extending Dynon wires

The instructions for connecting the dynon heated pitot control module say to extend the three non-terminated wires coming out back for power, ground and status light. They give a chart for length and recommended wire gauges. In my case, I'm looking at a total run of just over 16 feet, so by the chart I should use 12AWG. But the wires coming out of the control module look to be only 18 or 20AWG. Is this not a problem? All the current coming down that 12AWG wire will pass through the 18 or 20AWG wire to get into the control module. So if I need 20AWG wire for most of the run, why isn't this a problem when it gets funnelled into the small wires coming out of the control module??
Or is the idea that by using 12AWG for most of the run I keep the current from being too high, because the resistance across the long run will be lower... ok that must be it...
But it leaves the question - how do you go about splicing a 12AWG wire to a 20AWG wire? The butt splices I have are sized the same on both ends...

Look at it this way. If you connect 100' of 1/4" plastic tubing to your outside water faucet, the water flow and discharge force at the free end would be relatively low. But if you first had 100' of 1" garden hose and then 6" of 1/4" plastic tubing the water flow and force at the free end would be substantially higher. The difference in performance of the two setups is caused by the amount of pressure drop in the piping systems. The same is true for the circuit you have described. A long run of large diameter wire coupled with a short piece of smaller diameter wire will have overall less resistance than a long run of the smaller diameter wire.

One way to connect the different gauge wires is to use a butt splice that fits the 12 ga. wire. For the 18 ga. wire, strip it a little longer and then bend the stripped end over itself thereby making it effectively a larger diameter wire. Twist the end before inserting in the butt splice.

Hope this helps.

You are correct, the long run of the thicker wire will have less resistance than a thinner wire. 12 ga. still seems like an awfully thick wire for only 16 feet to power an EFIS but I don't know anything about your Dynon so if Dynon says so it must be right.

For connection you could tin both wires, then loop both ends into "hook terminals" (picture wrapping the 18ga. tinned wire around a 12 ga. sized post). solder the 18 ga. wire to the 12 like you would solder a wire to a turret terminal (the 12 ga. wire), slide a couple layers of heat shrink over the connection and your good to go.

If you can't envision what I am saying I could solder up a couple wires and take a pic..

Are you sure 16" is correct?

I have an RV7A. The Pitot heater is located about half way out the approximatly 10' wing, leaving around 5' for the run to the fuselage. After you get to the fuselage I figured another 4' to get to the circuit breakers.

I figured my total run at about 9'.

I don't understand how you figured 16'.

I don't understand how you figured 16'.

Click to expand...

8 feet from pitot tube controller to wing root (RV-9A), then 3 feet (allowing a margin for routing flexibility) to the switch, then about 4 feet tothe main bus (again allowing some margin for routing flexibility). That's 15 feet. Plus a foot safety margin my wire size calculation. I could probably get away with 14AWG no problem, but for a slight weight penalty I figure 12AWG and I'm sure.

Wire Length Pitot to Panel

This last question is still of interests to me as I'm about to buy either 16 or 14 gauge wire for my Dynon Heated Pitot. I measure 6 feet from pitot to fuselage and then 3 or 4 feet to panel. 3 feet gives me 9 feet total and I'll buy 16 gauge wire. But if it takes 4 feet that is 10 total feet and Dynon recommends 14 gauge. So can someone who knows what the wire length turned out to be from pitot to panel? The question has to do with how much extra length is needed once you get into the fuselage, in order to route around and get to the Dynon box.

thanks much

I think the comparison with the garden hose is a good one.
Translating this to electrics, it is the allowable voltage drop which is dependent on the resistance of the wire. Max drop is 0.5V for intermittend load and 1V for continuous loads if I recall this correctly.