Electrical Systems Simplified Part Two By Ron Alexander

Relays, contactors & solenoids

A relay or contactor is usually installed to activate an engine starter. It serves to eliminate the need to run a heavy cable from the battery and starter all the way to the master switch. A contactor or relay is simply an electromagnetic switch that operates a heavy current circuit. These are often called solenoids. A large wire (usually 4 gauge) is installed between the contactor, the battery, and the starter. A smaller gauge wire (usually 18 gauge) is then run between the relay and the master switch.

Voltage Regulators

A voltage regulator must be present to protect the battery. The charging voltage going into the battery must be controlled within a relatively small range—as an example, 13.8-14.2 volts for a typical lead acid battery at normal temperatures. This voltage varies with the ambient temperature. The voltage regulator is going to prevent battery overcharging by decreasing the alternator output as the battery nears a full charge state. Most other equipment within your airplane can withstand a wide range of voltage. Just about any solid-state regulator will control most alternators.

Circuit Protection

We must protect individual electrical circuits from overloads. This is usually accomplished using a fuse or a circuit breaker. Fuses and circuit breakers are not intended to protect the equipment rather they are installed to protect the cable (electrical wire) attaching the power source and the equipment. Of course, in protecting the cable the equipment is also protected. Fuses will be cheaper to purchase and lighter in overall weight than circuit breakers. During flight it is easier to reset a circuit breaker than change a fuse. If you closely analyze most electrical faults they are produced by failure of an electrical device that in turn causes it to draw an extremely high amount of current. In reality, how often will you be able to solve the problem by resetting a circuit breaker or replacing a fuse? Probably not very often. A short in a wire will also cause an electrical fault.

The bottom line is that you must select the right size circuit breaker or fuse for the size wire you are running to the component part. The circuit protection should open before the wire gets hot enough to begin smoking. Another point worth mentioning is that if the circuit protection is too small for the wire size you will get a nuisance protection requiring you to reset the circuit breaker or replace the fuse even though a major problem may not exist. The following chart found in FAA Advisory Circular 43-13 shows the comparison between wire size and circuit protection.

Wire AN gauge Copper
Circuit Breaker amps
Fuse amps

What would happen if we did not comply with these guidelines? Lets assume we install a 10-amp circuit breaker in a circuit connected by a 22-gauge wire. As long as everything is normal you will probably not know the difference. However, should an electrical fault occur in this circuit, the breaker may not trip until the wire gets so hot that it causes an electrical fire.

Electrical Wiring & Connectors

The primary concern is that you use the proper type of wire. Do not go down to your local electronic store and expect to get the proper wire for your aircraft. The best type of wire to use in your airplane is MIL-W-22759/16 unshielded wire or MIL-C-27500 shielded wire. Both types of wire are manufactured to the military specification number given and they are insulated with a Teflon™ type of material called tefzel. The wire is rated for 600 volts. Teflon™ and PVC wire is also often used but after researching the problems involved I would stay with tefzel wire. Sure you can probably save a few dollars on surplus electrical wire but is it really worth it? Wires are manufactured in various sizes with 22 gauge being the smallest you will normally encounter on your airplane. To provide an example, 22-gauge wire is about .025 inches in diameter and is comprised of several strands of smaller wires.

As mentioned under circuit protection, the size of the wire is of utmost importance. The wire size will be selected based upon the amount of current the wire will carry and the required length of the wire. The voltage drop resulting from a long length of wire can be significant. Fortunately, AC43-13 provides charts showing the relationship between wire length, size, and amperage.

A large number of electrical problems are the result of poor connections between the electrical cables and the various components of the system. Most cables are connected with “crimp-on” connectors, such as ring-end terminals or butt splices. Often a solder joint will be used to complete the connection. I would recommend the use of “crimp-on” connectors whenever possible. Solder joints are subject to breaking or weakening when subjected to vibration. If you use a solder joint, be sure to support the wire near the joint to prevent flexing. You will find the need for soldering to be more common during your radio installation.

Selection of the proper solderless connector is easy because the connectors are color coded. The coding is as follows:

Color of Connector
Size of wire
18-20-22 gauge
14-16 gauge
10-12 gauge

A special tool is used to crimp the connection. Approximately 3/16 inch is stripped away from the wire, using wire strippers, and the wire inserted into the connector. The tool is then used to make the crimp. You then check the connection by slightly pulling on the terminal and wire.

It is also recommended that you insulate the terminal connection using heat shrinkable tubing. You must remember to place the tubing over the wire prior to assembly.

Bus Bars

No doubt you have heard the electrical term “bus bar”. A bus bar is very simply a central point where wires from electrical equipment are grouped together and attached to a metal bar that is then attached to a power source. Without a bus bar we would have to connect every electrical component directly to the power source. This would be very complicated and impractical. Normally, one terminal of each circuit protector ( circuit breaker or fuse ) running from the electrical component is attached to the bus bar. The power to operate all of your electrical items is then obtained from one common point. This is generally accomplished using a strip of copper large enough to tap a hole for each circuit breaker required. This would usually be about 1/8 inch to ¼ inch thick and ½ inch wide. They must be long enough to accommodate the number of breakers. If using fuses, a heavy copper wire is often used that is soldered across one terminal of each fuse. Doing either of these creates a bus bar.


A lot of text could be written concerning electrical switches. You will need to determine what type of switch you desire and where you want it located. Even though there are many switch types they all provide the same type of function. That function is the control of an electrical component. Listed below are various types of switches you may encounter and their description.

Single-pole, single-throw
Used for one circuit