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Developing Your Electrical System Part 3 - Wiring Tips

By Tony Bingelis (originally published in EAA Sport Aviation, June 1990)

WELL, HOW ABOUT THAT! Here you have completed the wiring of the aircraft's alternator and battery circuits, installed the main bus and, at this point, have less than a dozen wires coming up to the instrument panel!

That wasn't so complicated, was it? The hardest part was probably getting organized and acquiring the right kind and size wires and the other necessary parts. After all, how many of us would just happen to have a battery, a voltage regulator, an over voltage relay, a split master switch and a couple of solenoids lying around the workshop. Or, for that matter, have the correct gauges of shielded wire and a sufficient quantity of insulated ring terminals?

You might have had a few insulated ring-terminals on hand . . . maybe enough of them to get the job started. But that would have been just about it. More likely, you probably had to order everything else from one of the homebuilt suppliers.

Other than that, you will have to admit that the basic wiring job was quite interesting, and not nearly as time consuming or as difficult as you had anticipated it would be.

However, as you well know, there will be more wiring than those few cables you had to install for the basic power system . . . especially if you intend to install radios, strobes and navigation lights.

All right, now that you have run the electrical power up to the panel, let's see how you can effectively distribute that power to operate all those gadgets, instruments, lights and switches needed to make your airplane fully operational.

QUICK DISCONNECTS . . . WHERE AND WHY

Keep this in mind. You will have to partially disassemble your airplane before you can haul it to the airport. That is, you undoubtedly will have to remove and reinstall the wings and tail surfaces at least one more time.

Before you can remove the wings, it will be necessary, for example, to disconnect the antenna cables, the navigation and strobe-light wiring . . . and maybe some other wires.

Ordinarily, this would not be a hard thing to do provided you had anticipated the need beforehand, and had fitted those wires and coax cables with quick disconnect terminals.

If you have not yet provided for this need, now is the time to do it.

Locate your quick disconnect points where they will be readily accessible and still be located near where the wires enter the fuselage.

The type of quick disconnect fitting you install is relatively unimportant . . . just so long as it is one of the insulated types. It could be in the form of phrenolic multiple circuit locking connector from Radio Shack, an insulated knife disconnect, a male/female disconnect, or the more recently available, bullet connector (available at most auto parts stores).

MOST CIRCUITS ARE SIMPLE . . .

A typical circuit can, indeed, be simple. For example, each of them, fortunately, will be powered by a wire having two ends.

One end will be connected to your aircraft power source (the bus). The other end, in effect, will be connected, ultimately, to the aircraft ground.

Between those two ends you can install a switch, if you need it, and, of course, the instrument or unit you want to operate.

Except for minor variations, all circuits in your airplane will be very much the same . . . quite simple really. What causes wiring complexity is not the design of the various individual circuits but, rather, the sheer number of circuits.

This apparent complexity develops rapidly as circuit after circuit is fabricated and connected. You suddenly find that you have created a maze of wires running every which way from different parts of the airplane to different parts of the instrument panel.

This criss-crossing soon becomes confusing and because of it tracing an individual circuit can become very difficult.

In the average homebuilt, more than 90% of the electrical wires (cables) will be less than 4 feet long. Therefore, except for a couple of the longer circuits, voltage drop is not generally something to worry about.

This should help simplify your wiring because you may be able to use No. 18 gauge wire for almost all of your circuits.

Actually, the smallest wire size you can use with any degree of confidence is the 22 gauge stranded copper wire. Anything smaller is absolutely too flimsy to withstand the operational demands the wiring may be subjected to in an aircraft structure.

Of course, the avionics folks often do use much smaller wires, especially for hooking up electrical instruments.

They seem to take great delight in using such tiny wires, much to my dismay. I find these flimsy wires too delicate to handle with confidence because they cannot tolerate much flexing or abuse before they break.

