FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 11. Aircraft Electrical Systems | 5. Electical Wire Rating | 11-69. Computing Current Carrying Capacity
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 6.6 amps and size
#20 wire should not carry more than 8.8 amps.
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 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 4: Refer to the “altitude derating curve”
of figure 11-6, 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.
STEP 2: Refer to the “bundling derating
curves” in figure 11-5, the 100 percent curve is
Par 11-67
Page 11-29