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stigaro

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Some devices require multiple power wires in order to share the high current (ie, heated pitot tube, GPS Nav/Com, etc). I am using a VPX Pro and would prefer one (appropriately sized) wire to come out of the VPX. However, what's the best way to splice this single wire into multiple inputs into the device? Can I put the two 18 AWG wires on the GAP26 pitot using a single 1/4" quick connector? Or should I solder two 18AWG wires onto the 14AWG wire that then use two quick connectors to the pitot wires? What about when there are more than 2 inputs like on GTN 750 (7!)? I'm guessing it's better to use a terminal block and have multiple wires come off that?
 
Someone told me that best practice is to have one protecting device per wire. When I ran out of VPX pins, I let one VPX pin power a blade fuse holder and let that do the distribution.
 
nilberg said:
Someone told me that best practice is to have one protecting device per wire. When I ran out of VPX pins, I let one VPX pin power a blade fuse holder and let that do the distribution.
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Yes, that's how I'm planning on wiring up my 750xi. 10A connector from VPX to fuse holder, then 3 outputs from there to GTN. But each output then needs to branch out to multiple inputs to the GTN. But it seems like the butt splices should work for this part.
 
Avanza said:
No. Just make sure that they are properly connected.

Good luck
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To be rigorous, the splices have a minimum and maximum conductor cross section but it’s pretty forgiving.

Also being rigorous when you splice wires of different sizes, the circuit protection should be sized to protect the smallest wire.
 
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Mikeyb said:
To be rigorous, the splices have a minimum and maximum conductor cross section but it’s pretty forgiving.

Also being rigorous when you splice wires of different sizes, the circuit protection should be sized to protect the smallest wire.
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I don't believe this is entirely true. If I go from, say a 16 AWG to two 18 AWG, the current carrying capacity is limited by the 16 AWG wire, not the 18 AWG as the current is halved going through the two 18 AWG wires and the capacity of an 18 AWG wire is greater than 50% of the 16 AWG wire. So you'd still want to protect it for the 16 AWG. But I agree, it needs to be carefully thought through.

My real world example, a heated pitot uses, 14 AWG wire on a 15A circuit from the VPX. But the 14 AWG wire gets spliced into two 18 AWG at the pitot. The circuit is still fused for the 14 AWG wire.
 
stigaro said:
Yes, that's how I'm planning on wiring up my 750xi. 10A connector from VPX to fuse holder, then 3 outputs from there to GTN. But each output then needs to branch out to multiple inputs to the GTN. But it seems like the butt splices should work for this part.
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How does a single 10A output from VPX cover a 10A (com) and 7.5A (nav/gps) CB or am I misinterpreting your plan?
 
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Walt said:
How does a single 10A output from VPX cover a 10A (com) and 7.5A (nav/gps) CB?
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750xi manual says maximum draw of 3.4 A (Main), 4.0 A (10W COM) and 0.9 A (Nav). That's 8.3A, so a I thought a 10A breaker should be sufficient with the extra margin. Why are you suggesting it's not?
 
stigaro said:
I don't believe this is entirely true. If I go from, say a 16 AWG to two 18 AWG, the current carrying capacity is limited by the 16 AWG wire, not the 18 AWG as the current is halved going through the two 18 AWG wires and the capacity of an 18 AWG wire is greater than 50% of the 16 AWG wire. So you'd still want to protect it for the 16 AWG. But I agree, it needs to be carefully thought through.

My real world example, a heated pitot uses, 14 AWG wire on a 15A circuit from the VPX. But the 14 AWG wire gets spliced into two 18 AWG at the pitot. The circuit is still fused for the 14 AWG wire.
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Again, being rigorous and using a worse case example, assume you had a 10A circuit and 18 gauge wire spliced into a 22 gauge wire that shorted to ground. that wire could potentially draw 10 amps and would overheat. Is that likely? Probably not. Is it worth it to save adding another circuit?
Up to you.
 
Mikeyb said:
Again, being rigorous and using a worse case example, assume you had a 10A circuit and 18 gauge wire spliced into a 22 gauge wire that shorted to ground. that wire could potentially draw 10 amps and would overheat. Is that likely? Probably not. Is it worth it to save adding another circuit?
Up to you.
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So you're suggesting using a 10A fuse on the 18 AWG, then breaking that down into two 5A (or less) fuses on 22 AWG wire? In theory, maybe, but I think it introduces a lot more potential problems with bad connections, not to mention the added weight and additional space considerations. I have no experience with this, would this method be typical?

Going one step further, from say a 15A (or 10A?) breaker on the VPX, I need to splice this out to 7A, 4A, 2A for the GTN. These then get spliced into 2 Main power connections, 3 COM connections and 2 NAV connections, all fed using 22 AWG wires. So, should I really have 3 layers of circuit protection? Again, I know this is probably good practice, but that's 11 fuses for one piece of equipment.
 
