Electrical Systems
Part 1
Voltage, current and wire size are just a few
things to consider for your wiring plan.
By Dave Prizio
Electrical wiring varies quite a bit
from one plane to another as builders
seek to place their personal stamps on
their projects with various accessories
and avionics. But most planes do have
some basic things in common. This
article and the next can serve as an introduction to several of the common elements shared by most projects and leave
a more detailed examination to a specialized publication such as Bob Nuckolls’s
AeroElectric Connection. There would
be little value in trying to compete with
such a comprehensive resource as that.
In fact, I will go so far as to say that anyone intent on wiring an Experimental
airplane should regard Nuckolls’s book
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KITPLANES May 2013
as required reading. Chapter 11 in the
FAA’s AC43.13B also contains a great
deal of useful electrical information. That
said, there are some basic things that are
still worth covering here, with the strong
suggestion that you supplement this
material with outside resources.
Equipment List
Before you can start planning your
wiring, you need to put together a
comprehensive list of every piece of electrical equipment in the airplane. The
list should identify the item, list its location and weight, and describe its voltage
and current requirements, including
recommended breaker size. Next you
RV-8 builder Eddie Rohwedder attached
the #6 alternator wire to a shunt on his
firewall. The shunt is used to measure
current flow by the ammeter in his plane.
In the photo you can also see his battery,
master relay and starter relay.
should decide if each item is something
that needs backup power, or if it can be
shut off in an emergency. For instance,
if you are going to rely solely on electronic flight instruments, you must have
backup power for these basic instruments, same for a FADEC or electronic
ignition system. These things need to
keep working no matter what happens.
Once you have put together a complete
list, things like battery and alternator
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size can be determined, as can breaker
(or fuse) size and wire size. Everything
comes back to the equipment list and
the current draw for each item.
Voltage must be considered because
many items will work with 14 or 28
volts, but some will not. You may decide
that it is best to go with a 14-volt system
but want to use one thing that requires
28 volts. In such a case, you must convert an appropriate amount of 14-volt
power to the higher voltage, or better
yet, find a substitute for the odd item
on your list. On the other hand, you
may decide to go with all 28-volt equipment, which is now quite common on
certified airplanes. But that will mean
that certain items you were considering may not work. Different battery
and alternator choices will also present themselves. Most amateur builders
opt for a 14-volt system due to its likely
lower cost, but you certainly do not
have to follow the crowd. Twenty-eightvolt systems are generally lighter, which
is why the big certified airplane builders
go with them.
Once your equipment list is complete,
you’re ready to start planning your wiring. You do not need to be an electrical
engineer to draw your own wiring diagram, but you do need to start off with
a complete knowledge of what you are
going to install. After that, follow some
basic rules and you should be successful,
even on your first attempt.
From this list, you can see that everything selected will work well with a 14-volt
system. The total connected load is only
9.75 amps, so even a 20-amp alternator
will be sufficient. Note that the primary
flight display and the com
radio both need at least 10
volts to operate, so in case
of an alternator failure, you
should expect to lose both
of these if the battery voltage
drops below 10. The weight
of your equipment has no
bearing on electrical wiring,
but it will be handy later
when you prepare the equipment list for your weightand-balance paperwork, and
that information is often
found in the same place as
the power requirements for
your electrical equipment. A
larger, more complex panel
will likely have a longer list
with more connected load,
but the concept is the same.
A ratcheting crimper is recommended for crimp-on
connectors such as these. Connectors shown here
include (bottom) ring-tongue or loop connectors and
(right) splice connectors. Red connectors are for 18- to
22-gauge wires, blue are for 16- and 14-gauge wires,
and yellow are for 12- and 10-gauge wires.
Some Basic Guidelines
Here are some basic bullet points to
guide you. From time to time, there
may be exceptions to these, but for
the most part, they apply across the
board. If you deviate from them, have
a good reason.
•Use aircraft-grade wire and
components to wire your airplane. They work better and will give you fewer headaches down the road.
•Minimize the use of the airplane structure for grounding electrical
components. The best choice is to not
use the airframe for grounding
anything. Whenever possible, run a ground wire to a common grounding
point in the airplane. You will have fewer problems in the long run, since Sample Electrical Equipment List – Cub N114DE
Item
Wiring
Voltage
Max. Current
Breaker
Weight
10-30 volts
2.0 amps
3 amps
2.75 lb.
PS Engineering PM1200
13.8-27.5 volts
0.2 amps
0.5 amps
0.75 lb.
Garmin 496 and mount
11-35 volts
1.2 amps
2 amps
1.0 lb.
Garmin SL40 com
10-40 volts
2.2 amps
3 amps
2.75 lb.
Garmin GTX 327
14-28 volts
1.9 amps
3 amps
3.1 lb.
AeroLED NS90 strobes
9-36 volts
2.25 amps
3 amps
0.5 lb.
Dynon D-180
Photos: Dave Prizio
poor grounding is the source of at least half of all electrical problems.
•Mount your battery as close to the engine as weight-and-balance
considerations will allow. Long wire runs to remote batteries are a waste of weight and money, unless a
battery must be located away from the engine for center-of-gravity adjustment.
