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12/14v vs 24/28v systems

Don

Well Known Member
While attending Oshkosh I heard Greg Richter (sp? - the Blue Mountain guy) give a talk on aircraft wiring for smart people. He made a strong case for using a 28v system. His argument is most electronics (radios and efis's...efii?) work on either and will stop working at 9.6v more or less. If you lose your alternator he argues that you'll have a lot longer use of your electrical system as your battery drops from 24v to 9.6v than you would if you had a 12v system. Actually he thinks 48v is ideal but 24v is the best you can readily do now. He also argues that you can save 6-10 lbs of wire using 24v and the lighter guage wire required.

I'm midway along with the fuselage and nearing a point where I need to finalize my wiring plans but early enough that I can change plans without any real impact (other than maybe a little money). I've got a set of Whelen position lights and strobes that are 12v that's my only real 12v electrical investment so far and I'm sure I could sell them for near what I have in them if I go 24v. But as I look around, the light weight alternators seems to be 12v (though I'm sure if l look harder I can find a 24v). The LED lights I've been thinking of switching to for weight and other reasons are strictly 12v. So right out of the gate I seem to be heading uphill. I believe my planned radios and purchased EFIS (a dual GRT Series 1) can operate on either.

So, this not is too long but my questions are:

1. Am I right that 12v systems are predominant today? This includes the assumption that putting together a 24v system will involve more scrounging and searching.

2. Is there a simple way to get a 12v system to last longer in the event of a battery failure other than adding a second battery?

3. Other than night flying, I don't really see an electrical failure as a big issue. About the biggest concern would be loss of the GPS and I expect I'd have a handheld that would suffice for getting me to an alternate airport. If all else fails there's always the chart and pilotage to get me to an airport. Does anyone see something I'm missing?

My inclination at this point is to resign myself to the 12v system and finish the airplane. In theory, Greg appears right. In practice, he may be right in the future. For now theory and practice don't seem to agree.

Don
 
Go with 12 volts! 6-10 lbs saved on wiring? I doubt it very much. Weight of 12v vs. 24v battery? Expense of 24v alternator system? Expense of 12v vs 24v accessories? Most amateur-built aircraft take advantage of many 12v automotive parts. 24v parts are almost exclusively aircraft. Compared the price difference?
 
I agree with Mel, go 12 volt. Your brand new aircraft with all new parts shouldn't be having any electrical problems any time soon. And if you do have problems...at night, how long will a 25 ah battery last you while running minimal equipment for minimal amounts of time? I doubt you'd have enough gas to run it all the way down. Good question tho!
 
I am going with a 28V system. I expect some will try to talk me out of it or tell me I'm stupid, but they're wasting their breath.

That said, yes, you will have a harder time finding some things in 28V. Most things can be found in both voltages, but if you plan on using automotive parts, go 12V.

I doubt the wire savings will really be 6-10 lbs. I know mine won't because I wired fairly conservatively. If you bought the Whelen System 6 that Van's sells, then it works on 28V, just need to swap the bulbs on the position lights. Plane-power makes a 70 amp alternator that does 28V and costs the same as their 14V model.

If you go 28V you are bucking the current trend and you will find yourself scratching your head over some items (landing lights, trim system, flap motor). All of these can be solved, it just depends on whether it's worth it for your situation and mission profile.

PJ
RV-10 #40032
 
You have to understand where Greg is coming from. That 6-10 pounds may be accurate on the composite plane that he is use to (Cozy Mk 4) but in our metal planes we can use the airframe as ground and effectively cut our wiring in half compared to his composite. While his thinking is technically correct, in the practical world of single engine airplanes, a 12/14 volt system beats out the technical superiority of 28 volts.

Side note; the composite Cirrus uses 28 volts but the Columbia uses 12/14. A couple of years ago the auto industry tried to move to 36/42 volts. But like the metic system, technical superiority lost out to day-to-day practically.
 
One other small thing that has happened to me. Ever been stranded at a small airport needing a "starter boost" and the airport doesn't have a 24v booster. Just about any car in the parking lot can give a 12v boost.
 
Automobiles use a 12v system. That means that there are many suppliers of instruments and other components that will work on 12v. The economies of scale mean that 24v stuff will be more expensive for equivalent products. The battery many (most?) of us are using these days costs about $70 on Ebay. I don't think you'll find a similar deal on a 24v battery.

In addition with 12v, you can easily find a battery tender or a battery charger that will work on your system. Shoot, you can jump the battery in your airplane from your car or from an automotive jump starter.

Finally, there are plenty of accessories (cell phones, CD players, etc.) you can run off of a 12v power plug installed in your panel. Not sure if you can do that with a 24v system.

