What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Electrical Buses

ptrotter

Well Known Member
I am finishing up the design of my electrical systems and would like some opinions on how people have set up their electrical busses. Basically I am putting in a main bus and an essential bus and am curious as to what devices people consider "essential". I am feeding my busses in pretty much the standard way by isolating the essential bus from the main bus with a doiode and providing a switch to connect he essential bus directly to the battery if I want to turn off the master switch to shut down the main bus. The question is what devices should I place on the essential bus so I am curious as to what others have done.
 
Good Question!

This should provide some interesting discussion....

I make no claim to having a "right answer" - just what I thought about in terms of an IFR essential panel. Note that I have a separate Avionics bus which can be switched between Main and Essential as sources....that's where the Navs, Comms and GPS's get their power....

Essential Bus Loads:

Trim
Flaps
Fuel Pump
Audio Panel
EFIS Displays (power feed B)
EIS
AHRS
Autopilot

Of course, for a VFR only plane, I'd probably think a lot different - heck, there isn't much that is essential for flight in that case!

Paul
 
Timely

This is timely and I'd like to chime in with my current plan and a request for feedback as well.

Battery Buss
Battery Contactor
Cigar Lighter Socket
Ignition - E-Mag / P-Mag incl. Maint Ann Light
Starter
Any avionics "keep alive" circuits
Panel flood light

Essential Buss
AHRS / Magnetometer
Audio Panel / Intercom
Autopilot
EFIS
EIS
Elevator Trim
Fuel Boost Pump
GPS(s)
Nav/Comm
Panel power for portable electronics
Transponder
Avionics fan / defroster

Main
Cockpit Light(s)
Add'l Panel Light(s)
Low Voltage Warning Light (also serves as a master switch reminder)
Flaps
Seat Heaters
Landing Light
Taxi Light
Position Lights
Strobes
Starter Contactor

Notes:
1. I have manual aileron trim.
2. I will be able to load shed individual items if needed.
3. This will be a "light" IFR bird meaning no planned enroute IMC, almost only to get up and down through a layer.
4. System design will likely be an Aeroelectric Z-11 or Z-13/8

Input is very much welcome.

George
 
Paul,

Thanks, I'm kind of using your panel as my model but there are a couple of breaker labels I couldn't read on the photos. If I can remember correctly from one of our discussions, your switchable avionics bus source was due primarily to not being able to get the switch you originally wanted and it just kind of ended up that way. If you were to do it again, would you do the same thing or would you just put all the avionics on the essential bus? I sometimes wonder which of the avionics are really essential as I worry about the load on the essential bus of all the avionics as I was only going to feed about 15 amps to the essential bus from the battery directly. I guess I could put my GNC300XL GPS on the main bus as my EFIS has a built in GPS that draws a lot less power and that will be on the essential bus.

Also, did you actually put a seperate breaker in for the AHRS? It seems to me that it could be fed off the same breakers as the displays since the displays are somewhat useless without the AHRS, so why power it seperately.

I'm also doing a small aux battery and 8 amp standby alternator from which I will provide emergnecy power to the EFIS (and AHRS) and my Pmags. I figure so long as I have an attitude display and ignition, I can get the plane down in a real emergency.

As I write this, I am beginning to wonder if an essential bus is really necessary as it seems that all the services that would be shed by shutting down the main bus have on/off switches of their own which could remove their load. Having a direct battery feed could still be useful to recover from a master relay failure, but it may not be necessary to seperate the bussess you can shed all the unnecessary things like lights individually.

As you said, this could create an interesting discussion.
 
Paul,

AS far as avionics loads go, they really aren't very large! Heck, when I am in cruise, with lights off, I am barely registering 6 amps with everything else (including strobes) powered on. The avionics load isn't that significant.

I would probably separate the avionics bus again if I repeated the design, because it does give me an option. I am conditioned by years and years of training to deal with "bus shorts", where you have to unpower a bus. Can't tell you that they are common - except in simulations! For that reason, I like being able to power down a shorted bus, and still be able o power the avionics. There are other ways to do it, but this is pretty simple. You're right that I originally was going to have an On-Off-On Avionics bus switch (wanted it lever locking), and settled for an On-On. No big deal.

I do occasionally power cycle the DU's without cycling the AHRS, so separate breakers are useful. Sometimes, if you want to do a hard reboot on the DU's, that saves an AHRS re-align. (Never had to do it in flight, but very useful for ground testing and software loading.)

If I were doing things again, I would give strong consideration to buying a bunch of power diodes and having all critical boxes powered through multiple feeds, like the GRT units are set up (they do it internally, but there is no reason you couldn't do it externally for other boxes). Then you'd have no systems management to perform at all...and where's the challenge in that?! ;)

Paul
 
George,

You have some interesting ideas. I like the idea of a panel light on the battery bus. I also find it interesting that you put your starter directly on the battery bus without going through the main contactor, I guess there is no real reason that it should. Other than that you came up with pretty much the same list as I did except that I have the Pmags on the essential bus as well as on my emergency battery. That is just a extra failsafe as they should work without power on their own.

