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Wire sizing temperature difference

rdamazio

rdamazio

Well Known Member
I'm beginning to wire my airplane, and have one related question: when sizing wires for temperature, what temperature difference should I use?

e.g. most aircraft wire is rated for 150˚C, but what's a reasonable estimate at the wire's temperature? I've seen some places using 20˚C (thus 130˚C difference), but that seems overly optimistic - if I decide to fly when it's 40˚C out, it won't be 20 inside my wings, right?

It also makes a difference for the voltage drop, since the resistance increases with temperature - e.g. for 20˚C it seems I can use a 12AWG for a 20ft 12A wire, but at 150˚C the voltage drop is greater than 0.5V so I'd have to use a 10AWG.

Am I overthinking this? :)
 
Get the Aeroelectric Connection book. It covers your concerns & tells you how to size for current, temperature, etc.
 
I've read it, but it only covers it briefly with that graph on temperature rise currents. Specifically, I'm not sure:
- What ambient temperature to use inside a conduit inside the wing (if the air outside is, say, at 40?C)
- should I derate temperature rise currents with the same altitude+bundling coefficients as used for the ampacity?

Thanks for your patience :)
 
My experience

My experience has been that using the optimium wire size for low power stuff doesnt really save much weight. Only for the high power stuff, like starter, main avionics bus, old style stobes and lamps, would this matter. For the low power stuff, i would not go too small since it becomes hard to work with and prone to breakage. JMHO
 
For the most part, use #22 for signal wires and #18 for power wires. Temperature rise is not a concern for intermittent loads. If you think that larger than #18 is needed, get another opinion.
 
If you size for minimal voltage drop, the temperature rise won't be much of a factor. But I think what the AEC charts are telling you is that for a given wire size and continuous current, you will see X amount of temperature *increase over ambient* temps. Ambient temps can be higher in a bundle where multiple wires are carrying significant current, because the individual wires can't shed their heat to the air as easily.

But again, voltage drop is a bigger issue than temperature rise, and if you keep voltage drop to a minimum, you won't get much temperature rise (because the resistance of the wire is what causes both the voltage drop and the temp increase).

The only wires that are in a significantly elevated ambient environment are the alternator B lead and the run from battery to starter (and back). #8 will handle almost any alternator you're likely to install on an RV, and #4, or #2, at most, is all you're likely to need for a starter. #4 should work for a firewall mounted battery; you *might* need #2 for an aft mounted battery, depending on the engine and starter you use.

For everything else, just use the voltage drop charts, and remember that heating is not a factor for intermittent duty items (as others have pointed out). Pitot heat could be considered continuous duty if you fly IFR. Most other stuff is intermittent at the current levels that would heat the wire.
 
It sounds like maybe you're cutting it a bit close, e.g., I would never plan on a 0.5 volt drop (except maybe starter current). If you plan on no more than a few tenths voltage loss, temperature concerns pretty much go away. And the weight penalty is very small.
 
Thanks for all the replies.

Yes, the loads I'm concerned about are "medium power" - 12A over 44ft (22ft each way, not grounding locally) for each set of wingtip lights (Aveo Ziptips) and up to 12A over ~34ft (17 each way) for the heated pitot (I do fly IFR). The other wires in the bundle are intermittent loads (like AP servo) which only "get in the way" of heat disspation.

Looking at voltage drops for those currents/lengths, it says I need #8 wire for the wingtips and #10 for the pitot, which seems quite large.
Also, (and this is where I may be overthinking it) if I calculate the voltage drop at a higher temperature (150˚C), then suddenly I need #8 for both to keep it under 0.5V, which is why I was wondering about the temperature.
 
If this document is for your wing tip lights
http://www.aveoengineering.com/downloads/ConformaZipTips-wiring.pdf
it shows #16 as the maximum wire size.
If your plane is a RV-10, why not use the airframe for the negative conductor?
_
Some electrical loads are voltage sensitive, like avionics. But I think that a pitot heater should work fine at 12 volts. Normal system voltage is 14. I would not worry about dropping 1 volt over long wires.
 