24v is superior in theory (more power, less weight, etc.), but the practicalities all point you to 12v.
 
12v vs 24v discussion

Thanks guys. This was exactly the discussion I wanted to get when I posted. I'm going to proceed ahead with a 12/14v system but I'm doing it knowingly. If there was an emoticon with a smile and a morter board it would be perfect for me - an education without having to go to the school of hard knocks.

Don
 
Avionics

While I still prefer 12v systems personally, there is one factor for 24v that no-one has mentioned yet.

Used avionics are cheaper in their 24v versions. Just look on e-bay, the price is bid up far faster for a KX-155 that is a 12v version than a similar (and probably newer) 24 volt version.

If you are planning lots of avionics, and like the older King stuff, 24v might save you $$ on avionics...

gil in Tucson
 
az_gila said:
While I still prefer 12v systems personally, there is one factor for 24v that no-one has mentioned yet.

Used avionics are cheaper in their 24v versions. Just look on e-bay, the price is bid up far faster for a KX-155 that is a 12v version than a similar (and probably newer) 24 volt version.

If you are planning lots of avionics, and like the older King stuff, 24v might save you $$ on avionics...

gil in Tucson
Sometimes! Anyone planning to use the Grand Rapids system with 24/28 volts, be advised that they charge about $150 more for 24/28 volt unit.
 
Cost savings, weight savings?

az_gila said:
While I still prefer 12v systems personally, there is one factor for 24v that no-one has mentioned yet.

Used avionics are cheaper in their 24v versions. Just look on e-bay, the price is bid up far faster for a KX-155 that is a 12v version than a similar (and probably newer) 24 volt version. If you are planning lots of avionics, and like the older King stuff, 24v might save you $$ on avionics...

gil in Tucson
I agree and disagree. Yes used king stuff like the comm 14 v 28, the 28v is a bargain but not by a large margin. On the other hand 28 volts stuff can cost way more.


Another advantage of 28 volts in the avionics/Inst area is an electric attitude gyro. Many of the small electric surplus units are either 28 volts or 115v/400hz. The 115v/400hz inverters only run on 28 volts. Going from 14 to 28 volts is a pain and impractical in cost and weight. So if that is your game plan is to use 28 volt equip, you are justified from that stand point.

Also Whelen only offers a 28v LED lights, not 12v.


28 volt is technically superior, heck 48 volts better from a power (V*I) transfer standpoint. When you have long runs (like on big planes) the saving in wire weight (because you can run lower amps due to higher volts and smaller wires) is significant. On a little RV the savings in little to none by going to 28 volts.

Also RV's are pretty simple with low electrical load systems. If building a Lancair with large electric motors, electric hydraulics 28 volt makes more sense.

14 volt is still the status quo. If you go buy a starter, alternator, battery, relay, avionics, lights you will find 14 volt is more readily available, new and used.

28 volts is technically superior. 14 volts is more practical. No of this is a turn on to me, so I went with a 14 volts system. :D
 
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Curious

George,

Would your vote differ based upon the kind of avionics you were going to fly behind? Not the brand, but the type and electical circumstances?

For example, if you were flying behind a VFR airplane with 6 pack with vacuum and a single nav/com with VFR GPS, I could see where the strong tendency would be for a 12/14v system - weight savings, avail, etc.

If you were flying behind a full IFR EFIS system with no vacuum, dual electronic ignition and fully redundant alternators, busses, and batteries. I'm curious if you'd still prefer a 12v over a 24v?

I went down the 24v route - mostly because if I do loose the main alt, the backup doesn't have to work so hard providing me "soft" access to the ground :).

I find it's interesting as well. Heated pitot tubes are half the price in 24v as they are in 12v.

Just curious, good discussion btw.
 
I've enjoyed this discussion. Lots of good points. I'd like to add another: If you are building an RV-10 and are thinking about air conditioning, the 24/28v makes a lot of sense. You can put an electric compressor aft and not bother with an engine driven compressor. Also, You can run the air conditioning on the ground with a GPU.
John
 
No one size fits all

aadamson said:
George,

Would your vote differ based upon the kind of avionics you were going to fly behind? Not the brand, but the type and electrical circumstances?
Absolutely, I forgot about the 24 volt pitot, and John's comment about the A/C.

Yes, that was the answer, there is no one answer; no one system voltage is "best". It depends on the equip and mission. Most of the modern avionics run on 10-30 volts anyway.

Also dropping down to 14 volts is easier than going up, so just because you have a 28 v system, does not mean you can't still run a few 14 volt items.

