Specifications

AC 43.13-1B CHG 1 9/27/01
Page 11-28 Par 11-68
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:
L
1
=24 feet maximum run length for
12 gauge wire carrying 20 amps from fig-
ure 11-2.
I
max
= 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.
ft 7.16
366
C)(24ft)5.254(
L
366
6108
C)(131+C)(234.5
C)(24ft)5.254(
L
)(T+C5.234
)C(L5.254
L
C 131=C)(-540)(150+C50
37/20( C)50-C(200+C50T
o
2
oo
o
2
2
o
1
o
2
ooo
ooo
2
==
==
=
==
AA
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