HANDS ON
What’s Your Voltage?
Choosing between a 14-volt and 28-volt system
BY RICHARD KOEHLER
EARLY IN THE BUILDING of an aircraft you will have to decide whether
to go with a 14- or 28-volt system. The advantage of a 14-volt system
is that it is common with your car, which means it is compatible with
the supplies available there, and it’s also common with most small
The 28-volt system is used on larger aircraft because it uses significantly smaller wire, reducing
the overall weight.
aircraft. The advantage of the 28-volt system
is that it uses wire that is significantly
smaller (around 80 percent less weight) than
the 14-volt system. This is why the higher
voltage system is used on larger aircraft.
For those interested in the details, a
14-volt system is designed to operate at 13. 75
volts of direct current (DC). This should be
the output of the alternator when the engine
is operating at a high enough rpm for the
alternator to carry the electrical load. The
battery for this system is rated at 12 volts,
and the higher voltage of the alternator
allows it to recharge the battery. For a
28-volt system the same idea applies, but the
fully charged battery puts out 24 volts, and
the alternator produces 27. 5 volts to
recharge it.
With all of the new avionics our aircraft
are acquiring these days, even smaller single-engine certified aircraft are coming with
28-volt systems, likely for the weight savings. If you estimate that a plane has 1,000
feet of wire in it, and you reduce the wire
size from 12 gauge to 20 gauge with a 28-volt
system, you’d save 16 pounds on the weight
of the aircraft!
