Wire size to E-Bus?

[idleup]

I am working with Bob's Z-13/8 and I am confused about the wire size between the main -> e-bus. In the drawing he has it as 20AWG. This seems way to small to me to power the e-bus. According to my load diagram my e-bus loads can reach 25A. Am I correct in thinking this wire should be 12AWG instead?

Also, the wire from my Batt. bus to the E-Bus is 14AWG on my schematic, should that also be 12AWG?

Thanks,

[humptybump]

The wire gauge is not only dependent on the current load but also the distance. If the distance from your main bus to your essential bus is measured in inches (or centimeters) then the wire gauge need not be huge.

[Bill.Peyton]

The wire size can be determined from calculation and/or table in AC43.13, Ch11, here http://www2.tech.purdue.edu/at/cours...-1B/CH11_5.pdf. Personally, I used 6 gauge stranded tefzel. I know it is a little large, but room for expansion is good.

[az_gila]

Originally Posted by idleup

I am working with Bob's Z-13/8 and I am confused about the wire size between the main -> e-bus. In the drawing he has it as 20AWG. This seems way to small to me to power the e-bus. According to my load diagram my e-bus loads can reach 25A. Am I correct in thinking this wire should be 12AWG instead?

Also, the wire from my Batt. bus to the E-Bus is 14AWG on my schematic, should that also be 12AWG?

Thanks,

Using the chart above, 20g wire is good for 4.5 ft at 10 amps.

But that is for a 0.5 volt drop.

Cut the distance to 3 inches - I think that is one line Bob N says to keep short - and the drop is only 28 milliVolts - no problem at all at 10 amps.

[idleup]

Gil, but my e-bus load is not 10amps it is 25 or so (worst case) so according to the chart my wire between bat bus and main bus to ebus should be 14AWG correct? just want to make sure I understand this...

[az_gila]

Originally Posted by idleup

Gil, but my e-bus load is not 10amps it is 25 or so (worst case) so according to the chart my wire between bat bus and main bus to ebus should be 14AWG correct? just want to make sure I understand this...

How long is this short length of wire?

IIRC Bob N. says to keep it less than 6 inches.

[MarkW]

I belive the intent of the E-bus is to power only essential items in an emergency. I also believe the design intent was for about 5-10 amps.
I know in an emergency I will want what battery I have left to last until I am on the ground. Therefore my E-bus will only have the essentials. About 6a.

[Bill.Peyton]

I guess the real question that one has to answer when deciding on a secondary bus is, what is its purpose? If the aircraft is an all electric aircraft designed for IFR IMC flight, the essential equipment becomes a longer list. There is no reason not to design an aux bus to address all of the avionics necessary to safely complete an IMC flight and approach if the primary source of power, the alternator, not the main battery, fails. Limiting the aircraft capability by purposely not powering equipment that would help to add to the safety of the flight, should not be considered an option when doing the analysis. Most avionics have on/off switches, so the option is always there to shed power if it becomes necessary.
Let's face it, your going to install a second battery that may have the same storage capacity as the primary, why not power the majority of the avionics and use that capacity. IMHO the purpose of the backup source is to get on the ground, not continue your cross country in IMC.
I guess all I am saying is don't blindly follow a recommendation until you have done your homework for your specific need. Perform a failure mode, result and corrective action analysis and then make an informed decision as to what you need. If you are a VFR only aircraft and pilot, then the need is totally different than an aircraft designed to be used for IFR/IMC use.

[Mike S]

Originally Posted by Bill.Peyton

I guess the real question that one has to answer when deciding on a secondary bus is, what is its purpose?

Bill has it spot on. Determine the mission, then build to suite.

Originally Posted by Bill.Peyton

IMHO the purpose of the backup source is to get on the ground, not continue your cross country in IMC.

I have a dual alternator, dual battery setup, and my goal was to have the ability to choose where I was going to land, be able to pick an airport within an hour of a main system failure. At RV speeds, this gives you quite a range of options.

I see absolutely no reason to push on to my original destination---which may be hours away---- with an aircraft that has a known fault in one of its major systems.

Not trying to tell you how to design your plane, but this is what fits into my personal comfort zone.

[Bill.Peyton]

Mike addresses a good point. A secondary alternator should be considered when evaluating the mission and the responsibility of the aux bus