My biggest problem is trying to keep from over-complicating things. I need to step back and decide what is really necessary.
 
Ironflight said:
Paul,

AS far as avionics loads go, they really aren't very large! Heck, when I am in cruise, with lights off, I am barely registering 6 amps with everything else (including strobes) powered on. The avionics load isn't that significant.

Paul
Paul,

That is good to know. Without any real experience, I tend to use breaker size to estimate power consumption which I guess is not a very accurate method :)

I agree with you on isolating bus shorts, but I guess you have to draw the line somewhere with respect to complexity.

The idea of diode isolated power feeds is a good one and I am going to power my Pmags through diodes off the essential bus and aux battery.

I think I'll create a avionics/essential bus and use a locking off/on/on switch as follows:

Off
Main
Main and Battery

This has the same effect as a seperate battery feed switch, while allowing an off position for the bus.

Too much to think about this late at night.
 
ptrotter said:
George,

You have some interesting ideas. I like the idea of a panel light on the battery bus. I also find it interesting that you put your starter directly on the battery bus without going through the main contactor, I guess there is no real reason that it should. Other than that you came up with pretty much the same list as I did except that I have the Pmags on the essential bus as well as on my emergency battery. That is just a extra failsafe as they should work without power on their own.

My biggest problem is trying to keep from over-complicating things. I need to step back and decide what is really necessary.

Paul:

When using a diode to connect two buses, remember that there will be a voltage drop. So be careful where you sense the bus voltage on your system, otherwise you may see it at .7 volts low and it may give you a warning depending upon which low voltage warning type of sensing you use. I originally used a diode in my split bus system, and changed over to a dual path relay which was rated at 4 times the power requirement that the essential bus draws.

I also have an essential bus power feed through a fuse wired directly to the battery. If the master contactor opens, all I need do is lift the essentail bus power feed switch guard and flip the switch.

The only advantage to having the spit bus is that it takes the decision making away from the pilot, only powering essential items by way of bus grouping. In reality, all the pilot need do is reduce the load by turning things off (or pulling breakers) and run with a reduced load until landing. Try to keep in mind that no two inflight distress situations are alike. There may be times when you may need a certain piece of equipment that is not powered by the essential bus. So the real need for installing an essential bus can be questioned, especially when put in the context of your comment about over-complicating things.

Simpler, lighter, cheaper and less prone to failure is often the best way to go.

Anyway, my .02c your mileage may vary.
 
I also find it interesting that you put your starter directly on the battery bus without going through the main contactor, I guess there is no real reason that it should.
If the starter current does not go through the main contactor, what is the procedure to use in the unlikely event that the starter relay ever sticks closed? If the starter current runs through the main contactor, you can simply select the Battery Master OFF, and this removes power to the starter. If the starter current does not go through the main contactor, your choices seem to be to let it spin until it drains the battery, which would likely result in the battery getting, very, very hot. Would the battery explode? Or, you could find some tools and unbolt the battery cable, but I wouldn't want to be the guy who pulls a battery cable off with over 200 amps going through it.
 
Here's mine and the reasons:

Essential:
  • GPS - Need to know where I'm going
  • NAV/COM - Need to talk to people to ask for help
  • TPDR - I want Center to be able to find me
  • INTERCOMM - Can't hear the radios without it
  • EI - Need to spin the big prop out front
  • EFIS - Need to keep greasy side down
  • AH - Backup instrument, same as above
  • Engine Monitor - Critical fuel information contained here
EVERYTHING else goes on the primary. I don't need it for continued flight ops. When I get ready to land, I can switch the Master back on to use the fuel pump, flaps, or trim motors.
 
Randy, almost exactly my thoughts---------one difference, my GPS has built in batteries, so it is not on my essential.

I agree with your turning on the main for landing idea------same thought. The idea is to shut off the main batt while there is still power left in it, so you have the landing reserve. This mandates the use, and monitoring, of a charge warning system.

Mike
 
Last edited:
Update

Gentlemen,

This is a terrific thread: very thought-provoking and useful. Based on this, my revised plan is:

Battery Buss
Battery Contactor
Cigar Lighter Socket
Ignition - E-Mag / P-Mag incl. Maint Ann Light
Any avionics "keep alive" circuits
Panel flood light

Essential Buss
AHRS / Magnetometer
Audio Panel / Intercom
Autopilot
EFIS
EIS
Elevator Trim
GPS(s)
Nav/Comm
Panel power for portable electronics
Transponder
Avionics fan / defroster
Low Voltage Warning Light (also serves as a master/e-bus switch reminder)

Main Buss
Cockpit Light(s)
Add'l Panel Light(s)
Fuel Boost Pump
Flaps
Seat Heaters
Landing Light
Taxi Light
Position Lights
Strobes
Starter & Contactor

My rationale is that I will fire up the Main Buss to land, or keep it fired up and load shed individually if needed. The E-Buss, though, is everything I need for continued comfortable flight if I'm running on battery only, and the switch makes it easy to quickly load shed.