Cars are going to be 42 volts not far down the road. If it was not for inertia (so many cars with 12 volts in service) we would be there. With the Hybrids and the increasing demand of electrical components in cars, the higher voltage makes sense, just like it does in a plane. Some cars have 130 amp alternator! However the inertia issue of 12 volts is real and a consideration.

Cars where 6 volts at one time, right. :D
 
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gmcjetpilot said:
Also dropping down to 14 volts is easier than going up, so just because you have a 28 v system, does not mean you can't still run a few 14 volt items.
This is the route I went. About 95% of my stuff is 28v. For those things that were either really expensive or hard to get in 28v, I've got a small 12v bus. None of the stuff on that bus is critical, so if my converter goes TU, no big deal.

PJ
RV-10 #40032
 
PJSeipel said:
This is the route I went. About 95% of my stuff is 28v. For those things that were either really expensive or hard to get in 28v, I've got a small 12v bus. None of the stuff on that bus is critical, so if my converter goes TU, no big deal.

PJ
RV-10 #40032

Yup, same with me.... I figure if Cessna can use this convertor, then I can too.... And they are *cheap*.

http://www.radiooutfitter.com/store/124056/N241212.html

N2412-24gif.gif
 
gmcjetpilot said:
Cars are going to be 42 volts not far down the road. If it was not for inertia (so many cars with 12 volts in service) we would be there. With the Hybrids and the increasing demand of electrical components in cars, the higher voltage makes sense, just like it does in a plane. Some cars have 130 amp alternator! However the inertia issue of 12 volts is real and a consideration.

Cars where 6 volts at one time, right. :D
This talk about cars going to 42 volts is new to me; where is a good site to go for discussions on this?

PS: My first car was a '50 Chevy with 6 volts....

John
 
Well,
I got on the web and answered my own question. You can buy 42 volt cars & trucks right now. You can get a Silverado pickup with hybrid assist that is 42 volt and acts as a generator.
I noticed that some cars/trucks use a 42 volt battery and others use 36 volt, but all the alternators are 42. It looks like it's here to stay. i found numerous articles talking about the problems with 12 volt and on-board computers in cars. The move towards 42 volt is not being driven by hybrids, it's the computers.
John
 
half the wire?

w1curtis said:
You have to understand where Greg is coming from. That 6-10 pounds may be accurate on the composite plane that he is use to (Cozy Mk 4) but in our metal planes we can use the airframe as ground and effectively cut our wiring in half compared to his composite. While his thinking is technically correct, in the practical world of single engine airplanes, a 12/14 volt system beats out the technical superiority of 28 volts.

Using the airframe as a ground all over the airplane instead of running ground wires and using a common airframe ground is something that will cause so many ground loops in your airplane that you will most likely chase gremlins forever. It's very important no try to prevent ground loops in all of your installations.

Regards,
 
Don said:
If you lose your alternator he argues that you'll have a lot longer use of your electrical system as your battery drops from 24v to 9.6v than you would if you had a 12v system.

Doesn't that sound like a law-of-physics violation? I'd wantt to see either some analysis to back it up, or better yet, a test using two batteries of identical weight, one 12 V and one 24 V.

If your essential avionics load is drawing P watts, and your battery stores E watt-hrs of energy, you will run out of energy P/E hours after the alternator dies.

Why does it matter if you draw P watts as 12 V * I amps, or 24 V * I/2 amps? To first order, the energy stored in a lead-acid battery is approximately proportional to its weight, isn't it?
 
Bryan Wood said:
Using the airframe as a ground all over the airplane instead of running ground wires and using a common airframe ground is something that will cause so many ground loops in your airplane that you will most likely chase gremlins forever. It's very important no try to prevent ground loops in all of your installations.

Regards,
As with all things, that depends. I'm not suggesting chassis ground for avionics. For avionics it is wise to ground everything to a common point. Since avionics are usually in close proximity, this does not represent a great increase in wire weight. But conventional design in metal cars and light plane is to go from the negative side of the battery to the chassis. Then all major power consumers, starter, flap motor, etc use the chassis for the negative side of the wiring. Maybe there are some, but I know of no production metal aircraft (or car) that run TWO massive #2 AWG wires from the battery to the starter? Same for the Pitot Heat, Flap motor, etc. These are the things that have BIG (heavy) wires that you have to run two of in a composite plane. These are the same wires that would be smaller (lighter) in a 28 volt system.

I've noticed some RV-10 builders running DUAL #2 AWG cables from the rear mounted battery to the firewall. I don't think this will reduce the number of electronics gremlins but I guess the think they are offering the electrons a better path--maybe they are since I've noticed some obsessive builders also dipping EACH rivet in primer prior to riveting.
 