Mich48041 said:
If this document is for your wing tip lights
http://www.aveoengineering.com/downloads/ConformaZipTips-wiring.pdf
it shows #16 as the maximum wire size.
If your plane is a RV-10, why not use the airframe for the negative conductor?
Click to expand...

Yes, that's the diagram, but I have no idea what parameters they calculated that with, and my interactions with Aveo have usually gotten replies saying "ask what other RV builders have done", so here I am :) It's much easier with the Garmin components where they proactively release specs and recommendations.

I don't want to use the airframe as ground on the wings because of the wingtip nav antenna and magnetometer there, though I guess if I manage to get them both to work together on the same side, I could at least ground to the airframe on the other side.

Mich48041 said:
Some electrical loads are voltage sensitive, like avionics. But I think that a pitot heater should work fine at 12 volts. Normal system voltage is 14. I would not worry about dropping 1 volt over long wires.
Click to expand...

That's a great point I admit to not having thought of, thanks!
I've been trying to stick to the 0.5V drop limit, but a lot of things will likely work with a much larger drop (possibly not the pitot heat controller electronics, but the wingtip lights probably will - out comes the bench power supply to try :) ). I guess this will make it all much easier.
 
I typically get the creepy crawlies when a manufacturer tells me to ask someone else what to use to make their own product work correctly. If the manufacturer doesn't know something as fundamental as how much current their product consumes in normal operation, and the range of permissible supply voltage.....
 
rv7charlie said:
I typically get the creepy crawlies when a manufacturer tells me to ask someone else what to use to make their own product work correctly. If the manufacturer doesn't know something as fundamental as how much current their product consumes in normal operation, and the range of permissible supply voltage.....
Click to expand...

Agreed, it was a little annoying that the wingtips came with zero documentation other than that diagram.
The currents are there as well as the "battery-side" voltages (12-18V), just not the admissible voltages at the other end of the wires, which they very likely calculated and tested (for some arbitrary length of wire?), just didn't publish. I guess since the lower end of the range is 12V, it'll be fine with at least a 2V drop from 14V.

The Garmin manual is much more friendly (to start, because there is a manual) and says "MINIMUM 14 AWG UP TO 12 FT, 12 AWG FROM 13 TO 20 FT, AND 10 AWG FROM 21 TO 30 FT" for their pitot heat.
 
rdamazio said:
I'm beginning to wire my airplane, and have one related question: when sizing wires for temperature, what temperature difference should I use?

e.g. most aircraft wire is rated for 150˚C, but what's a reasonable estimate at the wire's temperature? I've seen some places using 20˚C (thus 130˚C difference), but that seems overly optimistic - if I decide to fly when it's 40˚C out, it won't be 20 inside my wings, right?

It also makes a difference for the voltage drop, since the resistance increases with temperature - e.g. for 20˚C it seems I can use a 12AWG for a 20ft 12A wire, but at 150˚C the voltage drop is greater than 0.5V so I'd have to use a 10AWG.

Am I overthinking this? :)
Click to expand...

rdamazio said:
Thanks for all the replies.

Yes, the loads I'm concerned about are "medium power" - 12A over 44ft (22ft each way, not grounding locally) for each set of wingtip lights (Aveo Ziptips) and up to 12A over ~34ft (17 each way) for the heated pitot (I do fly IFR). The other wires in the bundle are intermittent loads (like AP servo) which only "get in the way" of heat disspation.

Looking at voltage drops for those currents/lengths, it says I need #8 wire for the wingtips and #10 for the pitot, which seems quite large.
Also, (and this is where I may be overthinking it) if I calculate the voltage drop at a higher temperature (150˚C), then suddenly I need #8 for both to keep it under 0.5V, which is why I was wondering about the temperature.
Click to expand...

Yes you are over thinking it.

Some of the "stuff" you are using is recommended to have separate grounds so that part is correct.

You do not need to calculate the wire length to and the wire length from the device. Only one direction is needed. The wire length calculation can be assumed to be to airframe ground even with the separate return ground wire.

It has been a while since I looked at AC43.13 wire charts. From what I remember, there is one chart for wire in free space and one in a bundle. The bundle is more likely the better chart to use for your application.

