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Wiring and electrical system design is a very different experience from the rest of the kit so far. This is the part of the project where there are no more step-by-step instructions, and no "electrical system" kit containing all the parts you need. Since every home-built plane is designed according to the builder's wishes, the exact list of equipment and how it is all wired up is unique and must be designed by you, the builder. There are a lot of resources out there to help... but unfortunately, much of that information is in conflict, and possibly rightly so, since the design criteria it is based on is different as well. My approach is to take the bits of information I've discovered that are relevant to my project, and try to put it all together in a way that will work for my airplane and my comfort level. I've found valuable information in the Aeroelectric Connection by Bob Nuckolls, as well as on other web sites and internet mailing lists. Although I haven't subscribed to Bob's methods entirely, I'm taking his advice for things that matter to me. My electrical system is designed around the ExpBus product, partly because I've seen other builders use them successfully. It might not be right for you (if you are planning on lots of IFR, for instance) but I think it will work for me. I will not use the master relay on the ExpBus but will use a normal master relay on the firewall. This is accomplished by removing the ExpBus relay and soldering a solid copper wire over the contacts. By doing this, the alternator wire does not have to run into the cockpit, and can terminate on the switched side of the master relay instead. I'm also using Van's 60A internally regulated alternator, but will add a continuous duty relay tied into the Alt field line to allow a complete disconnect of the alternator from the rest of the electrical system in case of a runaway over voltage failure. The ExpBus has overvoltage protection built-in and will automatically cut the Alt field circuit, which will in turn open the alternator relay cutting the alternator off from the system. I've posted my overall wire schematic below, however this is still subject to change as I continue to install components and receive feedback from others.
I've also been looking for a simple table that will tell me, given the distance of a wire run and the breaker size used, what size wire to use. The usual procedure is to start with the device being powered, and determine what size breaker is necessary. Then take the breaker size and distance of the wire run to determine the wire size. Since the ExpBus has fixed breaker sizes, all I had to do was determine that the size was adequate for the device being powered, and then determine wire size. All of the charts and graphs I've found for determining wire size were difficult to read or fairly cryptic, so I made my own table that makes it very easy.
The finished table looks like this: (please let me know if I've made any errors here, I'm new at this too!)
To determine the wire size you should use, start with the breaker size column, and read down to the distance of the wire run. Where the two intersect, you will find the wire size to use.
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