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
AC 43.13-1B CHG 1
9/27/01
The size #14 wire selected using the methods
outlined in paragraph 11-66d is too small to
meet the voltage drop limits from figure 11-2
for a 15.5 feet long wire run.
STEP 3: Select the next larger wire (size #12)
and repeat the calculations as follows:
L1=24 feet maximum run length for
12 gauge wire carrying 20 amps from fig
ure 11-2.
Imax = 37 amps (this is the maximum current
the size #12 wire can carry at 50 °C ambient.
Use calculation methods outlined in para
graph 11-69 and figure 11-4a.
T2 = 50 oC + (200 oC - 50 oC) ( 20 A / 37 A =
50 oC + (150 oC)(-540) = 131 oC
254.5 oC(L1 )
L2 =
o
234.5 C + (T2 )
L2 =
(254.5 oC)(24ft)
o
o
= 6108
(234.5 C) + (131 C) 366
(254.5 oC)(24ft)
L2 =
= 16.7 ft
366
The resultant maximum wire length, after ad
justing downward for the added resistance as
sociated with running the wire at a higher tem
perature, is 15.4 feet, which will meet the
original 15.5 foot wire run length requirement
without exceeding the voltage drop limit ex
pressed in figure 11-2.
11-69. COMPUTING CURRENT CARRY-
ING CAPACITY.
a. 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, will be installed in an
area where the ambient temperature is 60 °C
and the vehicle is capable of operating at a
60,000-foot altitude. Circuit analysis reveals
that 7 of the 35 wires in the bundle
(7/35 = 20 percent) will be carrying power cur
rents nearly at or up to capacity.
STEP 1: Refer to the “single wire in free air”
curves in figure 11-4a. Determine the change
of temperature of the wire to determine free air
ratings. Since the wire will be in an ambient
of 60 ºC and rated at 200° C, the change of to
temperature is 200 °C - 60 °C = 140 °C. Fol
low the 140 °C temperature difference hori
zontally until it intersects with wire size line
on figure 11-4a. 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 11-5, the 20 percent curve is selected
since circuit analysis indicate that 20 percent
or less of the wire in the harness would be car
rying power currents and less than 20 percent
of the bundle capacity would be used. Find
35 (on the abscissa) since there are 35 wires in
the bundle and determine a derating factor of
0.52 (on the ordinate) 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 11-6, look for 60,000 feet (on the ab
scissa) since that is the altitude at which the
vehicle will be operating. Note that the wire
must be derated by a factor of 0.79 (found on
the ordinate). Derate the size
#22 harness rating by multiplying
8.4 amps by 0.79 to get 6.6 amps. Derate the
size #20 harness rating by multiplying
11.2 amps by 0.79 to get 8.8 amps.
STEP 5: To find the total harness capacity,
multiply the total number of size #22 wires by
the derated capacity (25 x 6.6 = 165.0 amps)
and add to that the number of size #20 wires
Page 11-28
Par 11-68