Best Practices – Electrical and the Equipment List

Last month we looked at the best materials to use to wire your plane and examined the topic of grounding. This time I want to start out with the equipment list and then look at alternators and overvoltage protection, starters, wiring diagrams, circuit breakers and fuses. Because the topic of aircraft electrical is so large, we will have to save a few items for next time—batteries, backup systems, and electrical circuits.

Again, I emphasize that this is merely an introduction to the topic of aircraft electrical systems and refer you to the publications mentioned last month—Bob Nuckolls’ AeroElectric Connection, Marc Ausman’s Aircraft Wiring Guide, and the FAA’s AC43.13-1B.

The Equipment List

Before you start running wires and making connections, you need to make an equipment list: a list of every electrically powered item in your plane, its required voltage, location, and current draw. You may wish to list the weight of each item, too, but that is optional. Once you have an equipment list, you can determine wire sizes and circuit breakers or fuses. Below is an equipment list for a very simple airplane, but the same principles apply to more complex installations.

Note that every item has its own breaker. Fuses can also be used, but more on that later. The main point is that every item gets its own breaker, and every circuit will begin at the breaker panel. There should be no circuits without breakers except for the starter main power, and no doubling up of circuits on one breaker. That is why we want to list every single item on the equipment list. With this list completed you can now start to think about what materials you will need to buy.

Alternators and Overvoltage Protection

If we add up all the current requirements from the equipment list, not counting the alternator power, we get about 17.5 amps. However, we see that the starter relay draws 1.9 amps, and it is what could be called an intermittent load. Electric flaps or electric landing gear motors would be other examples of intermittent loads. Since these loads are only present for a short period of time, we do not need to size the alternator to cover them. That leaves us with a total continuous load of about 15.6 amps.

The basic rule for sizing the alternator is to have the total continuous load not be more that 80% of the alternator’s rated capacity. Based on this, an alternator with at least a 20-amp capacity would cover the load, but would not leave us with much extra to recharge a battery depleted by extended cranking at start-up. A B&C pad-mounted alternator (BC410-H) that puts out about 30 amps at cruise engine rpm would be a good choice, or you could consider a Plane-Power unit of similar size (FS1-14B). If your total continuous load was more than about 24 but less than 32 amps, you would need a 40-amp alternator. In every case, be sure to get an alternator with enough capacity, but don’t get carried away. Too much capacity just means too much weight, and weight is always the enemy in an airplane.

Alternators come with internal and external voltage regulation and overvoltage protection. In general Plane-Power units have internal voltage regulators, and most have internal overvoltage protection. On the other hand, B&C alternators rely on external voltage regulators and overvoltage protection. The Plane-Power approach makes for a simpler installation at a lower cost, but the B&C approach seems to be more robust because it removes the voltage regulator from the heat of the engine compartment. They are both popular choices with Experimental builders.

Always be sure to ask about overvoltage protection. Most manufacturers are including it as standard now, but you want to be sure. Voltage spikes can kill expensive avionics, and overvoltage protection is cheap. If your system doesn’t have it built in, be sure to add it. B&C makes an overvoltage protection module that you can buy for around $68. If you don’t have this protection by some other means, this is a sound investment for the protection it offers. It might save you thousands of dollars someday.

If you don’t need more than 30 amps of capacity in your charging system, a pad-mounted unit is worth your consideration. These attach to the back of the engine where the vacuum pump used to go before everyone got rid of vacuum-powered gyros. This eliminates the belt drive typically found on larger alternators, and it saves several ounces of weight. However, if you need 40 amps or more capacity, you will need to go to the belt-driven unit of the appropriate size. The pad-mount alternator can be added to a larger system as a backup.

Starters

Experimental aircraft with Lycoming-type engines come with Sky-Tec starters more often than any other brand by a wide margin. B&C and Hartzell (Hartzell Engine Technologies) also make fine products, but their sales are only a fraction of that enjoyed by Sky-Tec, which is now owned by Hartzell. They make an ultra-lightweight starter that only weighs 6.25 pounds, their standard and very popular LS series that comes in at 8.25 pounds, and the KPS series that weighs about 8.9 pounds, and includes kickback protection.

As for what is the best practice when it comes to starters, there is no hard and fast rule. It depends on what is important to you. If robustness and reliability are paramount, a heavier starter with things like kickback protection may best meet your needs. If the absolute minimum weight is more important, then the ultralight Sky-Tec starter is made for you. Most people split the difference and go with something like the Sky-Tec LS-series starter.

When purchasing a starter there is more to it than weight and price. Lycoming makes two different types of ring gears, one with 122 teeth and one with 149 teeth. It is very important that you get a starter that is matched to the number of teeth you have. To find out what is on your engine, you can count the teeth or just look at the shape of the teeth.