•Use crimp-on connectors whenever possible, and avoid, or at least
minimize, the use of soldered
connections. Soldered connections require more skill to do properly and are vulnerable to breaking from vibration.
•Always think about maintenance when locating items. How are you going to access and service them later?
The wire from your favorite hardware
store can be used to wire your airplane,
but airplane wire is better. It is not that
the conductor itself is better, but the insulation on airplane wire is thinner, lighter,
stronger and more heat resistant than
common hardware-store wire. Airplanewire insulation is also more resistant to
chafing and other damage, making it
safer. The biggest drawback to hardwarestore wire in airplanes is the propensity of
its insulation to emit toxic smoke when
KITPLANES May 2013
51
Aviation-grade wires shown from the bottom are 20, 18, 16 and 14
gauge, plus 16-gauge shielded wire on top. White is readily available,
but other colors may be hard to find in the size you want.
ignited. In the event of an electrical fire,
the fumes from burning wire can overwhelm the pilot. This won’t happen with
aviation-grade wire. However, it is your
money, so the trade-off is yours to make.
Larger wire is needed to carry more
current over greater distances. Tables
in AeroElectric Connection or the
FAA’s Advisory Circular 43.13B provide charts on wire sizes for various
combinations of current and distance.
A good-sized battery located far away
from the engine may require wire as
large as #2 AWG, but most individual
items can be served with #20 AWG
wire. Some circuits can be handled
with wire even smaller than #20, but
it is not really practical to buy so many
Larger wires require a special crimper like this hydraulic crimper from
Harbor Freight Tools. This crimper is set up for the #4 wire shown here,
but other jaws come with the tool to handle #12 to #0 wire sizes.
different sizes of wire and related
connectors, so using #20 or #22 as a
minimum wire size adds little weight
and makes things simpler. Also, most
crimp-on connectors don’t work for
wire smaller than #22.
Aviation wire comes unshielded
(MIL-W-22759) or shielded (MILC-27500). Most circuits do not need
shielded wire, but some items, such as
P-leads, do need to be shielded. To be
effective, the shield portion of such wires
needs to be connected to ground on
one end only. This certainly applies to
magneto P-leads. The exception to the
one-end-connection rule is for installations where the shield is actually used
as the ground conductor, as with many
Wire Sizes for Common Firewall Forward Items
Item
Wire Size
Alternator field
#20
Alternator power to main bus – 40 amps
#8
Alternator power to main bus – 60 to 70 amps
#6 or #4
Battery wires with battery on firewall
#4
Battery wires with battery in rear of plane
#2
Magneto P-leads
#16 shielded *
Master relay from master switch
#20
Sensor wires – fuel pressure, fuel flow, oil pressure, etc.
Starter power wire
#20 or #22
#2 or #4 match battery wires
Starter relay from starter switch
*Electrically #20 shielded wire will work, but #16 is more robust for this application.
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KITPLANES May 2013
#20
Garmin avionics installations. In this
case, there’s an obvious need to connect
both ends of the shield.
Wires terminate with ring-tongue
(loop) connectors or FastOn connectors,
both of which are made by AMP and
other vendors and are crimp-on connectors. These connectors are preferred for
aviation use because soldered wire connections are prone to breaking due to the
high level of vibration found in airplanes.
A ratcheting crimper is the preferred tool
for installing these connectors. Aircraft
Spruce, B&C and other aviation or electronics vendors sell these crimpers. They
do a much better job than the crimpers
you can get at a hardware store. For miniature and sub-miniature pins, such as
are used with Canon plugs, a much more
expensive crimper is required. Again,
B&C or an electronics supply store will
be the best places to buy these.
For any connector that might be
subjected to stress or high levels of
vibration, good practice is to support
the connector with a small length of
heat-shrink tubing. This tubing is available in a wide range of sizes and colors
from most electronic supply stores. A
heat gun works best for activating heatshrink tubing, but any number of heat
sources can be used in a pinch.
Avionics
Avionics change and improve with dizzying speed these days, presenting builders with a dazzling array of potential
choices. For that reason, there will be no
attempt here to cover anything beyond
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airplane. Choices made in these areas
can affect the size of the alternator, the
possible need for a backup alternator
and the battery requirements. The battery and alternator sizes and backup
requirements are driven largely by the
loads created by the avionics, so early in
the process you need to determine these
loads within reasonable limits.
Batteries
Two books you will need to build your plane: the more general FAA AC 43.13-1B and Bob
Nuckolls’s The AeroElectric Connection, which focuses exclusively on electrical systems.
the very basics. For an education in the
latest technology, builders should attend
AirVenture or Sun ’n Fun and meet with
as many vendors as possible to find products that will fit your taste and budget.
You might also visit aircraft-building
online forums (such as vansairforce.net)
or visit with your EAA chapter members
to get a feel for what builders are using
and how the equipment has worked for
them. Many vendors, such as Aerotronics, Inc. and SteinAir, Inc., can show you
the latest avionics, answer your questions
about features and costs, and give you
current pricing on everything from individual items to complete pre-wired panels. They can also make suggestions about
what works well, what doesn’t and what
equipment other builders of your type of
airplane are using. They can recommend
which antennas to use, which becomes
an important consideration in itself in
an era when amateur-built airplanes may
be made of anything from wood and
fabric to carbon-fiber composites. Talk
to people and gather as many opinions
as you can. The instrument panel may be
the largest investment you make in your
project. It pays to do your homework.