Paul & Kevin, good catch on the starter. It does belong on the main buss, an oversight in my first go, and for the purpose of killing a runaway. The fuel pump too as it is like flaps and should only be needed at the end of the flight.

I suspect we all want a system that is as simple and robust as possible, with very logical and easy to perform tasks for as many forseeable events as possible. How we each express that is quite different and good to pay attention to.

George
 
Personally, I would put the trim, and possibly the AP on the main buss - especially if you intend to fly mainly as VFR or IFR "lite" as your primary mission. If you intend to fly regularly in IMC or night conditions, then yes I can see the desire for those on the essential buss. Portable electronics should have (fresh) internal battery backup in any case - so panel power for them comes from the main buss.

My way of thinking is for the essential buss to be only what is absolutely critical for flight - not what is comfortable or nice to have. In the event things start smoking in the cockpit, you want to be able to kill all current flow and selectively bring items back online to isolate the trouble. A very lean essential buss gives you flight-critical items without a large probability of powering up the smoke-emitting item. For me, my idea of "essential" means about 30 minutes of emergency battery backup for the "Gotta have that" instruments and I'm looking for the nearest safe landing area - you don't need a lot of instrumentation for that kind of "essential" conditions - just enough to complete a night instrument approach. If you're in VFR conditions you can kill EVERYTHING and still get down - if IMC without vacuum backup, you simply have to take the chance that your essential buss will not smoke. If it smokes, you MAY still make it down - but without it your odds are very poor indeed, as the NTSB accounts will testify to.
 
Last edited:
If it were me, I would also take the alternator warning light circuit and the avionics fan off as well. My logic for the light is that I'm probably on the E-bus because my alternator failed. I don't need to see the light continously on to remind me and I want to save the current draw.

I don't have an avionics fan even though I have a stack of Garmin radios. I'm not sure one is needed but if I had one, I'm sure my radios could survive an hour without it.
 
Kevin Horton said:
If the starter current does not go through the main contactor, what is the procedure to use in the unlikely event that the starter relay ever sticks closed?


I have three warning lights installed:

1) Low Voltage -Yellow
2) Fuel Pump On -Blue
3) Starter Engaged -Red

So if the start contactor sticks you'll know it.
 
Do you really need it, really?

-Anything you MUST have to fly the airplane, to physically stay in the air.
-The idea is to extend the battery power by cutting out all unnecessary items, especially big drain items.

This is not an argument but a thought
-Flaps? you can land flaps up.
-Boost Pump? The engine runs fine without it.
-Transponder? Don't need to stay in the air (live near class B, C nice 2 have)
-Com? Probably a must in most builders books unless you have a handheld
-GPS? Probably a good thing and handhelds have low drain, but also can have their own battery
-Electronic Ignition? A must to stay in the air if you don't have backup (magneto)
-Glass?A big one now, since all pitot/static & mechanical gauges are all-in-one EFIS & Engine Monitors. (Can physically fly without it, but its a must for most).
-Lights? You can fly without lights in an emergency, must land, dark runway at night, Landing lights a must?​

The idea is extend the battery.
What do you need to stay in the air?
Don't confuse legal & min necessary to stay in the air.
The idea is extend the battery, for that item or items you MUST HAVE!
Have you ever tested you "essential items on the ground to see how long they last on battery power?

Autoengines with electronic fuel injection and ignition are the most demanding. Since the engine is so electrically dependant you may NOT want to have the EFIS as essential if you have backup mechanical/pitot-static instruments.

I see essential busses with everything on it, why? (especially if you can individually turn off individual items).

The question is DO you need an individual essential buss at all?

Why an essential buss, just turn stuff off you don't need when you want to conserve power. If you don't want to use, lights, flaps and boost, leave them off. (Not recommending this, but I know a guy in a Cessna that flew freaking 1/2 way across the US but turning everything OFF, wet compass, map, airspeed, altitude and watch. Piper J3 has no electrical system. I think the first goal with the electrical system is make it NON-essential if at all possible.

Food for thought to simplify wiring. Our planes are mostly day VFR (most fly day 90% of the time) and we are wiring them like a B737. Nothing wrong with it, but....
Keep it light
Keep it simple
Keep it per plans (has advantages)
 
Last edited:
I must say, I'm with George on this one. If I were setting up for VFR, I'd just as soon just hit the Master and be done with it. A big reason for the e-bus architecture is for redundancy 'cause loosing everything in the clouds is a bad place to be. For VFR, a handheld with VOR reception (like Sporty's), two eyeballs and a map (or even better, a cheapy handheld GPS) would give me all the navigation and communication I need to make any VFR flight with no pucker factor whatsoever.
 
