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05-21-2018, 05:43 PM
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Join Date: Aug 2010
Location: Newport News, Va
Posts: 325
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always hot wires
Quote:
Originally Posted by Pukauma
... I have been considering running an AWG 6 "fat wire" to the switches for the EFI bus. According to Nuckolls he feels it does not require fusing.
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Where is that EFI bus located?
Bob Nuckols' Aeroelectric Connection: "As a rule of thumb we try to avoid long runs of always hot wire fused at more than 7A. This is a crash-safety issue. If your E-Bus really needs a feeder protected at more than 7A, consider adding a power relay..." (rev 12A says in Appendix Z Page Z-6 under Figure Z-32)
In the Aeroelectric List I found: "Always hot wires are either crew controlled (relay or contactor) -OR- protected at low levels on the order of 7A fuse or 5A breaker. If you are attaching wires to the battery and they have lengths greater than the 6-inch rule of thumb then the design goal for low magnitude protection applies. In other words, a battery bus is located next to the battery. Any other location makes it something else."
Also an always hot wire can get you in trouble while working on the aircraft if you don't disconnect the negative battery terminal first.
The S704-1 20A relay sold by B&C has coil current of only 0.1A.
Bob Nuclols puts a "current limiter" in the alternator B lead so if there is a hard short at the alternator the current limiter will open. A fat wire shorted to aluminum sheetmetal will reportedly clear itself by burning the aluminum away like EDM machining but shorted to something thicker like alternator or engine case can conduct hundreds of amps. http://www.aeroelectric.com/articles/anl/anlvsjjs.html
Note also that Bob adds a starter contactor close to the battery contactor so there will not be an always-hot fat wire running to the starter.
Just my 2 cents as a Nuckols believer... flame on!
Last edited by johnbright : 05-21-2018 at 06:00 PM.
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05-21-2018, 06:56 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by johnbright
If your E-Bus really needs a feeder protected at more than 7A, consider adding a power relay..." (rev 12A says in Appendix Z Page Z-6 under Figure Z-32)
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I may not be understanding this sentence properly but an EFI E-Bus will always require a lot more than 7 amps feeding it- like triple that. Was this statement directed at EFI architecture specifically or other electrical systems like avionics?
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05-21-2018, 07:46 PM
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Join Date: Aug 2010
Location: Newport News, Va
Posts: 325
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Quote:
Originally Posted by rv6ejguy
I may not be understanding this sentence properly but an EFI E-Bus will always require a lot more than 7 amps feeding it- like triple that. Was this statement directed at EFI architecture specifically or other electrical systems like avionics?
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It's a generic statement... doesn't matter what's at the other end of the wire. He's saying don't run 20A from the battery bus to a switch on the panel; instead have a cockpit controllable relay close (6" or less) to the battery bus like the latest Z-13. An earlier Z-13 had a 7A fuse and no relay. The bus and relay is at the battery and the switch on the panel. https://drive.google.com/open?id=19v...MRef2DWwaqTbOp
Last edited by johnbright : 01-22-2020 at 05:38 PM.
Reason: remove link to outdated schematic, new link in signature
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05-21-2018, 08:05 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by johnbright
It's a generic statement... doesn't matter what's at the other end of the wire. He's saying don't run 20A from the battery bus to a switch on the panel; instead have a cockpit controllable relay close (6" or less) to the battery bus like the latest Z-13. An earlier Z-13 had a 7A fuse and no relay. The bus and relay is at the battery and the switch on the panel. https://drive.google.com/open?id=19v...MRef2DWwaqTbOp
I tried to follow his rule in my draft Z-14 modified for EFI. https://drive.google.com/open?id=1rQ...ncufqJo0o-iQuq
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Generic, ok, got that.
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05-21-2018, 09:24 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by rv6ejguy
I may not be understanding this sentence properly but an EFI E-Bus will always require a lot more than 7 amps feeding it- like triple that. Was this statement directed at EFI architecture specifically or other electrical systems like avionics?