The RV-6 that I build and have been flying 20-years (in a few more days) was done the way I list above and there have been no electric problems. I am an A&P but made my living with Electrical Engineering training as an engineer before retiring.
 
He did stipulate home-run ground wires. I'd agree that it's likely unnecessary for the pitot heat or the wingtip lights, but if home-run ground wires are used, then they must be included in the calculation.
 
RV6_flyer;1202859 You do not need to calculate the wire length to and the wire length from the device. Only one direction is needed. The wire length calculation can be assumed to be to airframe ground even with the separate return ground wire. .[/QUOTE said:
The above is not correct. The load does not care where it is in the circuit. The total wire length must be considered if you are calculating total wire resistance and the associated voltage drop etc.

Bevan
Click to expand...
 
Bevan said:
The above is not correct. The load does not care where it is in the circuit. The total wire length must be considered if you are calculating total wire resistance and the associated voltage drop etc.

Bevan
Click to expand...

I could be wrong, but my understanding has been that the voltage drop happens on both runs (half in each since it's the same current running through both, electrons come from the battery through the ground run to the device then back through your "main" run), and the device at the end "senses" the potential difference after both drops, so I believe I must include both. Also pretty sure 43.13 and the AMA say "including ground run". e.g. if you had a 1ft run to the device, then a 100ft ground run, there's no way that'd be equivalent to two 1ft runs.

When you say "the load does not care where it is", I understand it just means that if the two runs were different sizes, the total size would still govern, regardless of the split.

From my layman understanding, the reason the airframe resistance is not included when grounding locally is that such a large cross-section (if you were to accumulate the solid area of the wing's cross-section) gives negligible resistance compared to a wire - but it should be there, too, if you wanted to measure it.
 
That's correct. Perhaps the confusion arises because there are charts and calculators out there that tell you to enter the one way distance. But if you read their instructions carefully, they are building in the return length automatically. Most are written for facility wiring, where return runs can't be avoided, and are automatically assumed to be there..
 
Use #16 wire per the manufacturers pdf document.
Nearby electrical conductors (single or dual polarity) will have negligible affect on a nav antenna. However a single conductor could affect a magnetometer. I would use a twisted pair (positive and negative) within 3 or 4 feet of the magnetometer. There is no need to run the negative wire all of the way back to the source. At a safe distance from the magnetometer, ground the negative wire locally to the airframe at some point where there is access, like near an inspection cover.
 
Btw, also got a response from Aveo on this one:

"The Zip Tip module will run safely as low as 9VDC so you should be OK, I
have no reported problems of voltage drop in the lengths required for Vans
aircraft at the recommended gauge wire."
 
So it looks like your wire size will hinge on your decision to run a home run ground wire or not...

But I'm curious...where did you get the 12amp load in your wing tip light calculations? Combining circuits/returns?

rdamazio said:
Yes, the loads I'm concerned about are "medium power" - 12A over 44ft (22ft each way, not grounding locally) for each set of wingtip lights (Aveo Ziptips) and up to 12A over ~34ft (17 each way) for the heated pitot (I do fly IFR). The other wires in the bundle are intermittent loads (like AP servo) which only "get in the way" of heat disspation.
Click to expand...
 
FYI, I tested the ziptip lights today - most of them turn off when the voltage gets down below 8V (landing and taxi lights turn off below 7.8).
 
Tommycat said:
So it looks like your wire size will hinge on your decision to run a home run ground wire or not...

But I'm curious...where did you get the 12amp load in your wing tip light calculations? Combining circuits/returns?
Click to expand...

Yes, combining the two ground runs from the diagram above gives me 11A, 12 to be safe (or is there some reason to run two long wires instead of one slightly thicker wire?).
 
Yes

rdamazio said:
Yes, combining the two ground runs from the diagram above gives me 11A, 12 to be safe (or is there some reason to run two long wires instead of one slightly thicker wire?).
Click to expand...

If you run two wires one can use a smaller guage. This way only one or two sizes of wire need to be bought. I am Sizing for the smaller circuit and running two or three for the occasional higher current.
Also I am sizing for the circuit breaker size, not the application or load. JMHO
 
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