Why an essential buss, just turn stuff off you don't need when you want to conserve power
An advantage of using an essential buss is to take the master solenoid out of the loop. If you turn everything off individually but leave the master on, you are still depleting your battery. Just how much juice does the master solenoid take, I?m not sure but 1 amp sticks in my mind.
 
f1rocket said:
Here's mine and the reasons:

Essential:
  • GPS - Need to know where I'm going
  • NAV/COM - Need to talk to people to ask for help
  • TPDR - I want Center to be able to find me
  • INTERCOMM - Can't hear the radios without it
  • EI - Need to spin the big prop out front
  • EFIS - Need to keep greasy side down
  • AH - Backup instrument, same as above
  • Engine Monitor - Critical fuel information contained here
EVERYTHING else goes on the primary. I don't need it for continued flight ops. When I get ready to land, I can switch the Master back on to use the fuel pump, flaps, or trim motors.

After re-reading this, and other posts here, I just came to the realization that Randy and some others (myself included) probably have dual battery setups-----or dual alternators, or both---------and others dont.

It really makes a difference in how you look at the idea of an "essential" buss if you have a second, parallel power source.

Anyway, the idea of shutting off the main battery and running on the "essential" source is something that is much more adaptable to a dual system.

Food for thought.

Mike

Big batt, little batt, 60 amp alt, 8 amp alt.
 
Last edited:
The Rationale

Thanks Randy, the fan is off the e-buss now and back to the main, if I even put one in (desert living means I'd like as much air movement for radios as possible).

The low-voltage warning light has been a maybe from the get go since I'll have an EIS with low-voltage warning. I like the light, though, to remind me that the master and/or the e-buss feed is on, which is why I'd put it on the e-buss.

Great stuff all! Here's some more rationale...

1. If I were going VFR only, I would likely end up with the same electrical system but less avionics.
VFR
Transponder
Garmin SL40 or similar
Panel mounted Garmin 496 or similar
EIS (GRT likely)
Steam Airspeed & Altitude
Homebuilt LRI

"Light" IFR
Transponder
Garmin SL30 or similar
Handheld VHF as backup
Panel mounted Garmin 496 or similar
EIS (GRT likely)
EFIS (dual screen GRT is frontrunner)
Marker Beacon
Homebuilt LRI

At this point, I'm not likely to go with an IFR capable gps unless GRT is shipping theirs. If they're not, I'll probably put a Garmin 81 as a second backup. I also plan on the 496 as backup to the EFIS - for the level of IFR I'm planning, I believe that to be sufficient and will train accordingly.

2. I'm likely going E-Mag/P-Mag so will have something of an out in the P-Mag for total battery and alt. failure. Not quite magneto level independence, but close.

3. Alternator (likely Plane Power int. reg. w/int. OVP) failure leaves battery only and the need, if in IMC, to shed non-essential loads as fast as practical. Radios/glass/even auto-pilot are all low enough draw that putting them on an E-Buss is fine to me, more importantly I can shed them later if I choose. I don't, however, want to lose any of them if I want to shut the main down in a hurry. I can refire the main when I need flaps, pump, and lights to land. Even battery only with no alternator should leave 1-hour plus times with the loads and battery I am looking at.

4. Taking the main contactor offline saves 1 amp, quite useful if on battery and no alternator. This alone is reason enough to me for the simple addition of the E-Buss.

5. I am either going to have a backup vacuum pad alternator from the start, or be wired for it to be added later. It simply makes the main alternator failure a non-event assuming everything on the E-Buss can be powered entirely by the 8 amp backup (reasonable based on my load analysis thus far).

6. I'll likely put a backup battery only on the EFIS. Main battery failure, while unlikely, would lead to an instant desire on my part to land.

7. Smoke in VMC: power it all down right now. Smoke in IMC: power down main and see if it helps. Add or subtract items as needed/able. Hopefully not have to power down e-buss.

8. As complicated as this all sounds, the actual system will end up quite simple, the additional weight of the e-buss is negligible, same with the complexity. Dropping the e-buss would mean one less panel switch, a larger main fuse block and no e-buss fuse-block, and one less diode.

9. Unfortunately, simple, light, inexpensive, and IFR capable, don't all seem to be destined to live and play well together. I think I'm going to have to settle for relatively simple, relatively light, stupidly expensive, and IFR capable.

George
 
Last edited:
I've been going through a similar analysis over the last few weeks. I'm outfitting for IFR and plan to use the Z-13 design. One significant factor I haven't been able to decide on is whether pitot heat should go on the E-bus. At 7 amps, this would seem to drive the need to go with a 20 amp backup alternator instead of the 8 amp.

I'd appreciate any thoughts on this.
 
Pitot Heat

Mike,

I passed on the pitot heat altogether, I am very icing averse and don't intend to get near any possibility of it.

That said, if I did get into icing I'd want out ASAP so leaving the pitot heat on the main buss seems just fine to me.

The 20amp backup might make sense if you planned on hard regular IFR or were willing to take more risks with icing. Neither appeals to me nor does it seems to fit the mission/ability of the RV's.

George
 
MTBehnke said:
I've been going through a similar analysis over the last few weeks. I'm outfitting for IFR and plan to use the Z-13 design. One significant factor I haven't been able to decide on is whether pitot heat should go on the E-bus. At 7 amps, this would seem to drive the need to go with a 20 amp backup alternator instead of the 8 amp.

