Building an electrical system in your aircraft? These tips should help
Homebuilders frequently install switches rated for 125 volts and 15 amperes, thinking they are suitable for service in, for example, a 12-volt, 5-ampere electrical circuit. In this instance, the 125-volt rating is for alternating current (AC) and the 12-volt reference is direct current (DC). Is there a difference between AC and DC electricity? Absolutely, and misunderstanding this often leads to trouble—possibly big trouble like an electrical fire.
A safety seminar for A&P mechanics at Oshkosh ’92 included this subject, and several attendees were EAA technical counselors. According to reports, about 75% of the homebuilt airplanes inspected turned up with AC-rated switches, or worse, non-rated switches in DC circuits. This amounts to handwriting on the wall.
AC vs. DC
One difference between AC and DC electricity is the way electrons flow through a conductor. Because of internal effects, AC tends to flow along the outer surface (called skin effect) while DC flows throughout the cross-sectional area.
As frequency increases, this phenomenon becomes more pronounced, and at very high frequencies, all current flows within a few thousandths of an inch of the conductor’s surface. Because DC moves virtually all electrons in a conductor, several million additional collisions of those rascals begets more heat; therefore, all wires and electrical components in DC circuits need greater mass.
Wires used in DC circuits are massive when compared to those in AC circuits. For example, wires connecting the battery in an automobile are much larger than conductors interconnecting a reading lamp and wall receptacle. Yet the lamp operates on 120 volts and the car’s system is 12 volts. Clearly, the criterion for selecting proper wire size is not voltage alone.
Visualizing a sine wave symbolizing AC, it becomes apparent that current returns to zero potential twice during each cycle. Electrical engineers frequently call these cooling cycles because heat dissipation is one unique characteristic of AC electricity.
Combining AC circuit skin effect with cooling cycles explains why they produce considerably less heat. This places great importance on exercising caution when selecting wire size and component ratings for DC electrical systems.
The effective value of current or voltage is often called the root-mean-square (rms) value. Many electrical and electronic specifications include rms notations, which, as the name implies, is derived mathematically.
Fortunately, practical electrical wiring—even designing electrical circuits—seldom calls for rms computations; therefore, integral calculus is not one of the tools homebuilders need in their bag of tricks. Yet it’s important to clear the fog of mysticism surrounding this lexicon.
An easy way to understand rms is to remember that comparative heating capacities of AC and DC circuits are important. Applying an AC voltage to a resistor results in dissipated energy in the form of heat; the amount of DC voltage, applied to the same resistor and producing the same amount of heat, is the rms value of the AC voltage.
Typically, one switch has a rating of 10 amperes at 125 or 250 volts AC. Many find it surprising that this same switch can carry only 0.3 of an ampere at 125 volts DC, and increasing the DC voltage to 250 reduces the current rating to 0.15 amperes. This represents less than 1/60 of the original load-carrying capacity, and all we did was go from AC to DC electrical power.
Turning a switch off, or opening the circuit, is actually parlayed in AC circuits by the very nature of sinusoidal current. Since 60 Hz AC changes direction 120 times a second, no current flows during the frequent times of zero potential.
Current helps turn itself off when it sees an open circuit, even though briefly, and switch designers use this phenomenon to help reduce the cost of manufacturing AC switches. Arcing and welding of switch contacts, even those made with exotic high-temperature alloys, is a common problem with AC-rated switches in DC circuits.
Unlike AC current, DC has no periods of zero potential, so electromotive force (voltage) is always present, and contacts tend to arc upon opening a switch. That is one property of DC electricity; it’s the nature of the beast.
Thinking that fuses and circuit breakers offer protection against welded switch contacts is a fallacy. Fuses and circuit breakers provide overload protection, and welded switch contacts will not, by themselves, cause an increase in circuit load. To complicate matters, when switch contacts become welded, the heat generally destroys internal springs, cams and other critical parts.
