Wire gauge. 0 1 2 3 4 5 9 6 7 8 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Wire