Avionics News July 2020 - 76

Figure 3: FAA Figure 11-5: Bundle derating curves, which is used to determine the derating factor for wires in a bundle.

THEORY & PRACTICE
Continued from page 75

chart are amperage draws. Our device draws 10 amps, so
we will draw our red line rightward to the 10-amp diagonal
and then go straight down. It seems to work perfectly,
giving us a wire size of 20 AWG.
20 AWG would work on the ground at 68 degrees, in a
bundle where no other wires are carrying current. These
perfect conditions won't exist. The aircraft could be
flying at 15,000 feet. The temperature could be as high
as 122 degrees Fahrenheit (50 degrees Celsius) and two
of the wires in our bundle carry current. This means we
aren't done yet.
Let's move on to Figure 3, which in AC 43-13-1B is
known as Figure 11-5. We need to be concerned with the
combined heat of multiple wires within the same bundle.
Our wires are in a 10-wire bundle, and two of them
(20%) are carrying current. On this chart, we start at the
bottom with the line representing 10. We move up to the
20% line and then move to the left. Once on the left side
of the chart, we have a decision to make. The FAA allows
technicians to interpolate, but people tend to interpolate a
little differently. To avoid any possibility of an argument
with someone over whether or not our line arrived at 0.75
or 0.76, I advise people to move downward to the next
figure marked clearly. In this case, 0.7. This move is in
the direction of more safety, and no one can make any
argument we are interpreting this in an unsafe manner.
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2020

So what do we do with the 0.7? We simply take the 10
amps our device draws and divide it by 0.7. This gives
us an answer of 14.29. For combined heating purposes,
we will need to select a wire that can carry 14.29 amps
safely. But before we go back to Figure 2, we need to do
one more derating.
Figure 4, which is FAA Figure 11-6, will help us
compensate for altitude. The insulation of our wire works
with air. In addition, air helps with dissipation of heat.
Our sample aircraft can reach an altitude of 15,000 feet
where there is much less air than at sea level. On this
chart, we start at the bottom and find the line associated
with the service ceiling of our aircraft. We move upward
from the 15,000-feet line to the reference line and then
left to the derating factor. Like the bundle chart, if my
work ends up between lines, I'll move downward (in the
safe direction) toward the next clearly marked line. In
this case, the line comes out clearly at 0.925. Now we
divide again.
Previously, we derated to 14.29 amps. So now, we
divide 14.29 by 0.925. Our answer is 15.45 amps, and
we will need to find a wire capable of handling 15.45
amps. Now it's time to go back to Figure 2. We make
a continuation from our red line. We continue to just
beyond the 15-amp line. Now we have ended up between
the 18 and 16 AWG wire sizes. We move to the right and
find that now we need to use 16 AWG wire.
I've known many technicians who stop here, but
they shouldn't. Below the note in Figure 2, there is a

Avionics News July 2020

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