From a firewall-forward perspective,
we care about avionics and the entire
instrument panel, because the engine
and the panel must work together as
an integrated system in the finished
The trend in batteries for amateur-built
airplanes is toward smaller and lighter.
Recombinant gas (RG) batteries, such
as Concorde’s RG-25XC, are still popular with many airplane builders, but the
move toward smaller RG batteries, such
as the PC680 Odyssey, is undeniable.
These batteries began life in the off-road
vehicle world, but their rugged, sealed
construction makes them ideal for small
airplanes too. It is now common to see
these batteries as the primary electrical
power source for many amateur built airplanes. At first it seemed they were too
small to do the job, but time has proved
them to be adequate even for the largest
four-cylinder Lycoming, the IO-390.
The 16-amp-hour capacity meets most
amateur-built airplane needs. Concorde
also makes great backup batteries.
For operators who expect to work in
very cold weather, especially if they are
A Shorai 36-amp-hour battery is shown on the left next to a typical Odyssey PC680 battery. The Shorai lithium battery has twice the
amp-hour capacity at one-third the weight.
KITPLANES May 2013
53
using six-cylinder engines, a larger battery like the Concorde is still a better
option, because it has more cold-cranking capacity than the Odyssey battery.
The larger battery also gives you the
capacity to better handle larger electrical loads from things like fancy avionics
packages and pitot heat. The downside
to the Concorde battery is its heavier
weight (23.5 pounds versus 15.4 pounds)
and its larger physical dimensions.
One thing to keep in mind with any
battery is to not discharge it below 9
volts (assuming a 12-volt battery). The
more a battery is discharged below 9
volts, the harder it will be to get it back
to full power. In fact, a fully discharged
battery may never recover from such a
trauma. As you are working on your
project, keep the battery topped off
with a charger and remember to turn
things off when you go home. If you
come back several days later and find
your battery stone cold dead, you have
no more than a 50/50 chance of bringing it back to life.
Lithium Batteries
Lithium-ion battery technology is
finding wide application in vehicles
of all kinds, but it remains largely
unproven for aircraft. Shorai Power
and Voltphreaks LLC are two companies now producing lightweight
batteries potentially suitable for Experimental aircraft. They are using a technology called Lithium Iron Phosphate
(LIP or LiFePO4) to produce 12-volt
batteries weighing only a few pounds.
The LiFePO4 construction makes the
battery more stable than other types of
lithium batteries, so they’re supposedly
less susceptible to fire when improperly
charged. However, even at that, Cessna
withdrew LiFePO4 batteries from its
Citation fleet after one caught fire on
the ground and seriously damaged a
new airplane.
The one weakness of LiFePO4 batteries is that their life expectancy
is significantly shortened by high
temperatures such as you would find
inside an engine compartment. They
therefore need to be mounted inside
the cabin for best results. We eagerly
await the results of real-life airplane
use to see if these batteries live up to
their early promise. American Legend
now includes a lithium-ion battery
made by Voltphreaks in its new Super
A big wiring project will go better if you
have everything you need in one place. An
inexpensive cart from Harbor Freight holds
wire, connectors and tools for just about
any airplane-wiring job.
Legend Cub, which should make the
company the first airplane manufacturer to employ this technology.
Flight Design, an LSA manufacturer,
is also using Li-ion technology. In the
Legend, the Li-ion battery saves more
than 10 pounds compared to a conventional Odyssey battery. Still, there
is an unknown risk here. Concorde,
for example, hasn’t entered the Li-ion
market yet because it’s not comfortable with battery safety. As we go
to press this month, Boeing’s new
787 Dreamliner has been grounded
because of two Li-ion battery fires.
Don’t Stress Over Wiring
If wiring your panel seems too daunting to contemplate, there is still the rest
of the airplane to deal with, and much
of this work is simpler and within the
means of most any airplane builder.
Do not feel intimidated by wiring just
because you have limited experience in
this area. Most builders have limited
experience in at least one or two major
areas of airplane construction when
they start. Education is half the purpose
of Experimental aviation, so dive in and
let the learning begin.
In the next article, we will continue
with the electrical system, exploring
such topics as alternators, switches
and relays, bus bars, grounding, circuit
breakers and fuses and more. We will
briefly touch on sensors too, but sensors
really deserve, and will get, their own
article later. J
AeroElectric Connection
(Bob Nuckolls) www.aeroelectric.com/Catalog/
pub/pub.html (or Amazon.com)
Aerotronics, Inc.
www.aerotronics.com
B&C Specialty Products
www.bandc.biz
Concorde Batteries
www.concordebattery.com
Lithium (LIP) batteries
www.lithiumaviationbattery.com
Shorai Power (batteries)
www.shoraipower.com
SteinAir, Inc. www.steinair.com
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KITPLANES May 2013
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