Because of mechanical damage in these instances, cycling a switch lever has no effect, and the circuit remains hot. This type malfunction invites several potentially dangerous conditions. Imagine what might happen if a switch controlling a fuel transfer or boost pump should have its contacts welded closed.
Either of these pumps might continue to run even though the pilot thinks, because of the switch’s apparent position, they had been turned off. Another horrifying possibility is a malfunctioning switch in either an elevator or aileron trim tab circuit. Ironically, these examples are but the tip of an iceberg; think about it and I’m sure you’ll agree this is a serious problem in a flight environment.
Most high-quality electrical components have both AC and DC ratings, such as the circuit breaker shown in Photo 1. Fortunately, many electronic stores and auto supply houses stock these items so availability is not a problem. Logically, prices are higher; generally they are about three times the amount of AC only or non rated counterparts. Considering the blood, sweat, tears—and money—going into a homebuilt project, can one really afford to take a chance?
Another case in point is a pair of push button switches I used in a friend’s RV-6 project, shown in Photo 2. These are PTT switches in a 9-volt, 0.018-ampere (18 milliamperes) circuit. Manufactured by GC Electronics, these switches are stocked in many electronic stores but not Radio Shack. Look-alike switches in “America’s #1 Technology Store” are rated for 3 amperes at 125-volts AC, which takes us back to the beginning of this article.
Because of the circuit’s low amperage, these AC-rated switches may be adequate, but, in my opinion, only marginally. Nothing replaces the warm feeling of confidence in an aircraft electrical system. Consequently, the AC/DC rating of the pictured switches appealed to me. So there will be no misunderstanding, I’m not maligning Radio Shack, because I use many of these products. But PTT switches in an aircraft com system is not one of them.
Basic Ohm’s Law equations provide simple methods for computing electrical loads of rudimentary DC circuits. They’re easier to use than weight-and-balance calculations—and certainly less cumbersome. Just remember that E = IR, I = E/R and R = E/I where E = electromotive force in volts, I = intensity (current) in amperes and R = resistance in ohms.
Physical Dimensions of Electrical Cable
Wire tables contain information needed to compute physical dimensions of conductors in an electrical circuit. Developed through more than a century of experience, the electrical industry used sound engineering logic compiling these tables.
When standardizing a wire’s cross-sectional dimensions, its convenient to use a small unit of measurement because the diameter of a wire is usually only a small fraction of an inch. Accordingly, the diameter of wire is expressed in terms of a unit called the mil, which is one thousandth (0.001) of an inch.
That is, there are 1000 mils in an inch. This is easily remembered because the mil is simply a milli-inch. For example, using 64 mils to express the diameter of Number 14 wire is more convenient than using 0.064 of an inch.
Cross-sectional areas of round conductors are measured in terms of the circular mil. A circular mil is the area of a circle whose diameter is 1 mil, and areas of circles vary as the squares of their diameters. For example, a circle whose diameter is 2 inches has four times the area of a circle having a diameter of 1 inch.
Similarly, the area of a circle whose diameter is 0.003 inch (3 mils) has nine times the area of a circle having a diameter of 0.001 inch (1 mil). Because, by definition, the circular mil is the area of a circle with a diameter of 1 mil, it’s evident that a circle with a diameter of 3 mils must have an area of 9 circular mils.
Hence, the area of a circle can be expressed in circular mils by squaring the diameter, providing, however, that the diameter is expressed in mils. Conversely, if the area of a circle is expressed in circular mils, the diameter in mils can be found by extracting the square root of the area.
Using circular mils for the data base, wire sizes are designated by numbers in a system known as the American Wire Gauge (AWG)—formerly Brown and Sharpe gauge. These numbers, ranging from 0000, the largest size, to 40, the smallest, are based on a constant ratio between successive gauge numbers.
Number 10 wire makes a convenient reference because it is nearly 1/10 inch in diameter and has a cross-sectional area of approximately 10,000 circular mils. Also, its resistance is nearly 1 ohm per 1000 feet. As wire sizes become smaller, every third gauge number results in one half the area and, therefore, double the resistance.
