Firewall Forward: Electrical Systems, Part 2

Alternators, busses, breakers and fuses form the core of aircraft electrical systems.



Even though many builders are skeptical of their ability to deal with electrical issues, basic wiring is really not that complicated once you get into it. And the good news is that the tools for electrical work are not expensive, unlike those required for many other types of aircraft construction.

The B&C L-40 alternator can meet the needs of most amateur builders and sets the standard for quality in the industry. It requires an external voltage regulator and over-voltage protection module. Some Plane Power units have these features built in.

In a previous article in this series, we dealt with basics, such as the equipment list and basic wiring. We looked at batteries in more depth and mentioned avionics briefly. In this article, we’ll dig into the major components of the electrical system such as alternators, sensors, switches and relays, busses and bus bars, grounding, circuit breakers and fuses, and offer some final thoughts.


Lycoming engines used to come with alternators by Prestolite or Electrosystems (now Hartzell Engine Technologies) and others that are quite heavy, but new Lycomings for amateur builders come with either no alternator or can be ordered with lightweight alternators by suppliers such as Plane Power or B&C. The B&C models L40 and L60 are especially well-suited to the needs of amateur builders, providing 40 and 60 amps of power respectively, and they have set the standard in the industry for quality. Both Plane Power and B&C also make smaller backup alternators that are driven off the vacuum pump mount on most Lycoming engine models. These are becoming increasingly popular as more builders go all-electric with their flight instruments.

An old-style Prestolite alternator tips the scale at 12.5 pounds. Replacement units by B&C or Plane Power can reduce that by up to 50%.

A budget-minded alternative to these aviation alternators are the automotive Nippondenso (ND) alternators. Some amateur builders use these common auto parts for their electrical needs. At one time, Van’s sold these units to their RV customers. They provide a low-cost alternative for the amateur builder. If you are going to use an ND alternator, it is important to be sure that it has the correct rotation for airplane use, since many units do not. Many, but not all, ND alternators have an internal voltage regulator, but none have any overvoltage protection. To be safe, you should add an overvoltage protection circuit when using an ND alternator because some types of alternator failures could absolutely destroy your expensive avionics. Bob Nuckolls’ book, The AeroElectric Connection, has information on this modification.

As mentioned above, alternators can come with internal voltage regulators or not. It is important to know what you have in this regard. The Plane Power experimental 60-amp alternator comes with internal voltage regulation and overvoltage protection. Their 70-amp unit comes with or without internal regulation. The B&C units look to external devices to perform both of these functions, as do the Kelly Aerospace alternators (now Hartzell Engine Technologies). B&C has a line of very sophisticated voltage regulators and overvoltage modules, but these items are external to the alternator and come at an added cost. If you do use an external voltage regulator, it is best to mount it in the cabin, away from engine heat and possible moisture leaking through the cowling. As always, be sure to consult with the manufacturer for proper installation instructions.

Here are some possible choices for your consideration:

The B&C model SD-8 has a permanent magnet for pretty much fail-safe back-up protection. The benefit of this is that it needs no power to energize the field, but because of this, it is critical that overvoltage protection be used.

B&C makes a backup alternator with a permanent magnet that mounts on the vacuum pump pad on Lycoming engines. The magnet allows it to operate without power to the field windings, unlike traditional alternators that must have power to energize the field.

When you go to select an alternator for your project, the big question is, how big (amps) an alternator do I need? When dealing with airplanes, the answer is always just big enough to get the job done, but let’s get more specific. Look at the equipment list. What is the total amperage of everything on the list if it were all switched on at once? Add to that any likely items that you plan to install and get a total connected load. Do not use the sum total of the circuit breakers; use the actual current draw of each item. That number should not exceed 80% of the rated capacity of the alternator you choose. For example, if your total connected load is not more than 48 amps, then a 60-amp alternator will do the job. However, if your load is only 30 amps, you should really be looking at a 40-amp alternator rather than a 60-amp model. There is really no advantage to using an alternator that is too big. It just adds extra weight.

Use a torque wrench to set the alternator belt tension. The belt should start to slip when torque exceeds the value listed in Lycoming Service Instruction 1129A.


Wiring engine sensors depends greatly on the sensors and gauges or engine management system being used. Any plan for such wiring must begin with the manufacturer’s installation instructions. Many heat sensors (CHT and EGT) have leads that should not be cut or altered in any way, although this is not always the case. Be sure to check with the manufacturer before altering these wires. It is also important to remember with any engine management system that programming is required prior to use. Many systems have settings for different brands of sensors, thus incorrect programming could cause erroneous readings.

Since most of the wiring forward of the firewall will be for engine sensors, it is important to think about how these wires will be routed, protected, and secured in the engine compartment. Here are a few good general rules in that regard.

• Keep all wires at least 1½ inches away from exhaust pipes and mufflers; farther away is better.

• Neatness counts. Bundle groups of wires together with ties or spiral wrap to keep them neat and tidy.