I'd appreciate any thoughts on this.

Kind of depends on your setup and what you're using for backups. For example, if we build a panel with a Dynon as the backup instrument (which it's wonderful for with it's own internal battery) then we like to see the pitot heat have some sort of backup power/redundancy or be on the same buss as the Dynon - as a Dynon with no pitot is useless as a flight instrument (before anyone flames me, I'm not taking anything away from the usefullness of the instrument because I fly behind one....I'm just pointing out that if you remove the pitot from that instrument in the clouds and it's your only attitude indicator bad things are likely to happen). Now, if you have a different kind of attitude indicator for your backup, then don't bother with the pitot tube. My opinion is the Ebuss is there to get you on the ground quickly and safely....not keep you flying for extended periods or time continuing on in crappy conditions.

Just my 2 cents as usual!

Cheers,
Stein.
 
SteinAir said:
My opinion is the Ebuss is there to get you on the ground quickly and safely....not keep you flying for extended periods or time continuing on in crappy conditions.

Just my 2 cents as usual!

Cheers,
Stein.

Stein, I also adhere to the same interpretation of the Ebuss, use it to make a safe and expedient landing somewhere you can get the plane fixed.

I hear, and to some extent, understand Aerolectric Bobs idea of finishing the flight to the original destination, but as far as I am concerned I dont feel like flying a plane with a known fault to be a good path to longevity as a pilot.

And, I mean no disrespect to Bob, but I have to build and fly to my own level of comfort.

This is one of the things that make this hobby so great-------------freedom to "do it my way"........

Mike
 
Stein and Mike,

Great points. I've been back and forth on the Essential (get on ground) vs. Endurance (finish the flight) bus goals as well. Think you've helped me settle on the no-pitot heat / land quickly concept. Save a little money and weight as well.

Thanks to both for your help!
 
Z11 ESS diode or no diode

I have been following the Z11 basic design but had planned on including an avionics master switch similar to Paul Dye so that I could start the engine and then fire up the avionics which would include the D-120 EMS (I have a oil pressure light to know that oil pressure is good ), it just struck me reading this thread that if this switch is a ON-OFF-ON to select between the main buss and the essential buss then the diode pack is not required, have I got this correct?
 
McFly said:
An advantage of using an essential buss is to take the master solenoid out of the loop.

I'm a complete novice when it comes to electrical stuff, so I hope this isn't a dumb question:

If isolating the master solenoid is such a good idea, does anybody know why Cherokees and 172s aren't wired this way? Is there a downside to taking the master solenoid out of the loop?
 
You didn't mention how you are equipping your plane.

If day VFR only, the list is very short. If IFR, things get a bit more complex.

EFIS - If yours has a battery, leave it off the Ebus. It should last two hours or more. (probably 1 1/2 hours longer than you will need.)

EMS - probably not needed. As long as that thing up front is making noise, all is good. Besides, you should be looking for a place to land. Again, this one is a comfort level thing.

Landing lights - Include them but no other light. This could well use your last few ounces (?) of juice.

Radio - Yes

Interior light - Yes

Trim - your call but don't use it because motors can drain a battery FAST.

Nav Radio - Yes, if you don't have a handheld GPS w/ internal battery.

Electric Fuel Pump - What George said. Besides, this is to be used only when the engine drive pump fails. Could put it on the bus and not use unless needed. Kind of like the landing light, only turn it on when you have the runway made.

E/P-Mag - No. The P-mag is self powered so your engine will continue to run w/o being on the Ebus. The E-mag will not function but that is why you have duel ignition. A LOT of airplanes have landed with only one operational ignition.

Let us know how you set yours up.

PS. My -9 is a a day/night VFR ship and I elected to skip the Ebus thing. Just my choice but did install duel P-mags and put every item on it's own breaker so I can selectively power down items as needed.
 
My load analysis using a 13/8 architecture:

B-Bus: .8 amps continuous in alternate mode
E-mag .7
P-mag 0
EFIS clock .1
power jack 0
e-bus relay 1 (in normal, 0 in alternate)

E-Bus: total draw 9.1 amps in alternate mode
GRT EFIS
EIS-4000
MAP sensor
TruTrak A/P
Trim
SL-30
LED strip panel light
Pilot map light

M-bus:
SL-40
GPS
Audio panel
exterior lights
Transponder
flaps
boost pump
Seat heaters
copilot map light
AoA
indicator lights
battery contactor
TruTrak ADI with internal battery

My audio panel in PSE PMA-8000B, when power is lost, it defaults to com 1, nav 1 so don't need it on the e-bus, this save 2.5 amps.

At cruise power, the P-mag uses no outside current and the e-mag uses only .7 amp. Seems better to me to expend the .7 to keep a 2nd mag operating.

I think an operable A/P is a safety item in IMC. Don't want to loose that when the chips are down.

The ADI works off it's backup battery with the flick of a switch, so it's not needed on the e-bus.