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stigaro said:
So you're suggesting using a 10A fuse on the 18 AWG, then breaking that down into two 5A (or less) fuses on 22 AWG wire? In theory, maybe, but I think it introduces a lot more potential problems with bad connections, not to mention the added weight and additional space considerations. I have no experience with this, would this method be typical?
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The guidance in FAA AC43-13 par 11-49 allows you to go one fuse size over the recommended value if the wires are in open air. So in most cases using a 10 amp fuse on a 5 amp wire is allowable. Same for the 15/10 amp wires for the pitot
 
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stigaro said:
750xi manual says maximum draw of 3.4 A (Main), 4.0 A (10W COM) and 0.9 A (Nav). That's 8.3A, so a I thought a 10A breaker should be sufficient with the extra margin. Why are you suggesting it's not?
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See figure 11.2 of the install manual for recommended breaker sizing and power wiring.
 
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stigaro said:
Yes, that's how I'm planning on wiring up my 750xi. 10A connector from VPX to fuse holder, then 3 outputs from there to GTN. But each output then needs to branch out to multiple inputs to the GTN. But it seems like the butt splices should work for this part.
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Why make it so hard. Skip the VPX and just use a 10A and 7.5A breakers (or fuses) For the GTN?
Carl
 
Carl Froehlich said:
Why make it so hard. Skip the VPX and just use a 10A and 7.5A breakers (or fuses) For the GTN?
Carl
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Yes, that's the other option and maybe the easiest. But it raises my original question again. The NAV/GPS section of the interconnect diagram shows 20 AWG from the fuse, then spliced to four 22 AWG to the GTN. So how to splice the four 22 AWG to the one 20 AWG?
 
For what it is worth, I find it just as easy to run 2 20ga wires to the nav/gps breaker in case a situation arises where the breaker pops and you want to isolate the nav from the GPS section. Then an 18ga for the com.
 
stigaro said:
Yes, that's the other option and maybe the easiest. But it raises my original question again. The NAV/GPS section of the interconnect diagram shows 20 AWG from the fuse, then spliced to four 22 AWG to the GTN. So how to splice the four 22 AWG to the one 20 AWG?
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No spliced required or desired. Crimp the wires on a single ring connector that goes on the breaker. From there the #22 wires go to the multiple pins on the GTN-650 tray.

Carl
 
But - to the original question on splicing wires a solder joint covered in heat shrink is simple and effective. I immobilize the solder joint in a wire bundle so it never flexes.

Carl
 
Carl Froehlich said:
But - to the original question on splicing wires a solder joint covered in heat shrink is simple and effective. I immobilize the solder joint in a wire bundle so it never flexes.

Carl
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Now you’ve done it Carl
It would have been my first choice but Walt hates them and he is OG :)
 
Carl Froehlich said:
But - to the original question on splicing wires a solder joint covered in heat shrink is simple and effective. I immobilize the solder joint in a wire bundle so it never flexes.

Carl
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Everything on an airplane vibrates and flex’s, even wire bundles!
Not to mention that I could do a boatload of butt splices in the time it would take you to do a single solder splice.
 
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Walt said:
Everything on an airplane vibrates and flex’s, even wire bundles!
Not to mention that I could do a boatload of butt splices in the time it would take you to do a single solder splice.
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Well Walt - there you go again…..

Any joint will fail from flexing. But failure requires relative motion between the wires. Fix one end of the solder (or butt splice) and flex the other end and it will fail. Don’t fix one end and eliminate the relative motion.

I’ve fixed many failed butt splices because the builder used solid wire or fixed one end leaving the other to bounce around and fatigue fail. The most common butt splice problem I run across is splicing a resistor (solid wire) an ignition wire to feed RPM signal to the EMS.

There is a place for crimped butt splices as in single wire to wire joint. But as example a splice to make multiple “pig tails” is best done using solder and heat shrink. Personally I have no need for crimped butt splices or solder sleeve.

Carl
 
You should let Boeing know you have a better way then their approved repair methods!
 
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Some good advice here. Thanks everyone. I did not mean to start a new "primer war". Butt splice or solder joint?

But one further question about soldering, do you use flux? I solder a lot of electrical wire with flux, but don't know about long term corrosiveness.
 
Just use rosin core of an appropriate size for what you’re trying to solder. Paste is just messy.
As an electronics tech early in my career, soldering is a walk in my sleep kind of a thing. However, solder sleeves are easier in tight spaces and when you’re done, you have a soldered joint covered in heat shrink. Same as if you did it the “old fashioned” way. For the amateur, fewer cold joints than trying to hold loose wires together to manually solder. They are a bit bulkier.
 
Solder sleeves are a preform of solder in shrink tubing heated with a heat gun. The good ones have a color temperature indicator that tells you when they are hot enough. They have been around since the 60’s. They are approved for spacecraft use.

They can be difficult to get the wires aligned and heated properly. A small heat gun with a curved heat deflector and an alligator third hand tool makes everything wonderful. Buy the good ones from AS and try it.
 
Most folks who use solder sleeves don’t have the right temperature controlled heat gun and end up with poor connections.
In addition solder joints should be mechanically secure prior to soldering (not ‘glued’ together with solder).
 
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I’m just lucky. I found them extremely easy to use and obvious.
Oh well. To the OP - try em all and see what you like.
In my airplanes, I have multiple wires into a lug as shown in Aeroelectric. Mechanical Butt Splices. Solder Sleeves, and hand soldered joints.
They have all worked and held up to the test of time….
If you do it right, they all work.
 
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