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Ross, what is the total load ...dual ECU, two coils, two fuel pumps, six injectors, all together?
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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05-22-2018, 06:12 AM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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A round figure on a 540 would be 20 amps with both (standard) pumps on.
Depends on fuel pressure, pumps used (we offer 3 different pumps), rpm, MAP, number of injectors and coils you're driving.
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05-22-2018, 07:00 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by rv6ejguy
A round figure on a 540 would be 20 amps with both (standard) pumps on.
Depends on fuel pressure, pumps used (we offer 3 different pumps), rpm, MAP, number of injectors and coils you're driving.
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Got it.
Returning to the very simple backup you described...
Quote:
Originally Posted by rv6ejguy
My RV6A also has no alternator connection to the backup battery- just a 30 amp ATO fuse and toggle switch to the engine bus. I isolate that bus from the primary battery and alternator by turning off the master.
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Operating description here:
http://www.sdsefi.com/air45.htm
So the basic architecture looks like this?

__________________
Dan Horton
RV-8 SS
Barrett IO-390
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05-22-2018, 07:40 AM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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That's correct Dan.
I must admit I haven't read this page for over a decade but it's still pretty much valid. The backup battery is now an 18 amp hour AGM one and I charge it the 1st of every month on the ground. Load test every annual. Voltage is verified prior to engine start, on my checklist.
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05-22-2018, 09:50 AM
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Join Date: Jul 2014
Location: West Covina, Ca
Posts: 52
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Thanks, John. Yes, in my myopic quest for the SIMPLE power design, I neglected to address the always hot wires. (approx. 20A max load per Ross).
So the only way I can reliably keep panel switches for the two EFI power feeds is to have relays of some sort. This does add two relays and connections to the simple concept.
Is there any way to avoid the relays? Reading the Nuckols' comment, it appears ill advised. Still, they can be independently tested, and the power paths to the EFI bus are redundant.
And is there a "most" desirable location for the EFI bus?
I appreciate the critical comments. Really.
__________________
Cliff Langlois
Meridian, Idaho
Cliffsrv10.blogspot.com
Project Sold 9/2018
RV-10
Empennage finished!
QB Fuselage and wings in hand
Taking a bite of the elephant every day.
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05-22-2018, 09:53 AM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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Quote:
Originally Posted by Pukauma
Much cleaner diagram(s), Dan. I have been considering running an AWG 6 "fat wire" to the switches for the EFI bus. According to Nuckolls he feels it does not require fusing. Eliminates another component and connections.
http://www.vansairforce.com/communit...uckolls&page=2
Any complaints with that?
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Can I take a stab at that? I think the other responses were good, but I don't think the basic philosophy is clear yet. The following is my interpretation of what Nuckolls is saying.
It's not a universal truth that 'fat' wires don't need fusing (protecting). If it's a very short run (say, 6" or so), the danger to the wire is so low that you can make a 'sure bet' that you can get away with no protection. But if it's an extended run, there are more opportunities for faults. Now there are various ways to protect, and a contactor (relay) is one legit way to protect. It just requires a human in the loop to trigger the protection. That's what we do with a battery & master contactor in the back, and the fat feeder running to the front of the plane.
For my E-dependent engine bus, I wanted the engine to be as independent from the rest of the plane *as possible*, as we're accustomed to with a magneto ignition. My power paths:
battery>fusible link >fat wire>high current switch>engine bus<>high current bus tie switch<>main a/c bus<fat wire<master contactor<battery.
I'm contemplating a high current diode wired across the bus tie switch, pointing toward the engine bus. This will allow uninterrupted power to the engine bus if the engine power switch fails. If the master contactor fails, the airframe would go dark (EFIS has its own battery backup) until the bus tie switch is closed.
The bus tie switch provides one-switch redundant power feed to both the engine bus (from the main bus), and to the main bus (from the engine bus).
This is a two identical alternators, single battery system.
FWIW...
Charlie
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