Conversely, as the wire size becomes larger, every third wire gauge results in twice the circular mil area and half the resistance.
From an electrical viewpoint, three factors govern the selection of wire size to be used for transmitting current:
The safe current-carrying capacity of the wire.
The power lost in the wire.
The voltage variation, or the volt age drop, in the wire.
It’s important to remember that the length of wire, for the purpose of computing wire resistance and its effects, is always twice the distance from the source of power to the load (outgoing and return loads). This means that in composite and wood airplanes, length of the return, or ground wire must be taken into consideration.
Resistance of the current path through adequately bonded metal aircraft structures is generally considered negligible. Maximum allowable resistance measured between the ground point of the alternator, or battery, to the ground terminal of any electric device is 0.005 ohm (5 milliohms).
Computed when the alternator is carrying rated current or the battery is discharging at the 5-minute rate, volt age drop in the main power cable is critical. This should not exceed 2% of the regulated voltage.
At Your Service?
Besides wire gauge and length, the type of service becomes important in aircraft electrical systems. Is the circuit used continuously or intermittently? Are the wires in a bundle, inside a conduit, or is it a single cable in free air? All these things affect heat intensity produced by electrical conductors. Shielding on a wire, by the way, is a type of conduit.
Clearly, wire tables alone do not give all the answers homebuilders need. Circuit designers use several methods when combining variables, and my favorite is the graphic representation shown in Figure 1.
This illustration shows AWG numbers (or wire size) and lengths, with types of service curves for maintaining specified voltage drops. Plotting data came from current military specifications for copper airframe wire.
A common use of the chart deter mines the required AWG number when known factors are wire length and ampere load. In this example, it’s assumed that the length is 50 feet and the continuous current is 25 amperes. From the left scale, follow the horizontal line designated 50 feet until it intersects the diagonal 25-ampere line.
This line is slightly more than mid way between the 20- and 30-ampere lines because of the logarithmic scale. The first vertical line to the right of this intersection denotes that AWG 8 is the smallest suitable conductor.
Also note that the point of intersection is above curve 1, indicating that this conductor may be installed inside a conduit or bundled with other wires and carry the specified current without over heating.
Knowing the wire size and ampere load illustrates another typical use of the chart. With this information, one may determine the maximum wire length that may be used without exceeding a 1-volt drop. For example, consider an AWG 2 wire in a continuous-current circuit carrying a 150-ampere load.
Starting at the bottom scale, follow the vertical line for 2 AWG to the intersection of the diagonal 150-ampere line; then project that point to read 38 feet, by interpolation, on the left scale. Since the point of intersection is below curve 1, this must be a single wire in free air.
After a little study, it becomes apparent that there are several other useful applications of this chart. I hope some of you make photocopies of Figure 1 for use in your workshop.
House wire, auto wire, industrial building wire and even electronic “black box” wire installed in an air craft electrical system is an accident looking for a place to happen. None of these wires, designed for ground-related applications, is suitable for the extraordinary demands made on air frame wire.
Equally important—perhaps more important—a pilot cruising at 10,000 feet cannot simply pull off to the side of a road because of smoke coming from the instrument panel. To paint the picture more dismally, smoke from this wire’s insulation is, more than likely, toxic. Think what could happen inside a closed cabin.
Poly vinyl chloride (PVC) is the most common insulting material used on the wires mentioned previously. Although PVC is a good insulator, and it does resist abrasion, it does have two disqualifying properties for air craft use: It’s flammable and it produces toxic vapors when burned.
Ironically, all electrical wires contain an ignition source because of the current carrying function. Another undesirable property of PVC is that its plasticizers can penetrate other insulating materials causing a large dielectric loss, even on other cables in close contact.
Type II PVC, a non-contaminating compound, resolves this problem; but I have never seen Type II touted as nonflammable or nontoxic. Frankly, I have never bothered to investigate flammability or toxicity of this material, because, as far as I’m concerned, there is a better solution.