With this set up, I've got full IFR/night capability at 10 amps with com, nav, engine power, trim, full instrumentation, panel lights, a map light and dual attitude references. I think the SD-8 may cover this draw, if not, the battery may contribute 1 amp. Definately enough to use a full bag of gas in the goo.

I thought long and hard about keeping the GPS vs. the nav but in the end, the Nav won out. I carry a portable GPS.

Jekyll
 
Last edited:
At the risk of sounding like an advertisement, this whole topic is a non-issue with the Vertical Power system (the VP-200). The "endurance bus" is built into the system, and can power avionics (or any device you choose) via the battery directly or via the battery contactor.

You can specify -individually- when devices turn on and off, and change them later if you don't like it using the setup menus. No changes to the wiring.

You can also specify what stays on and what turns off when the alternator fails and you go into a load shed condition. Don't like it? Change it using the setup menus. No changes to the wiring. You can also specify a VMC vs IMC load shed, so you're not locked into one hard-wired configuration. And it's very easy to use in an emergency.

You can also see the real-time current draw of the entire system, or *each* electrical device individually. It really helps to understand what your system is doing. Turns out most things draw less than you think, but now you have real data to make informed decisions.

See us at OSH (Hangar B) for a demo. :D
 
This has turned out to be a very interesting discussion with the usual variety of ideas. After reading all the responses and discussing the ideas with a couple of other builders, I am coming to the conclusion that multiple buses may not be necessary. I am building a fairly complex aircraft capable of IFR flight (although I don't expect to use this capability) and even with this complexity of systems, I'm not sure multiple buses will gain me anything except more complexity. The first problem is that I cannot decide what is "really" essential. If I choose everything that "might" be needed then I end up with so much on the essential bus that it makes no sense to seperate it from the main bus. Since I have seperate breakers for every system, I can shed whatever load I need to using these breakers. In this way I can pick and choose the loads to shed based on what I need at any particular time, and can change that in different phases of flight as necessary. The only disadvantage to this is that I cannot shed the load quickly and I need to think about it a little rather than just flipping a switch. The only situation that this does not cover is a bus short or main contactor failure (which I could mitigate by using redundant main contactors). Neither of these events is very likely, but I plan to cover this situation by using a small auxillary battery to power just my EFIS and ignition. This is all that is needed to get the plane on the ground as fast as possible if there is a situation that requires the entire electrical system to be shut down. Perhaps I should just consider this auxillary battery my essential bus:)

The problem is that designing a system to overcome any possible problem adds so much complexity that this complexity itself adds more risk that then must be addressed. This becomes a vicious circle that has no real end. Sometimes you have to step back and say "is this 'really' necessary?" and design a system that meets your basic needs without overcomplicating the situation.

Rather than ask myself what I really "needed" to get the plane down, I asked myself what I didn't need. When I removed all the items I didn't need there really wasn't anything left so that kind of eliminated the need for an essential bus.

I'm also coming to the conclusion that the idea of an essentail bus comes from the Bob Nucholls designs, which while excellent, generally uses fuses rather than breakers which does not allow one to selectively shed load. Multiple buses make a lot of sense when fuses are used, but not so much when breakers are used.

I'm still not sure which route I will take, but this discussion has gotten me to look at the issue in different ways and that will help me come up with the best solution for me.

Thanks for all the comments.
 
How many buses are too many?

Gents,

I have been following this post for some time. I too am very close to deciding on the electrical layout of my RV-7A. After reading the AeroElectric Connection I learned all about buses. But how many does one really need?

I am building an VFR airplane with a Dynon EFIS/Engine Monitor, Garmin radio and transponder stack. Topping off with a Garmin GPS 396/496. I am using toggle switches, circuit breakers and fuses and now after reading this post I need multiple buses?

Wait a minute. The idea of having a E-bus vs a Main bus is that you can shut down a group of systems to minimize power consumption if needed. I have switches that can do that but I do have to spend the time thinking of which switch I need to shut off or which breaker to pull. I believe I can manage that pretty well without the complexity of multiple buses.

When I go to stir the Cryo* tanks in my RV-7A and something goes horribly wrong I believe I will be able to figure out what should or shouldn't be shut down.

No disrespect intended to those with more complex aircraft with multiple buses.

* This is a reference to stirring the cryogenic tanks on the Apollo 13 command module which caused a near catastrophic end to the Apollo 13 mission and crew-Reference provided for those young RV builders out there.

Food for thought.

Paul

RV-4/RV-7A (Finishing Kit)
 
Last edited:
Wait a minute. The idea of having a E-bus vs a Main bus is that you can shut down a group of systems to minimize power consumption if needed. I have switches that can do that but I do have to spend the time thinking of which switch I need to shut off or which breaker to pull. I believe I can manage that pretty well without the complexity of multiple buses.

The primary reason for the E-bus on my RV-6 is to prevent a total power loss if the master contactor fails. If the master fails, it won't matter what kind of switches or breakers you have. Unless you have independent battery backup either on each important device or by the ship battery via an E-bus, you are out of power.
 
