applications where they are subjected to excessive vibration, repeated bending, or frequent disconnection from screw termination. [Figure 9-116] 

## Current Carrying Capacity 

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. 

## Maximum Operating Temperature 

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. 

1. Single Wire in Free Air 

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] 

2. Wires in a Harness 

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. 

3. Harness at Altitude 

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. 

4. Aluminum Conductor Wire 

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. 

## Computing Current Carrying Capacity 

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. 

## Example 1 

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 

