nuts & bolts
building basics
12 or 24 Volts
Choosing the best system for your aircraft project
GEORGE R. WILHELMSEN, EAA 442664
Working through the various stages of building
your own airplane can be an exciting and
challenging experience. Whether your plane
is built from a kit, plans, or scratch, there are dozens of
decisions to be made as far as which direction to go with
the construction.
One of the areas that
many builders struggle with
is which electrical system
to select for their aircraft—
either a 12-volt system
or a 24-volt system. To be
fair, each system has its
strengths and weaknesses.
The key to knowing which
system to use is first in what
you need, and second, in
what you want. To help you
sort out the best system for your aircraft, I will provide
the pros and cons of each electrical system.
12-volt System Advantages
There are several advantages to using a 12-volt electrical
system for your custom plane. The strongest reason
for using 12 volts is the availability of components.
There is a plethora of alternators, batteries, and circuit
breakers available in the new and used market for
12-volt systems. Whether it is in the form of new gear,
overhauled equipment, or materials salvaged from
aircraft, it is readily found and is generally available for
reasonable prices.
There are other advantages of a 12-volt system. As an
example, since the majority of cars use 12-volt systems,
you have a good source for battery chargers. The same
goes for jump-starting the aircraft—since cars have the
same voltage, they can be used to jump-start the plane in
a pinch. In addition, the majority of avionics these days,
including the legacy gear that you may intend to use,
were designed to operate on 12-volt systems.
The strongest reason for using a
12-volt system is the availability of
components. There is a plethora of
alternators, batteries, and circuit
breakers available in the new and used
market for 12-volt systems.
12-volt System Disadvantages
The primary disadvantage of a 12-volt system is the
actual voltage of the system. In both Ohm’s law and the
Power law, current and power are inversely proportional
to voltage. That is to say, as the voltage goes down, the
current required or power used goes up for non-pure
inductive components.
This simply means that you will have a greater current
and power demand for all equipment powered by a
12-volt system, roughly double what it would be for a
comparable component on a 24-volt system. The higher
current and power draw means you will have to buy larger
wires and de-rate the wires more when you calculate their
bundling, as called out in Advisory Circular (AC) 43.13-
1B, Acceptable Methods, Techniques, and Practices.
