Avionics News July 2020 - 75

Figure 2: FAA Figure 11-2: The continuous flow conductor chart. This chart can be used to determine
the wire size necessary for a certain length of wire and current draw. Alternatively, it can be used to
determine how long a wire could be, starting with a wire size, and current draw.

for 115-volt systems. At 200 volts, the continuous allowable
voltage drop is 7 volts and the intermittent allowable
voltage drop is 14 volts.
When resistance drops voltage, heat is generated.
Modern avionics equipment may be tolerant to low voltage
operation, but we still need to worry about heat. Staying
below the voltage drop requirements is the first step in
ensuring we don't generate a dangerous amount of heat
in the wiring. Not only do we need to be concerned with
the resistance of the wire, but also the effects of altitude,
ambient temperature, and combined heat of multiple wires.
Fortunately, AC 43.13-1B has charts to help.
For explanation purposes, we will use a 28-volt aircraft, with
a service ceiling of 15,000 feet. We plan to have two wires
(power and ground) supplying power to a device that draws
10 amps continuously and will require 10 feet of wire to make
the connection. In addition, we are planning to route these
wires as part of a bundle with eight other wires. Our wire has a
heat rating of 150 degrees Celsius, and the environment in the

Figure 2A: The calculation required to determine T2

aircraft could get as hot as
50 degrees Celsius.
I tell my students to
start with AC 43.13-1B
Figure 11-4a, shown
as Figure 1. In order to
cover a range from zero to
1,000 amperes, the FAA
has provided two charts.
These charts do not take
length into consideration,
so technicians get data
from them that may seem
rather strange. In our
example, we start with
the difference between
the wire rating and the
highest possible ambient
temperature. In this
case, this difference is
100 degrees Celsius. We
draw a straight line to the
right. Then we start at the
bottom of the chart, the
abscissa, and draw a straight line up from the 10-amp point.
We observe where they cross and then select the next wire
shown to the right, which in this case is 24 AWG. The chart
warns us 24 AWG is not to be used as a single wire. But
that's OK; ours is in a bundle. We will find out soon that
24 AWG wire is too small for us to use. We can move on to
the next chart. As long as our next chart gives us a wire size
greater than 24 AWG, we will not have any worries.
As you look at these charts, please note the wire sizes
shown are the wire sizes available. There are no oddnumbered sizes.
Time to move on to Figure 2, which in AC 43.13-1B is
called Figure 11-2. This is another one of two charts. This
figure is for continuous current draw; its companion chart
is designed for intermittent (less than two minute) current
draw. On the left side of Figure 2, we have wire lengths
shown in columns, with system voltage at the top and
allowable voltage drop on the bottom. Since our example
aircraft has a 28-volt system, we will use the column
with 28 at the top and 1 at the bottom. We
estimate or interpolate where 10 feet of
wire starts and draw a line toward the right,
as shown in red. The diagonal lines on the
Continued on following page
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Avionics News July 2020

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