Because of the high current draw of the starter motor, it is always activated by means of a starter relay or solenoid. That way only a small current needs to be switched to turn on the starter. If not for this, the starter switch would have to deal with 200-300 amps and would be huge. The starter relay differs from the master relay in two ways. One, it need not be rated for continuous use. And two, it is switched with positive power. The power wires in and out of the starter relay are the big 2- or 4-gauge wires. The relay is activated with a 20-gauge wire from the starter switch. The relay must also be grounded. Just remember that the starter and master relays are different, and try not to get them mixed up.

It is good practice to install a diode between the two small posts of the starter relay to extend the life of the relay. Similarly, a diode can help extend the life of the master relay if placed between the activation wire (negative) and the hot (positive) side of the relay. Any 1N5400 or 1N4000 series diode can be used and should be connected so the cathode end (end with silver stripe) is toward the positive terminal.

As a side note, if you plan to do aerobatics with your new plane, it is better to mount the starter relay upside down. That way high positive G’s will not accidentally activate the starter relay.

Wiring Diagrams

Wiring diagrams come in many forms from schematics to more pictorial wiring diagrams to simple pin-out diagrams. Each has its function. The important thing is to create a wiring diagram that will work for you if you, or someone else, ever needs to go back into your electrical system and change or repair something. Schematics tend to be rather abstract, using electrical symbols that you can find in AC43.13-1B or any number of electrical handbooks. Their weakness is that they don’t usually depict the actual location of items, but merely show how they are connected together conceptually.

A more pictorial wiring diagram will typically show where items are in the plane and will often use symbols or images that are more like the actual parts involved. These drawings tend to be larger, but that can be managed by breaking them down into sections.

If you have a pre-wired instrument panel from someone such as SteinAir or Aerotronics, you will most likely get what are called pin-out diagrams. These panels come with pre-wired Cannon plugs or other multi-pin connectors that then must connect to the various items in the airplane. It will be your job to bring those wires to the other side of the connector and make them line up with the pre-wired connector provided. To do that you must know the pin location for each wire you are bringing to the connector. The pin-out diagram shows you that information. The only hard part is working with those annoying D-sub pins and getting them in the correct holes.

Whenever you have multi-pin connectors, the pin-out diagram will be your best friend if you ever have to go back and fix or modify something. Without it you are up a creek without a paddle, as the saying goes. Be sure to keep these somewhere handy after you complete your project.

The best practice in this regard is to make up a wiring diagram as you go, and then clean it up when you are finished. Keep it along with any schematics and pin-out diagrams you might have in a safe place for future reference. Along with these things you should retain any literature that came with any electrical item in your plane. Try to put it all in a big notebook if you can. You will be glad you did down the road.

Circuit Protection

For years Bob Nuckolls has been advocating for the use of fuses instead of aircraft circuit breakers for circuit protection. At first almost everyone dismissed that as a crazy idea, but with the introduction of the Van’s RV-12, his idea finally got the respect it deserves. Fuses save money and weight, and who doesn’t want to save money and weight when they are building an airplane? That said, the urge to use aircraft circuit breakers is so ingrained in most of us that it is almost irresistible. But, come to think of it, how many times have you actually reset a circuit breaker in flight? And if you did, how long did it stay reset? In truth, if a fuse blows or a breaker trips, there is something wrong that needs to be fixed that isn’t going to magically repair itself by resetting a breaker or replacing a fuse. In any case, if you do decide to use fuses, you will need to carry some spares that can be readily accessed in flight. You will need enough of each size to replace half the number of that size, with a minimum of one of each size.

The important thing to remember is that each circuit begins with a fuse or breaker. From there the power wire goes to each device needing electrical power. A ground wire then comes back from that device to ground, preferably a ground bus. There are no fuses or breakers on the ground side of the circuit. The same size of wire is used coming and going. The total length of the wire for size calculations needs to consider the entire round-trip length.

Breakers or fuses are powered from a bus. This can be a bar with a number of holes that line up with the powered (line) sides of a row of breakers or fuses, or it can be simply a set of wires that loop together to the powered sides of those breakers or fuses. The master relay sends power to the bus and thus provides the breakers or fuses with positive electrical power. Oftentimes breakers or fuses are grouped into sub-buses, such as an avionics bus that turns on all avionics without requiring each unit to be switched on separately, or an essential bus that makes it easy to shed non-essential items in case of an alternator failure. This isn’t anything complicated. It is just a group of breakers or fuses that are powered together and connected to the main bus through a relay that switches them as a group.

Getting back to fuses and breakers, they are there to protect the wire going to the various devices. That means that a breaker or fuse should never be so large as to allow a wire to overheat in case of a short. It is always permissible, but rarely best practice, to use an oversized wire. It is never permissible to use a breaker or fuse that is oversized for the wire being used.

There is much more to look at next month. In the meantime, check out some of the very helpful EAA Hints for Homebuilders that deal with electrical issues, or many others for that matter. Here is where you can find the electrical hints.