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Avionics buss wire size

Rv8bill

Member
I'm Trying to determine proper wire size to the avionics buss. A single wire will carry power to the buss from the batt contactor through the avionics switch. However, the ground return will consist of numerous wires from the various devices. I'm using a single point ground so the ground wire distance from the devices may vary from as little as 2' (comm radio) to as much as 15' (A/P servos). Since the total wire length of a given circuit is used to determine wire size, how do I come up with a total when the lengths of ground wires vary so much? Should I consider the average length, longest, or...? I'm guessing it will be in the neighborhood of 16awg or 18awg but would still like to know how it is determined.
 
I'm Trying to determine proper wire size to the avionics buss. A single wire will carry power to the buss from the batt contactor through the avionics switch.

Add up the total amperage all the various devices connected to the buss, size the wire for that.

Ground run is not part of the equation as far as I have ever done it.
 
In principle your ground return wires should be sized using the same chart as the power input: current carried before CB pops, and wire length. However, for most installations, a true single point ground is difficult to obtain for the power to most avionics. Is the frame of the radio isolated from ground? Usually not. Do you use metal screws to connect the rack to the airframe? usually yes. e.g., Most of your ground return current is likely flowing thru the very large, low resistance airframe.
 
Thanks to all for the feedback:

Mike - I believe you are correct when the airframe is used as a common ground. It essentially becomes wire with a massive cross section. However, I intend to use a single point ground buss in accordance Bob Nuckols' Aeroelectric Connection.

Bob - Perhaps I'm confused, but why would a single point ground be difficult to obtain? Instead of grounding to the airframe, the ground wire is simply routed back to the ground buss. Perhaps requiring a bit more wire, but also proving a more robust ground connection.
 
Thanks to all for the feedback:

Bob - Perhaps I'm confused, but why would a single point ground be difficult to obtain? Instead of grounding to the airframe, the ground wire is simply routed back to the ground buss. Perhaps requiring a bit more wire, but also proving a more robust ground connection.

Most avionics have their internal ground connected to the metal box. To get a true single point ground you have to insulate the box from the metal panel. Possible, but a pain that most builders don’t do.
 
Thanks to all for the feedback:

Mike - I believe you are correct when the airframe is used as a common ground. It essentially becomes wire with a massive cross section. However, I intend to use a single point ground buss in accordance Bob Nuckols' Aeroelectric Connection.

Bob - Perhaps I'm confused, but why would a single point ground be difficult to obtain? Instead of grounding to the airframe, the ground wire is simply routed back to the ground buss. Perhaps requiring a bit more wire, but also proving a more robust ground connection.

And creating a path of least resistance...
 
Grounding

But unfortunately the whole point is not to minimize losses; it’s to eliminate ground loops, of any resistance.

So the problem can be broken down into three groups.
1. Power returns that are delivering essentially DC current used to power a component
2. Data busses and signal wire returns that are balanced or unbalanced to airframe or a single point reference.
3. RF coaxial that is either balanced about the airframe reference or single sided to the airframe reference.

Large transport aircraft have a grounding and bonding process defined as part of the engineering and manufacturing processes to support all three of the three groups of connections. For us guys in the experimental world it really depends how you thought through the priming, bonding and grounding processes you chose to use. In the large airplane world saving weight, cost and time running short ground returns is a really big deal so a lot of thought goes into planning during the design stage. For us in the small plane experimental world the penalty is not so significant so running power return ground wires and data bus and signal returns to a forest of tabs type ground pad behind the instrument panel means you dont have to worry about employing a comprehensive bonding and grounding process. More attention needs to be paid to bonding and grounding RF cables by using isolating back to back connectors at disconnect points but that is a separate subject all on its own. Minimizing ground loops to the greatest extent possible is always a good plan but it may not always be possible depending on how manufacturers arrange grounding to case within their equipment and the configuration of the switch mode power supply doing the power conversion. There are no guaranteed methods to avoiding EMI problems just industry practices to be followed. Sometimes solving EMI interference issues takes a lot of work and expensive test equipment………….

KT
 
I'm Trying to determine proper wire size to the avionics buss. A single wire will carry power to the buss from the batt contactor through the avionics switch. However, the ground return will consist of numerous wires from the various devices. I'm using a single point ground so the ground wire distance from the devices may vary from as little as 2' (comm radio) to as much as 15' (A/P servos). Since the total wire length of a given circuit is used to determine wire size, how do I come up with a total when the lengths of ground wires vary so much? Should I consider the average length, longest, or...? I'm guessing it will be in the neighborhood of 16awg or 18awg but would still like to know how it is determined.

I think you need to look at it as 2 separate "circuits". The total load on the buss is for everything that it will run (for the amps), and the distances from the buss and the single point ground back to the battery (contactor for the +) for the buss "circuit" total distance. The individual circuits are then sized as if the buss and the single point ground are the "battery" for total run/distance/load computation.
 
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