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9/27/01 AC 43.13-1B CHG 1 multiplied by the derated capacity (10 x 8.8 = 88 amps) and multiply the sum by the 20 percent harness capacity factor. Thus, the total harness capacity is (165.0 + 88.0) x 0.20 = 50.6 amps. It has been determined that the total harness current should not exceed 50.6 A, size #22 wire should not carry more than 6.6amps and size #20 wire should not carry more than 8.8 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. b. Example 2. Assume a harness (open or braided), consisting of 12, size #12, 200 °C rated copper wires, will be operated in an am bient of 25°C at sea level and 60°C at a 20,000-foot altitude. All 12 wires will be op erated at or near their maximum capacity. STEP 1: Refer to the “single wire in free air” curve in figure 11-4a, determine the tempera ture difference of the wire to determine free air ratings. Since the wire will be in ambient 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 respec tively. Follow the 175 °C and the 140 °C tem perature difference lines on figure 11-4a until each intersects wire size line, the free air rat ings of size #12 are 68 amps and 61 amps, re spectively. STEP 2: Refer to the “bundling derating curves” in figure 11-5, the 100 percent curve is selected because we know all 12 wires will be carrying full load. Find 12 (on the abscissa) since there are 12 wires in the bundle and de termine a derating factor of 0.43 (on the ordi nate) from the 100 percent curve. 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 26.2 amps, respectively. STEP 4: Refer to the “altitude derating curve” of figure11-6, look for sea level and 20,000 feet (on the abscissa) since these are the conditions at which the load will be car ried. The wire must be derated by a factor of 1.0 and 0.91, respectively. STEP 5: Derate the size #12 in a bundle rat ings by multiplying 29.2 amps at sea level and 26.6 amps at 20,000 feet by 1.0 and 0.91, re spectively, to obtained 29.2 amps and 23.8 amps. The total bundle capacity at sea level and 25 °C ambient is 29.2x12=350.4 amps. At 20,000 feet and 60 °C ambient the bundle capacity is 23.8x12=285.6 amps. Each size #12 wire can carry 29.2 amps at sea level, 25 °C ambient or 23.8 amps at 20,000 feet, and 60 °C ambient. 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 ex ceeded, select the next larger size wire and re peat the calculations. Par 11-67 Page 11-29PDF Image | AFS-640
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