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FWIW, I knew better and still wired my landing lights without a seperate ground wire to the lights. In a former life I used to earn my living fixing problems like these in airplanes that had problems like we are talking about. Now when the landing lights are switched on the hash appears in my headsets every time. Add more systems wired this way and enjoy more of these kinds of problems. Do one of these complete wiring jobs and it will drive you and your avionics nuts.

JMHO,
 
For many of the reasons (pro 28V) stated above I have built my current project with a 28V system.

If I had it to do over again I would not.

Probably saved 2-3 pounds in wiring picked up a heck of a lot more in battery and converter. Saved a little on the heated pitot.

No vacuum system and ship will be IFR so have to have 28v backup and you guessed it a separate 28v batt and converter for 14v for the backup bus. As well as a way to get the 14v and 28v from the backup busseS to their respective busseS.

My 14v items are trim fuel boost and the oil scavenge pump.

I do not consider trim or fuel boost non essential for IFR.

Because trip is 14v and flaps 28 V I need a separate relay board for the flaps.


28V equals more weight, more stuff, more complexity
and when it comes to trouble shooting its a what voltage at what point when nightmare.

Stick with 14Volts and save yourself a lot of grief.
 
I’m wondering how the information here has aged. I’m in early stages of building and planning. It seems like 28v is “better” on paper but the disadvantages noted are not insignificant.

If one is planning electric AC and vertical power vp-X pro, it seems that 28v is what to do. The vp-X seems to allow 28v with built-in capability to run trim and flaps at 14v. But what about EI? It seems there are still a few wrinkles and the inertia of 14v still lives on.

Any recent experiences with trying to do 28v, EI, AC, VP-X?

Thanks for the input and sorry if resurrecting this old thread was bad form…?

Patrick
 
While I still prefer 12v systems personally, there is one factor for 24v that no-one has mentioned yet.

Used avionics are cheaper in their 24v versions. Just look on e-bay, the price is bid up far faster for a KX-155 that is a 12v version than a similar (and probably newer) 24 volt version.

If you are planning lots of avionics, and like the older King stuff, 24v might save you $$ on avionics...

gil in Tucson

I got a great deal on an old King 24V comm radio. I made an inexpensive buck converter array to up the 12->24 volts. I needed two units to handle the current and instead wired four of them in parallel in a mini box to provide redundancy and lower their load for longevity. Total cost about $25. Not sure I would do this for a single radio installation, but met my tolerance for a 2nd comm.

Larry
 
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It seems like 28v is “better” on paper but the disadvantages noted are not insignificant.


Patrick

Go look at the price for a 28V alternator and compare that to the $50 many of us pay for an automotive 12V version. Then tell us if it is still better on paper. Then go find trim motors, flap motors, USB ports, map lights, etc. that will work on 28V. I don't see any advantage unless you have something like an elec AC system or deicing. The vAST MAJORITY of wire in my plane is 22 gauge and wouldn't go any thinner even if I could. It is just too hard to work with. I don't think you would save more than a pound or two with 28V. YOu could save more weight by drilling lightening holes in steel parts.

Larry
 
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Larry,

Certainly all good points. Mainly due to EI, I will probably have to go 12 volt.

But something to consider- the vp-X pro, when configured for a 24v system still has outputs to control 12v flaps, trim, and a few other items.

Depending on one’s preference for Ignition systems, using a 24/28v system architecture to drive an all-electric Air-conditioning system may be the way to go.

Also, I don’t plan to use a $50 auto alternator. I’ve replaced my auto alternator on my “American-made” SUV twice now. The first went after 100,000 miles, the second after 5,000 miles and one year, and third is doing better… but I’m not planning on going cheap on the electrics system when planning electronic ignition.

I do appreciate the thoughts, and I’m planning on 12/14v now but maybe my ideas on EI will evolve as I build too.

Patrick
 
Gonna step in it but it should be said. If you’re EI and/or EFI, strongly (re)consider anything that adds complexity in those circuits. Won’t debate but as I mentioned, it needs to be said/heard/considered. Lots of related threads here. Build safe, Sir.
 
12 volts - unless you're planning on an electrically driven AC compressor. Most use an engine driven compressor.
A couple of decades ago, high current usage was pushing more aircraft toward 24 volts. But then came LEDs, temperature controlled pitot heaters, etc., and power requirements dropped. 12 volts works fine today, and, as others have mentioned, is quite a bit less expensive.
 
12 volts - unless you're planning on an electrically driven AC compressor. Most use an engine driven compressor.
A couple of decades ago, high current usage was pushing more aircraft toward 24 volts. But then came LEDs, temperature controlled pitot heaters, etc., and power requirements dropped. 12 volts works fine today, and, as others have mentioned, is quite a bit less expensive.

SofieLite has an all electric 12V AC system.
 
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