Sam has made the same point I would - my essential bus gives me power independent of the master contactor - and I have had those fail before.

I do agree that "load Shed" is probably not sufficient reason for me to build a complex electrical system....I can power everything down individually, and frankly, all of my "normal" loads are so small that it isn't really a big deal. All of the big loads (lights) are usually off anyway....

Paul
 
Single Bus with dual feed

I am new to electrical systems and I have been trying to educate myself...

Mission: Day with some night VFR flying. (RV-8, MVP-50 engine monitor, TruTrak EFIS (maybe), GPS, COM, Transponder, AOA.)

Everything will have its own switch.

Question:

If you used a single bus could you provide an alternate (second) power feed from the battery to the single bus through a potter & brumfield circuit breaker switch?

Thanks for any comments

Paul
 
Extra stuff is minimal...

Pmerems said:
Gents,

I have been following this post for some time. I too am very close to deciding on the electrical layout of my RV-7A. After reading the AeroElectric Connection I learned all about buses. But how many does one really need?
.....Paul
Paul,

One way of looking at it is the extra parts/work needed to implement the two busses.

It's cost is one diode, one extra switch and a partition (no extra, a division only) of your fuses and/or breakers.

In the grand electrical scheme of your RV, it's pretty minimal in both cost and extra work... I say go for it, even for a simple VFR panel.

gil in Tucson

PS I like "busses" with 3 "s" total even if spell check doesn't... :)
 
E-bus

Reviving an old thread cuz I don't understand something.
I'm using the Aircraft Flight Systems ACM. It specifically forbids powering the P-Mags.
That means I need another BUSS. Makes sense to have the P-mags on an E-BUSS.
What I don't understand is what does the E-BUSS look like?
I envision a feed from the battery, but is there another contactor and switch? Seems overkill for a few amps.
Is it just a battery feed with an ANL or other circuit protection then a switchable breaker(s) inside for the circuits?
Is there an actual BUSS inside? Maybe the BUSS is just a bar connecting the breakers?
I need two breakers for the two P-mags and maybe a third for E-BUSS items like a stand alone EFIS. Just enough to get back on the ground. It's not IFR (yet).

All advise appreciated.
 
A "bus" is just a fuse holder or some other method of power distribution - even a wire with a lot of splices could be considered a bus - although not exactly what we want to do in our aircraft.

Here's how I did it.

https://vansairforce.net/community/showpost.php?p=1403570&postcount=55

It's pretty simple - run a connection from the battery to something like this:

https://www.steinair.com/product/12-circuit-fuse-block/

You can connect the fuse holder to the battery directly (always on) or have a switch between the battery and the fuse holder.

I just have two fuse holders connected to the battery contactor, which is activated by a switch.
 
A "bus" is just a fuse holder or some other method of power distribution - even a wire with a lot of splices could be considered a bus - although not exactly what we want to do in our aircraft.

Here's how I did it.

https://vansairforce.net/community/showpost.php?p=1403570&postcount=55

It's pretty simple - run a connection from the battery to something like this:

https://www.steinair.com/product/12-circuit-fuse-block/

You can connect the fuse holder to the battery directly (always on) or have a switch between the battery and the fuse holder.

I just have two fuse holders connected to the battery contactor, which is activated by a switch.

Or this -- except I really don't like to use FASTON terminations, but that's what was available.
 

Attachments

  • Screen Shot 2021-04-29 at 3.16.02 PM.png
    Screen Shot 2021-04-29 at 3.16.02 PM.png
    1.3 MB · Views: 161
Just curious - why not? Bob Nuckolls recommends them for a variety of what seem to be good reasons.

Paranoid perhaps :)

There's no belt/suspenders holding them in place, just spring tension. I suppose they're fine if they are only installed ONCE, or re-formed after every removal to keep the original tension (drag? friction?) force on the tongue.

I like ring terminals, they aren't coming out if the wire gets tugged on...And yes, they can come loose if not secured with screw/locking washer|correct torque, but so can alot of things...
 
Cable protection

Seems like there should be some sort of breaker, fuse, current limiter or something on the cable between the battery and entering the cabin.
What's the best way to protect the cable for a low amp buss like this?
Maybe 10 amps. A simple blade fuse seems the easiest.
How about a simple relay. Is there such a thing as a switchable relay/breaker?
That would allow the E-Bus to be switched on/off.
Thjs is getting complicated!:D
 
Last edited:
What you need is an engine bus with no other loads on it.
Bob Nuckolls has a new electrical drawing called Z101B.
The engine bus is drawn in pink. Notice that it is not protected
by a fuse or circuit breaker which could be failure points.
The engine bus also gets power from both sides of the master contactor.
If the battery fails, the engine bus still gets powered by the alternator.
If the alternator fails , the engine gets powered by the battery.
If the master contactor fails, the engine gets powered either by the
battery or alternator, whichever has the highest voltage.
 
