Avionics News July 2020 - 74

series
B Y

T H O M A S

I N M A N

Selecting Wire
T

he engineers at aircraft and avionics producing
14-volt system, the voltage drop is too high for safety.
companies do a fine job, but they can't
Avionics and other devices are designed to work properly
anticipate everything. Occasionally, technicians
when connected to a proper voltage source. In the example
need to do some engineering in the field. This month,
above, 13.75 volts on the buss would drop to below 13.3
we look at the Federal Aviation Administration guidance
volts at the device. Most modern avionics are quite tolerant
for choosing wire of an appropriate size. In most cases,
to voltage variations, but some of the older equipment is
technicians can follow the manufacturers' documentation.
not. The FAA places limits on allowable voltage drops. In
But sometimes, the situation is different, and the technician
14-volt systems, the allowable drop is 0.5 volts for anything
must be the one to determine whether or not a wire is safe.
that can be operated for more than two minutes and 1.0
FAA Advisory Circular 43.13-1B contains charts and
volts for any intermittent load. An intermittent load is
instructions on selecting wire. Over the years, this guidance
defined as a current draw lasting under two minutes. These
has changed. Many years ago, there were only two charts
values are doubled for 28 -volt systems and doubled again
and a formula. Now, there are six charts and a
formula.
When connecting equipment to the voltage
buss, there are two main factors to which we
must pay attention. The first is voltage drop.
Often, we don't think of wire as having any
resistance. But in fact, it does, even though it
will be a small amount. Small diameter wire
has more resistance than large diameter wire.
For example, an 18 American wire gauge wire
constructed of stranded copper may have a
resistance of 6.44 Ω per 1,000 feet. Copper
stranded wire of 20 AWG (smaller) has a
resistance of 10.25 ohms per 1,000 feet. If
we have 10 feet of 20 AWG wire between the
buss and the equipment we are connecting,
the wire will have a resistance of 0.1025 Ω. If
our equipment draws 5 amps, we will have a
Figure 1: FAA Figure 11-4a:
voltage drop of 0.5125 volts. If this were in a
A temperature derating chart for a single copper wire in free air.
74

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Avionics News July 2020

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