• Zip ties with stand-offs can be used to secure bundles of wires to engine mount tubes or other handy places, but cushioned Adel clamps are much better. Never simply zip tie wires to anything where abrasion between wires and other objects could produce damage over time.

• Support crimped connections with heat-shrink tubing any place where frequent movement is likely, such as on wires from the airframe to the engine.

Please note that a future article will deal specifically with engine sensors, so both articles should be considered together as you contemplate tackling your electrical system.

Master Switches and Relays

Relays and switches go together. The master switch controls the master relay and the power to the alternator field windings. The ignition switch controls the magnetos (or electronic ignition) and the power to the starter relay. These four components all work together to get the airplane running. Since the ignition switch has already been covered at some length in a previous article, let’s look at the other components.

The master switch usually comes split into two switches joined together in one unit. Half the switch controls the master relay and the other half controls the alternator by switching power to the alternator field windings. Without power to the field, the alternator will not produce current. What may not be intuitively obvious is that the battery side of the switch controls the ground (negative voltage) connection to the master relay, but the alternator side of the switch controls positive voltage to the alternator. With a remote battery installation, the advantage of this arrangement becomes more obvious. By switching the ground, it is then not necessary to run a positive lead all the way up to the master switch and then back to the master relay, which should always be mounted as close to the battery as possible. However, even with a firewall-mounted battery, it is necessary to switch the ground to activate the master relay, because standard practice is to have the master relay internally connected to the positive lead coming into the relay.

Van’s recommends connecting the master relay and the starter relay together with a copper bus bar. This makes for a clean installation on this under-construction RV-8. The positive lead from the battery goes to the master relay. The bus bar connects the master relay to the starter relay. The positive lead (not shown) goes to the battery. A partially wired ammeter shunt is visible below the master relay.

The alternator side of the master switch gets power from the main bus through a circuit breaker or fuse. The power then goes directly to the alternator if it is internally regulated or, if it is not, to the voltage regulator first and then to the field of the alternator. An overvoltage relay may be added to this circuit in front of the voltage regulator or internal to the regulator or alternator, depending on the particular installation. In any case, overvoltage protection should be provided somewhere to protect your avionics.

The master relay typically connects to the battery with a very short lead. The purpose of the master relay is to allow a small panel-mounted switch to control the heavy current from the battery. The solenoid inside the relay has an internal connection to the positive powered (hot) side of the relay. The small external post connects to the master switch through which it completes the circuit to ground, thus switching on the relay. A relay that does not have an internal positive connection can also be used, but it is not standard practice for the master relay. In that case, the connection to positive voltage must be made externally. In any case, it is important that the master relay be one that is rated for continuous duty.

The starter relay also allows you to control the very large current flow (300 amps or more) with a small switch. In this case, the starter switch controls the positive side of the relay. For this application, it is common to see a relay with two large posts and two small posts. The large posts are for positive voltage in from the battery and out to the starter, and the small posts connect to the solenoid that activates the switch. One small post connects to ground and the other connects to the lead from the starter switch (positive). Not all relays are wired the same, so it is a good idea to test yours before wiring it into your plane.

It is good practice to install a diode between the positive and negative posts of the starter and master relays to extend the life of the relays. Any 1N5400 or 1N4000 series diode can be used, and should be connected so the cathode end (end with silver stripe) is towards the positive terminal.

The latest development in relays is the solid state relay. It has no moving parts, and at least in theory, should enjoy a much longer service life. The downside is the $145 price tag. These can be separate units, such as those made by B&C, or part of an integrated electrical management system such as those made by Vertical Power.

Busses and Bus Bars

An electrical bus is a grouping of devices that can be switched on and off together. A bus bar is a physical device, usually a copper bar that ties these devices together and connects them to a common power source. Most bus bars are on the positive side, but ground (negative) bus bars are also useful. Almost all airplane electrical systems have a main bus energized by the master switch. This connects to the various circuit breakers or fuses that power the many electrical components in the airplane. Many airplanes also have an avionics bus controlled by an avionics master. This allows all the radios to be switched on or off together, saving wear and tear on the individual on/off switches of the various components. Like the master switch, the avionics master controls a relay that handles the large current load to the avionics bus, thereby allowing for a smaller switch.

In this homemade ground bus in a Sportsman, the large bolt goes through the firewall to the engine ground cable.

With many airplanes now going all-electric, it is becoming more common to see an essential bus added to the system. This allows the pilot to switch off all non-essential items quickly to preserve battery power in the case of an alternator failure or other mishap. For those who want maximum protection, the essential bus can be connected to a back-up battery and/or backup alternator.

Shown here is a typical shunt used for measuring current (amps). The small wires go to the engine management system or ammeter and both should have inline fuses.


Good grounding is a vital part of a good electrical system. It receives too little attention considering its importance, which is probably why such a large portion of electrical problems are grounding-related. An innovative design idea championed by Bob Nuckolls is the ground bus. It is not switched on or off like the main bus, but it serves as a central grounding point for the entire electrical system in your airplane. By using a ground bus, you eliminate using the airframe for grounding. This helps to prevent a host of electrical problems, such as loose or corroded connections to ground, or simply poor contact between major structural components. The ground bus is indispensable in a composite airplane where there is no metal structure for grounding. In planes with steel tube cages, it also goes a long way towards preventing the cage from becoming magnetized, something that can really play havoc with your magnetic compass. The negative is that it does add wire to the electrical system and therefore extra weight. However, the extra weight seems to be a reasonable price to pay for the benefits received.