Reviving an old thread cuz I don't understand something.
I'm using the Aircraft Flight Systems ACM. It specifically forbids powering the P-Mags.
That means I need another BUSS. Makes sense to have the P-mags on an E-BUSS.
What I don't understand is what does the E-BUSS look like?
I envision a feed from the battery, but is there another contactor and switch? Seems overkill for a few amps.
Is it just a battery feed with an ANL or other circuit protection then a switchable breaker(s) inside for the circuits?
Is there an actual BUSS inside? Maybe the BUSS is just a bar connecting the breakers?
I need two breakers for the two P-mags and maybe a third for E-BUSS items like a stand alone EFIS. Just enough to get back on the ground. It's not IFR (yet).

All advise appreciated.

Reminder, pMags have an internal generator, they keep working if you loose all electrical power (RPM above ~800), just like a mag. Power is only needed for start up. EFII engines require the extensive backup power capacity, not pMags.

A simple pull breaker powered from your standard aircraft buss that gets power when you turn on your master. An ON/OFF switch to control the pMag (just like a mag). Test the pMag internal generator from time to time by opening the breaker(s) with the engine running.

No “engine buss” required or desired.

Now - keeping the panel up under IFR conditions when you have a power distribution failure is a very different issue.

Carl
 
Aircraft Buss

Reminder, pMags have an internal generator, they keep working if you loose all electrical power (RPM above ~800), just like a mag. Power is only needed for start up. EFII engines require the extensive backup power capacity, not pMags.

A simple pull breaker powered from your standard aircraft buss that gets power when you turn on your master. An ON/OFF switch to control the pMag (just like a mag). Test the pMag internal generator from time to time by opening the breaker(s) with the engine running.

No “engine buss” required or desired.

Now - keeping the panel up under IFR conditions when you have a power distribution failure is a very different issue.

Carl

Agreed but my problem is the ACM is the aircraft buss and it has no provision to power the P-Mags. I have to have some sort of auxiliary buss. Battery buss makes sense but I like KISS. The simplest would be a fused circuit fed from the Master Contactor buss to the P-Mag switches. As you mention, they keep running if the fuse blows and it's not an IFR airplane.
I will ask AFS what their P-Mag customers do. Their schematic shows a simple battery circuit.
 
Last edited:
Larry, you don't really need your PMAGs on an endurance bus. They can just be connected to your main bus. When you turn off the main bus if needed in the case of an electrical partial shutdown they are self powered. I realize this is a decision folks have to make based upon their level of comfort, but that is the way I have mine and the suggestion from Bob K. if you review his schematics.
 
Agreed but my problem is the ACM is the aircraft buss and it has no provision to power the P-Mags. I have to have some sort of auxiliary buss. Battery buss makes sense but I like KISS. The simplest would be a fused circuit fed from the Master Contactor buss to the P-Mag switches. As you mention, they keep running if the fuse blows and it's not an IFR airplane.
I will ask AFS what their P-Mag customers do. Their schematic shows a simple battery circuit.

Interesting that buying a box like the ACM or perhaps VPX that is supposed to make life easier - does not always fit the bill.

Many builders like an electrical power management box that does everything. For me none of the current offerings fit my design objectives.

Carl
 
Interesting that buying a box like the ACM or perhaps VPX that is supposed to make life easier - does not always fit the bill.

Many builders like an electrical power management box that does everything. For me none of the current offerings fit my design objectives.

Carl

We maxed out a VPX Pro and still have 16 circuit breakers for our Ebus. It is a little disheartening when you try to consolidate with the latest tech and still end up having to put an array of old school CBs in your panel because the new tech can't do everything.
 
Larry, you don't really need your PMAGs on an endurance bus. They can just be connected to your main bus. When you turn off the main bus if needed in the case of an electrical partial shutdown they are self powered. I realize this is a decision folks have to make based upon their level of comfort, but that is the way I have mine and the suggestion from Bob K. if you review his schematics.

If I had 2 pmags, I would consider some form of b/u power. A scenario to consider: Elec issue causes the need to disconnect master, battery dies, etc. Pilot on final pulls power back to idle and pmags stop self-generating power and engine dies. If a go around is necessary or additional power is necessary to make the runway, problems ensue. Different story if the pilot understands this and follows an engine out approach, but can you expect every pilot to understand that situation and react accordingly given that the engine had previously been running just fine without ship power before he/she pulled the power back?

Note I don't have Pmags and don't really know at what RPM they stop producing power, but I thought it as above idle RPMs (800-1000 is what I have read).

Larry
 
Last edited:
I need two breakers for the two P-mags and maybe a third for E-BUSS items like a stand alone EFIS. Just enough to get back on the ground. It's not IFR (yet).

All advise appreciated.

If KISS is your goal, maybe it looks like this. & remember most EFIS systems have their own internal BU batteries.
Check the P-Mag manual, it suggests it would be operational anything above 900 rpm & sufficient to land without outside power.
 

Attachments

  • acm wiring.jpg
    acm wiring.jpg
    21.9 KB · Views: 142
Last edited:
Back
Top