current should not exceed 55.2 A, size 22 wire should not  carry more than 7.2 amps and size 20 wire should not carry  more than 9.6 amps. Step 6—Determine the actual circuit current for each wire in  the bundle and for the whole bundle. If the values calculated  in step 5 are exceeded, select the next larger size wire and  repeat the calculations. Assume a harness (open or braided), consisting of 12 size  12, 200 °C rated copper wires, is operated in an ambient  temperature of 25 °C at sea level and 60 °C at a 20,000- foot altitude. All 12 wires are operated at or near their  maximum capacity. Step 1—Refer to the single wire in free air curve in  Figure 9-117 , determine the temperature difference of the  wire to determine free air ratings. Since the wire is in ambient  temperature of 25 °C and 60 °C and is rated at 200 °C, the  temperature differences are 200 °C – 25 °C = 175 °C and  200 °C – 60 °C = 140 °C, respectively. Follow the 175 °C  and the 140 °C temperature difference lines on  Figure 9-116 until each intersects wire size line. The free air ratings of size  12 are 68 amps and 59 amps, respectively. Step 2—Refer to the bundling derating curves in  Figure 9-118 .  The 100 percent curve is selected because we know all 12  wires are carrying full load. Find 12 (on the horizontal  axis) since there are 12 wires in the bundle and determine  a derating factor of 0.43 (on the vertical axis) from the 100  percent curve. Step 4—Refer to the altitude derating curve of  Figure 9-119, look for sea level and 20,000 feet (on the horizontal axis)  since these are the conditions at which the load is carried. The  wire must be derated by a factor of 1.0 and 0.91, respectively. Step 5—Derate the size 12 in a bundle ratings by multiplying  29.2 amps at sea level and 25.4 amps at 20,000 feet by 1.0  and 0.91, respectively to obtain 29.2 amps and 23.1 amps.  The total bundle capacity at sea level and 25 °C ambient  temperature is 29.2 × 12 = 350.4 amps. At 20,000 feet  and 60 °C ambient temperature, the bundle capacity is  23.1 × 12 = 277.2 amps. Each size 12 wire can carry 29.2  amps at sea level, 25 °C ambient temperature or 23.1 amps  at 20,000 feet and 60 °C ambient temperature. Step 6—Determine the actual circuit current for each wire  in the bundle and for the bundle. If the values calculated  in Step 5 are exceeded, select the next larger size wire and  repeat the calculations. The voltage drop in the main power wires from the 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 the 5-minute rate.  The tabulation shown in  Figure 9-121  defines the maximum  acceptable voltage drop in the load circuits between the bus  and the utilization equipment ground.  The resistance of the current return path through the aircraft  structure is generally considered negligible. However, this  is based on the assumption that adequate bonding to the  structure or a special electric current return path has been  provided that is capable of carrying the required electric  current with a negligible voltage drop. To determine circuit  resistance, check the voltage drop across the circuit. If the  voltage drop does not exceed the limit established by the  aircraft or product manufacturer, the resistance value for  the circuit may be considered satisfactory. When checking a  circuit, the input voltage should be maintained at a constant  value.  Figures 9-122  and  9-123  show formulas that may be  used to determine electrical resistance in wires and some  typical examples.  Step 3—Derate the size #12 free air ratings by multiplying  68 amps and 61 amps by 0.43 to get 29.2 amps and 25.4  amps, respectively. Allowable Voltage Drop Example 2 Figure 9-120.  Current-carrying capacity and resistance of  aluminum wire. Figure 9-121.  Tabulation chart (allowable voltage drop between  bus and utilization equipment ground). #8 #6 #4 #2 #1 #0 #00 #000 #0000 30 40 54 76 90 102 117 138 163 45 61 82 113 133 153 178 209 248 1.093 0.641 0.427 0.268 0.214 0.169 0.133 0.109 0.085  Wire size Continuous duty current  (amp) wires in bundles, groups,  or harnesses or conduits  Max. resistance  ohms/1000 feet Wire conductor temperature rating @ 105 °C  @ 150 °C  @ 20 °C  Nominal system voltage  Allowable voltage drop during  continuous operation  Intermittent operation 14 28 115 200 1 2 8 14 0.5 1 4 7 9-75