Furthermore, I find that insulated crimp type connectors for these delicate wires are difficult to obtain.

It's understandable, therefore, that such small gauge wires are more often installed using pin terminals and uninsulated terminals which can be soldered.

Electrical
WIRING STRATEGY

Making the connections behind the instrument panel will tax your patience, believe me. For every connection you successfully complete, you will drop at least one screw and maybe the lock washer. If, on the other hand, a nut is used to connect the wire to a terminal stud and you fumble and drop the nut, you can bet that the washer will quickly follow.

And where do they drop, pray tell? Into the dim recesses of that dark cockpit floor. They might as well have dropped into a bottomless pit because there they instantly become invisible, never to be found until long after you have found replacements for them.

I suggest you outwit the little rascals by spreading a white cloth on the cockpit floor (tucked up well around the edges) to keep the hardware from bouncing into some dark corner. It's my recommendation that you take this temper-saving step before you attempt to fiddle with that first connection.

You don't believe me? O. K., try making a few connections .. . and then spread that catch-all cloth! Here's some more good guidance.

    1. Be sure the insulated ring terminal connector selected is the correct size for the wire you are installing. Otherwise, you might have trouble with the wire pulling out of the terminal.
      
    Also, be sure that the eye of the terminal is not too small to fit over the screw or stud to which it is to be secured.
      
    Every now and then the eye of an insulated ring terminal I crimp to a wire turns out to be too small to fit the screw or stud. By unthinkingly allowing this to happen I am forced to cut off and discard the ring terminal and replace it with one that has a larger eye. Not only is that wasteful, it can also become costly. Don't snicker . . . it can happen to you, too.
     

    2. Fabricate and fit one circuit at a time. Complete it and then check it out before undertaking another circuit. Use a temporary 12 volt shop battery if your aircraft battery is not yet installed.
     

    3. When a particular electrical instrument or avionics unit has a number of wires to route, keep all of those wires together for as far as you can.
      
    Tie them together by wrapping a temporary "wire twistie" around the cables (wires) to form a temporary "bundle" (assuming we can call two or three wires a bundle).
      
    NOTE: The wire twisties I refer to are the same colorful wire twisties you use at the supermarket to secure the plastic bags you fill with fruit and vegetables. Find an economical source for them or start stockpiling those you can salvage from your bread wrappers, etc. Using these temporary wire twists instead of the more permanent plastic tie wraps is a quicker and cheaper way to initially organize your wiring.
      
    Don't waste your time trying to twist the wire twist ends together to secure the bundle. Merely gather the circuit wiring you want to keep together and wrap a twistie around them a couple of times. That temporary restraint will hold up quite well. And yet it will allow you to easily separate the bundle should you need to change the routing of one of the wires.
      
    At any rate, don't be overly eager to install those expensive plastic ties. Long before you are satisfied with the routing of the various circuits, you will find you are making and separating the same wire bundles a countless number of times. Each time you do, there goes another expensive tie wrap. You'll waste a hundred of them before you realize what is happening.
       
    After you are satisfied with the final routing of the wires, then you can replace the temporary quickie wire twists with those classy permanent plastic tie wraps.
     

    4. Always label both ends of each wire (approximately 4 inches from the end) using a short tab made from a piece of adhesive tape folded around the wire.
      
    The common practice of sticking a piece of masking tape to the wire as an identification marker is O. K. but, at best, is a short term solution. This is especially true when you mark with a nonpermanent ink that will fade or smudge away.
      
    Furthermore, masking tape will become brittle after a year or so. However, If you do use masking tape tabs, at least be sure to use a permanent pen (Pilot, Sharpie or similar).
      
    If you prefer, you can use the "peel and stick" number and letter wire markers sold for this purpose (Radio Shack and some electronics shops). In other words, you simply number each of your circuits instead of labeling them.
      
    In that case, of course, you will have to have made a record of each numbered wire on your schematic wiring diagram for future reference.
     