For builders who don’t want to make their own, B&C makes a ground bus with FastOn connections.

The standard way to ground your airplane engine is to use a woven grounding strap fastened to the engine case on one end and bolted to the motor mount on the other. This method is certainly tried and true, but a better way is to run a large wire (#4 AWG minimum) to the ground bus connection mentioned above. At this point, the negative lead from the battery, the ground wire from the engine and the ground bus all come together. You should secure all of this to the firewall, thus grounding that, too. A fireproof connection through the firewall can be made with a brass bolt or threaded rod, tying this all together. It is important to note that you do want to ground the airframe, if you have a metal airframe, but you do not want to conduct current through the airframe if you can avoid it.

An overvoltage module is a small but important addition to any airplane electrical system. Voltage spikes can ruin expensive avionics in the blink of an eye.

You may want to have a small ground bus on the engine side for any firewall forward items such as landing lights, and a longer ground bus bar on the cabin side for everything else. These bus bars can easily be made from a copper bar one-half inch wide by one-eighth inch thick by whatever length you need. A number of 8-32 or 10-32 threaded holes spaced about one-half inch apart will allow loop connectors to attach to the bar. Aircraft Spruce & Specialty sells this copper bus bar material, as do a number of other vendors. Bob Nuckolls also offers a ground bus bar with FastOn connections through his web site.

This Glasair Sportsman has a traditional breaker panel similar to what you’d find in a certificated airplane.

Fuses and Circuit Breakers

The standard means of circuit protection in aviation has been the circuit breaker. Every device in the airplane ties to its own breaker. This practice has served aviation well for a long time. However, Nuckolls and others have in recent years advocated for the use of automotive fuses instead. Van’s now uses these fuses in their ELSA, the RV-12. They are light, cheap, and easy to install. But when they blow they cannot be reset; the fuse must be replaced. According to fuse fans, you should never reset a circuit breaker in flight anyway, so what’s the problem with a blown fuse? Not everyone agrees with this line of reasoning, which is one reason why most airplanes still have circuit breakers, but there is a certain logic supporting the fuse advocates that is worth considering. In any case, a blown fuse can be replaced in flight, but it will be harder to find a blown fuse in most cases than it is to find a tripped circuit breaker.

Van’s RV-12 kits and the prototype of the RV-14 use automotive fuses in lieu of traditional circuit breakers. They are lighter, cheaper and effective. Bob Nuckolls (Aeroelectric Connection) has been promoting fuses for years.

This Texas Sport Cub has a simple switch/breaker panel. From top left: a toggle switch for the master switch, two combination switch/breakers for avionics and strobes, and the rest are traditional aircraft circuit breakers. Note spare breaker for possible future addition.

With simple electrical systems, a builder can save money by using a circuit breaker/switch combination unit. These devices are specially designed to stand up to the rigors of frequent switching, which regular circuit breakers are not, and serve as circuit breakers, too. On the other end of the cost spectrum, Vertical Power has a system of electronic circuit breakers combined with a complete electrical system monitoring and switching center that can be set up to stand alone or interface with a number of popular EFIS/EMS systems.

When deciding what size wire and breakers or fuses to use refer to AC43.13-1B, Chapter 11. It contains a lot of useful information that should serve as your guideline for these decisions.

Quality and Workmanship Count

It is best to assemble airplane electrical systems with airplane components. Airplanes are built a certain way and with certain materials because they work better than the alternatives. That does not mean that there is never any room for innovation or improvement, but be sure that you are really making an improvement when you deviate from aviation standards. Good workmanship and materials can save you a lot of headaches later.

Final Thoughts on Electrics

Many builders feel a bit intimidated by electrical systems, but there is really no reason to be. With some good support and the right tools, any builder can install a workable electrical system. Pre-wired instrument panels or harnesses from a number of competent shops and others can go a long way towards untangling the more complicated aspects of airplane wiring for you. From there, it is mostly a matter of hooking up simple components and installing antennas. If you are still uneasy, look to your local EAA Technical Counselor, your EAA chapter, and the builder support group for your particular type of airplane for some help. Chances are, it won’t be that hard to find.

Last, electrical items—especially avionics—can really be budget-busters and weight-adders. Even though each item doesn’t weigh much, they do pile up, adding to your empty weight, even as they subtract from the weight of your wallet. By all means create the airplane of your dreams, but remember that weight and complexity detract from overall airplane performance and reliability. The battle against spending too much money has a way of taking care of itself, but the fight to save weight and complexity must be fought every day right to the end.

Coming Up Next Month…

Next time we will finish up the electrical system by taking a more in-depth look at engine sensors.

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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 four—two GlaStars, 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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