applications where they are subjected to excessive vibration,  repeated bending, or frequent disconnection from screw  termination.  [Figure 9-116] In some instances, the wire may be capable of carrying more  current than is recommended for the contacts of the related  connector. In this instance, it is the contact rating that dictates  the maximum current to be carried by a wire. Wires of larger  gauge may need to be used to fit within the crimp range of  connector contacts that are adequately rated for the current  being carried.  Figure 9-117  gives a family of curves whereby  the bundle derating factor may be obtained. The current that causes a temperature steady state condition  equal to the rated temperature of the wire should not be  exceeded. Rated temperature of the wire may be based  upon the ability of either the conductor or the insulation to  withstand continuous operation without degradation. Determining a wiring system’s current-carrying capacity  begins with determining the maximum current that a  given-sized wire can carry without exceeding the allowable  temperature difference (wire rating minus ambient °C).  The curves are based upon a single copper wire in free air.  [Figure 9-117] When wires are bundled into harnesses, the current derived  for a single wire must be reduced, as shown in  Figure 9-118 .  The amount of current derating is a function of the number  of wires in the bundle and the percentage of the total wire  bundle capacity that is being used.  Since heat loss from the bundle is reduced with increased  altitude, the amount of current should be derated.  Figure 9-119  gives a curve whereby the altitude-derating  factor may be obtained. When aluminum conductor wire is used, sizes should be  selected on the basis of current ratings shown in  Figure 9-120 .  The use of sizes smaller than #8 is discouraged. Aluminum  wire should not be attached to engine mounted accessories  or used in areas having corrosive fumes, severe vibration,  mechanical stresses, or where there is a need for frequent  disconnection. Use of aluminum wire is also discouraged  for runs of less than 3 feet. Termination hardware should  be of the type specifically designed for use with aluminum  conductor wiring.  The following section presents some examples on how to  calculate the load carrying capacity of aircraft electrical  wire. The calculation is a step by step approach and several  graphs are used to obtain information to compute the current  carrying capacity of a particular wire. Assume a harness (open or braided) consisting of 10 wires,  size 20, 200 °C rated copper, and 25 wires size 22, 200 °C  rated copper, is installed in an area where the ambient  temperature is 60 °C and the aircraft is capable of operating  at a 35,000 foot altitude. Circuit analysis reveals that 7 of the  35 wires in the bundle ( 7 ⁄ 35 = 20 percent) are carrying power  currents near or up to capacity. Step 1—Refer to the single wire in free air curves in  Figure 9-114 . Determine the change of temperature of the wire  to determine free air ratings. Since the wire is in an ambient  temperature of 60 °C and rated at 200 °C, the change of the  temperature is 200 °C – 60 °C = 140 °C. Follow the 140 °C  temperature difference horizontally until it intersects with  wire size line on  Figure 9-113 . The free air rating for size 20  is 21.5 amps, and the free air rating for size 22 is 16.2 amps. Step 2—Refer to the bundle derating curves in  Figure 9-118 .  The 20 percent curve is selected since circuit analysis indicate  that 20 percent or less of the wire in the harness would be  carrying power currents and less than 20 percent of the bundle  capacity would be used. Find 35 (on the horizontal axis), since  there are 35 wires in the bundle, and determine a derating  factor of 0.52 (on the vertical axis) from the 20 percent curve. Step 3—Derate the size 22 free air rating by multiplying 16.2  by 0.52 to get 8.4 amps in harness rating. Derate the size 20  free air rating by multiplying 21.5 by 0.52 to get 11.2 amps  in-harness rating. Step 4—Refer to the altitude derating curve of  Figure 9-119 . Look for 35,000 feet (on the horizontal axis) since that is the  altitude at which the aircraft is operating. Note that the wire  must be derated by a factor of 0.86 (found on the vertical  axis). Derate the size 22 harness rating by multiplying 8.4  amps by 0.86 to get 7.2 amps. Derate the size 20 harness  rating by multiplying 11.2 amps by 0.86 to get 9.6 amps. Step 5—To find the total harness capacity, multiply the  total number of size 22 wires by the derated capacity  (25 × 7.2 = 180.0 amps) and add to that the number of size 20  wires multiplied by the derated capacity (10 × 9.6 = 96.8 amps)  and multiply the sum by the 20 percent harness capacity factor.  Thus, the total harness capacity is (180.0 + 96.0) × 0.20 = 55.2  amps. It has been determined that the total harness  4. Aluminum Conductor Wire 3. Harness at Altitude  2. Wires in a Harness 1. Single Wire in Free Air Maximum Operating Temperature Current Carrying Capacity Computing Current Carrying Capacity Example 1 9-71