    5. Consider prewiring your master switch and ignition switch before installing either of them. The ignition switch, especially, is the more troublesome to install because it will have 5 or more wires to be attached to its terminals. Crowded as the terminals are, it will be difficult to correctly identify and connect all these wires after the switch is installed in the instrument panel. So, why not do it the easy way?
      
    Simply pre-cut the wires allowing sufficient length for each to reach its destination from the swtich. Then fit each of the wires with an insulated crimp on terminal (on one end only for now) and attach each wire to its respective terminal. The terminals are marked for easy identification on the back side of the ignition switch.
      
    After all the wires are secured to the switch (and labeled), you can bundle them . . . at least for the first several inches. Beyond that distance one or more of the wires may have to be separated and routed in some other direction.
      
    Again, I would remind you to route related wires as far as you can along a common route. It will make trouble shooting easier later . . . should that be necessary.
      
    When you are ready to connect the opposite ends of the ignition switch wires to their respective destinations, cut the wires to the required length and then you can attach the appropriate terminal connectors.
    6. Electrical
     

    7. Consider installing a terminal strip or terminal block in a convenient location. It will enable you to better organize the wiring.
      
    The wires to that terminal strip will generally be coming in from various directions. Therefore, it serves as a convenient point (terminal) where the circuits may be separated and easily removed when necessary.
      
    In addition, all the wires from the terminal block to the instrument panel can be tied into a neat bundle and routed in one direction directly to the aircraft bus. See Figure 2 to get an idea of how a terminal strip works for you.
      
    You can make your own terminal strip or get one of the types sold by the Radio Shack stores.
     

    8. Anchor your wires/cables with plastic tie wraps or clamps at reasonably close intervals wherever you can. This will help immobilize and strengthen individual wires as well as wire bundles.
      
    Remember, an unsupported single wire is rather weak and vulnerable to damage. Therefore, if the wire is clamped every 6 to 8 inches, the risk of an unsecured wire developing a short circuit, or some other problem, will be greatly diminished.
     

    9. Running wires and cables through the cockpit area is never easy. Any airplane will look disorganized and cluttered when randomly routed electrical wiring is visible here and there.
       
    You could make your wiring installation much neater if you would provide, during construction, for the installation of a conduit through the cockpit area. If this is not practical, consider routing all of your wires and cables through the center of the aircraft (especially if it is a low wing design). You will already have a useable opening through the spar and bulkheads for the control system. Take advantage of that arrangement by making room for the electrical wiring also.
     

    10. Be sure to protect the wires and cables from interference with the control system and unprotected sharp edges by using a firewall shield, rubber grommet, cushioned clamp, etc.
      
    Any opening in a bulkhead, rib, skin or other structure through which wires pass must be fitted with some sort of protection to safeguard the wiring from abrasion.
     

    11. Allow a few inches of slack in each wire you connect to the bus, to other instrument panel switches, and the like. At best, you will only save an insignificant amount of weight by cutting your wires to the exact lengths without allowing a few inches of working slack. Without some slack in the wire, you will find it difficult to remove a switch, fuse block, circuit breaker or whatever from the instrument panel and lower it enough that you can easily work on it.
     

    12. As you complete each circuit, turn on the master switch and activate that circuit to see that it is working properly.

If your battery is not yet installed you can hook up your car battery, or use a temporary shop battery for testing the circuits.

ABOUT SHORT CIRCUITS

A short circuit results when a positive (hot) wire touches any part of the aircraft ground. When a wire shorts to ground, a potential fire hazard is created when the wire gets hot, starts to smoke or burn.

A short circuit can be caused by a wire that becomes disconnected and drops against some metal part. More often, though, a short is caused by a wire that becomes frayed from rubbing against some sharp metal opening or unprotected bulkhead opening.

Short circuits are, therefore, more likely to occur in metal aircraft than they are in composite or wood structures for obvious reasons.

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