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  #1  
Old 01-19-2021, 11:23 PM
TASEsq TASEsq is offline
 
Join Date: Aug 2017
Location: Melbourne, Australia
Posts: 302
Default Wire Size Round Trip or Not

I am planning wiring for my wings and want to use the Garmin GAP26 pitot.

I measure it probably to be around 20ft in wire length from the panel out to the pitot. (40ft would be the return length).

The G3X manual says 12amps at -40C initially, and 10amps at 0C, so i think 10 amps is a reasonable value for the current draw.

Looking at AC43 for 20ft at 14V, the table says a 13AWG wire (so 12 AWG). If i consider the total wire run as being 40ft it would be 10AWG wire!!

The 12AWG wire needs a 30A breaker according to AC43, but the garmin manual says 20amp.

Am i supposed to consider the return path of the wire?

I know that most people on the RV14/RV10 use 14AWG wire I'm not sure what to make of the wire charts. Thoroughly confused.
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  #2  
Old 01-20-2021, 12:10 AM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
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Okay, two different things:
1. The 12 awg wire is rated at a current limit of 30 amps, due to heat generation (you don’t want to melt the insulation). This is regardless of length. That’s where the 30 amp CB recommendation comes from. Using a 20 amp CB is just more conservative.
2. The longer the wire is, the greater the voltage drop there will be. Some stuff really needs nothing less than 12 volts, other stuff can run fine on 10 volts. Look up the resistance per foot, multiply by 40 feet, calculate V = 10 amps x resistance, this is the loss, decide if 14 minus the loss is still adequate. Probably is fine for a pitot heater.

Last edited by BobTurner : 01-20-2021 at 01:09 AM.
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  #3  
Old 01-20-2021, 12:42 AM
BobTurner BobTurner is offline
 
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I ran the numbers. 14 awg has a resistance of 2.5 ohms per 1000’. So 40’ gives 0.1 ohms. At 10 amps you lose 1 volt, so your 14 volt buss voltage becomes 13 volts at the pitot. No problem.
From a heat perspective, 14 awg can handle 15 amps continuously. No problem
I’d use #14, protected by a 15 amp CB.

Last edited by BobTurner : 01-20-2021 at 01:14 AM.
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  #4  
Old 01-20-2021, 04:26 AM
TASEsq TASEsq is offline
 
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Thank you for the explanation. Makes sense.

What about the 0.5v voltage drop maximum? I didn’t think you were allowed to have more than this due to the heat increase in the wire would be too much?

Follow up question:
AC43 table 11.3 specifies a 20a breaker for 14awg wire. But if you look up table 11.9 for 150c wire it says the max current capacity is 14amps? Am I missing something?
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Last edited by TASEsq : 01-20-2021 at 04:38 AM.
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  #5  
Old 01-20-2021, 08:19 AM
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Greg Arehart Greg Arehart is offline
 
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Trent,

Simple answer to your question about 0.5V drop. There is no absolute "minimum drop" value, but a lot of folks like to size wire for less than 0.5V. For some devices, the proper voltage is critical, and they will usually say in the specs somewhere what the allowable voltage is. For example (from memory, so would need to look this up to be sure), my Dynon operates on anywhere from 9-28V. So in your 12V system, a drop of 0.5V would still be well within the parameters for the Dynon. For a heater (like your pitot), usually voltage drop is not a critical concern. Lower voltage will result in less heat, but it will still work as long as there are any electrons going through the wire.

In terms of heat, there are tables (or calculators) on the internet that relate resistance and wire size to heat generation based on current. Be sure to take into consideration whether the wire is in "free air" or in a bundle, as that makes some difference in the temperature.

I don't know why ACS specs are what they are, maybe someone else will chime in.

Caveat, I'm not an electrician, just have done work with some of this stuff.

Cheers,
Greg
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  #6  
Old 01-20-2021, 08:33 AM
lr172 lr172 is offline
 
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None of your equip is that sensitive. Your buss voltage is 14.5 volts when the alternator is running and as low a 11 volts just before the battery dies. If your equipment cannot handle that range of voltage, I would be looking at a second and third alternator as backups I would not be worried about voltage drop on a pitot. Airframe ground has essentially no resistance, relative to small gauge wire, and using one for the pitot will eliminate half that calculated drop.
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Last edited by lr172 : 01-20-2021 at 01:23 PM.
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  #7  
Old 01-20-2021, 11:24 AM
Mtherr Mtherr is offline
 
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Canít the pitot or any device in the wings for that matter, be grounded locally to the airframe? Why running a ground back to the cabin?
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  #8  
Old 01-20-2021, 12:47 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by TASEsq View Post
Thank you for the explanation. Makes sense.

What about the 0.5v voltage drop maximum? I didnít think you were allowed to have more than this due to the heat increase in the wire would be too much?
You can calculate the heat generated given the voltage drop AND other information, e.g., how long is the wire, what gauge. But what you really want to know, with respect to melting stuff, is how much heat is generated per inch, per foot, etc. Since the same current flows thru the entire wire, no matter how long, you can make that calculation knowing just the current and the resistance per foot. The voltage drop is of more interest for whatever device youíre trying to power. For example, if you are powering LED nav lights, and only get 1 volt out to them, they wonít work at all. Not dim, zero.

Just a small correction to a previous post: unless your airframe is built out of a superconductor, it has some resistance, just small. Unfortunately that resistance often grows as the airframe ages, tiny amounts of oxide build between lap joints, etc. Hence itís usually recommended that sensitive circuits, like audio lines, do not use the airframe ground but rather a separate wire back to a single point common ground. Without knowing anything about the electronic regulation circuit in this pitot tube (assuming there is one) it is hard to make a definitive statement as to how much, if any, ac noise it might be generating. But my guess is that the noise is little or none, and using the airframe as ground is unlikely to cause noise issues here. Thatís what I did.
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  #9  
Old 01-20-2021, 12:56 PM
AlpineYoda AlpineYoda is online now
 
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The official tables are a great resource. Probably the most technically accurate guidance out there.

But, for another point of reference, page 5-4 of the Garmin G3X manual says the following for 14V installations: 14 AWG up to 12 feet. 12 AWG for 13 to 20 feet. 10 AWG for 21-30 feet.

I figured with the wire through the wing, then the fuselage, and to the panel, it is still a little under 12 feet. But I used 12 AWG just to be safe since I was pushing the upper distance limit
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  #10  
Old 01-20-2021, 02:17 PM
TASEsq TASEsq is offline
 
Join Date: Aug 2017
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Thanks for the replies,

As to local grounds - I had assumed from what I read on here that everything needed a ground back to the firewall to avoid noise? So I had planned my wings this way (especially since the magenetometer will be in the left wing).

Is this a misguided idea? I am finding that I will need massive wires otherwise.

I am unsure now if I should use the airframe only for the larger current draw items.

Another example is the landing lights (not as high current of course). These are led and will wig-wag and I had planned a ground on these all the way back as well.

How do you decide if an item would be ‘noisy’ or not?

The other consideration is that I have primed everything - does this mean my wing won’t be a good ground path back to the battery? Literally every part has paint between the mating surfaces.
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Aug 2021

Last edited by TASEsq : 01-20-2021 at 02:34 PM.
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