Firewall Forward: Electrical Systems, Part 1

Voltage, current and wire size are just a few things to consider for your wiring plan.

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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 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 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 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.

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.

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-eight-volt systems are generally lighter, which is why the big certified airplane builders go with them.

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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 10V. 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 weight-and-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.

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.

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 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?

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.

Wiring

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. Airplane-wire insulation is also more resistant to chafing and other damage, making it safer. The biggest drawback to hardware-store wire in airplanes is the propensity of its insulation to emit toxic smoke when 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 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.

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 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 (MIL-C-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 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 heat-shrink 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 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 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.

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.

Batteries

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.

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.

For operators who expect to work in very cold weather, especially if they are 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 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.

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Dave Prizio
Dave Prizio has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed three—a GlaStar, a Glasair Sportsman, and a Texas Sport Cub—and is helping a friend build an RV-8. When he isn’t building something, he shares his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also an A&P mechanic, Designated Airworthiness Representative (DAR), and was a member of the EAA Homebuilt Aircraft Council for six years.

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