DuPont developed an insulating material known by its trade name Tefzel. When DuPont’s patents expired, other chemical firms produced the resin ethylene tetra fluoro ethylene (ETFE). By either name—Tefzel or ETFE— this insulating material exhibits out standing performance characteristics for aircraft use.
It provides long-term aging stability, resistance to contaminating fluids, non-flammability and, of course, high dielectric properties. Understandably, these materials became standards for wire insulation in government-owned aircraft and space vehicles.
Military specifications include many detailed requirements besides insulating materials for airframe wiring. It takes several monotonous volumes of nitty-gritty to cover the subject in its entirety. Therefore, I’ve limited this article to a few recommendations for homebuilders, and a good starting point is Photo 3.
Of the many milspec wire types, Mil-W-2259/16 is well suited for gen eral aviation airframe wiring; besides technical excellence, its economical price is within reason for the average homebuilder. The two pieces pictured on the left illustrate this type.
The one on the extreme left is AWG 18, the adjacent one is AWG 20, and the full military part numbers are Mil-W- 2259/16-18 and Mil-W-2259/16-20 respectively. Shielded wire has many applications in homebuilt projects: leads to speakers, headphones, microphones and strobe beacon lights are common examples.
Looking at the picture again, the third example from the left illustrates Mil-C-27500 shielded wire. In this example the center conductor is AWG 20, making the military part number Mil-C-27500-20.
Coaxial cable is another type of wire needed to interconnect radios and antennas. Shown on the extreme right is a piece of coax manufactured to Mil-C-17 specifications, with a M17/28 RG58 part number. This coax is superior to the conventional RG58-A/U stocked at most aircraft supply stores. What makes it superior?
Dielectric of the center conductor shown in this picture is Teflon (another DuPont trade name), which accounts for a velocity factor of 0.72. RG58- A/U coax uses a solid polyethylene dielectric giving this coax a velocity factor of 0.66.
Density of the braided shield is about the same in both types (95%), but the outer jacket of the mil-spec coax is ETFE, whereas the other is Type II PVC. Now for the coup de grace, the price tag: I paid 16 cents a foot for the coax pictured here, and RG58-A/U is listed for 26-28 cents a foot in most catalogs. So, you see, using the best is not necessarily the most expensive.
Unfortunately, we live in an era of bogus parts, and counterfeit milspec wire abounds. For this reason, trace ability has become a buzzword in the aviation industry—especially among A&P mechanics. Accident investigators often wish to trace a part back to the original manufacturer, and a mechanic who keeps good records might win a medal.
But one who keeps poor records, or none at all, may wind up in a heap of trouble. Is there a lesson here for homebuilders? I think so. If you want genuine milspec wire, the best way to get it is to insist on full traceability through your supplier.
Generally, large national distributors specializing in this wire can offer this service, but minimum order requirements rule these suppliers out for an average homebuilder. I only know of one firm that caters to those of us in the small spender category, and I’ve included that name and address at the end of this article.
My last order from this company, shown in Photo 4, included 400 feet of wire, or 100 feet each of the four-wire types shown in Photo 3. Surprisingly, the invoice total was $48.20 (1994 prices), which includes freight.
It took five days delivery time, via UPS, and I have enough wire for about four homebuilt projects, perhaps more—making the numbers look like $10 or $12 per airplane (1994 prices). Significantly, the packing slip includes lot serial numbers, dates of manufacture, and manufacturer’s identifying number for each spool of wire. Beat that if you can.
Wrapping It Up
Designing and installing an aircraft electrical system is not the portentous task some make it out to be. Understand that there is a difference between AC and DC electricity, and use sound engineering logic when selecting components and wires.
Also understand that genuine milspec wire is superior to any other type for aircraft installations, and insist on getting the real thing. Following these guidelines need not be an expensive road to follow and can be the cheapest route to happy homebuilding.