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Instrument Earth Bus cable size

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MikeDunlop

Joined: 02 Jan 2012
Posts: 15

Posted: Wed Jan 25, 2012 5:59 am Post subject: Instrument Earth Bus cable size

Starting a new topic to avoid confusion with previous attempts at a simple explanation.



I'm building a Long-EZ with a Odyssey PC925 battery in the nose section, there is one 0.5 inch copper tube conduit that runs from the nose section to the firewall at the rear. 



The positive cables (# 2 welding cable) will run from the battery to the master switch in the nose section (6 to 7 inches), from the master switch through the earth conduit to the starter solenoid way back on the firewall (about 10 ft), then from solenoid to starter. 



The earth cable (# 2 welding cable) will run from the battery to the copper tab on the front of the earth conduit, just behind the nose section (20 inches). The earth is then picked up from the rear copper tab of the earth conduit, on the engine side of the firewall, by means of a copper braided strap between conduit and engine. So I presume (dangerous thing to do!) the cranking current runs through the copper conduit. 



My question is: 

What is the minimum size cable can I use from the copper tab on the FRONT of the earth conduit tab (just behind the nose section) to the instrument earth bus (approx 18-20 inches in length). I'm trying to avoid using the very heavy welding cable.

MikeD (U.K.)

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user9253

Joined: 28 Mar 2008
Posts: 1920
Location: Riley TWP Michigan

Posted: Wed Jan 25, 2012 6:12 am Post subject: Re: Instrument Earth Bus cable size

 	  Quote: 	
		  My question is:

What is the minimum size cable can I use from the copper tab on the FRONT of the earth conduit tab (just behind the nose section) to the instrument earth bus (approx 18-20 inches in length). 	


Use the same size wire for the negative as is used for the positive supply to the instrument panel.

Joe

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Joe Gores

Neal.George(at)hurlburt.a
Guest

Posted: Wed Jan 25, 2012 6:55 am Post subject: Instrument Earth Bus cable size

Mike - 



It depends.  Have you worked a load analysis for the equipment that

would be serviced by the panel ground?  



Referring to the wire chart in The AeroElectric Connection, Chapter 8, a

length of #8 copper wire is expected to carry about 22 amps continuous

with less than 10-deg C rise in the wire temp.  



neal



--

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mrspudandcompany(at)veriz
Guest

Posted: Wed Jan 25, 2012 7:20 am Post subject: Instrument Earth Bus cable size

My question is: 

What is the minimum size cable can I use from the copper tab on the FRONT of

the earth conduit tab (just behind the nose section) to the instrument earth

bus (approx 18-20 inches in length). I'm trying to avoid using the very

heavy welding cable.



		Mike,



		It appears that the current running through this wire is

only that used in the instrument panel.  You really need to add up all the

loads from the avionics, lights, and other electrical devices that connect

through this ground, then calculate the conductor size.  Your + feed wire

should be of the same gauge.



		Once you know the total load you can see Bob's Wire size

calculator here: http://www.aeroelectric.com/articles/wiresize.pdf



		Hope this helps.



		Roger

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nuckolls.bob(at)aeroelect
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Posted: Wed Jan 25, 2012 9:47 am Post subject: Instrument Earth Bus cable size

 	  Quote: 	
		  My question is:

What is the minimum size cable can I use from the copper tab on the 

FRONT of the earth conduit tab (just behind the nose section) to the 

instrument earth bus (approx 18-20 inches in length). I'm trying to 

avoid using the very heavy welding cable.


	


   See VIEW -B- in this drawing:



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z15K1.pdf

   In your case, the 2AWG ground wire is a piece of copper tube.

   Your design does not call for a firewall ground bus so the

   engine ground strap goes right to the copper tube. Your design

   does not mention the use of a forward high-power ground bus

   so the battery (-) wire goes right to a tab on the copper

   tube. An instrument panel ground bus could be fabricated per

http://aeroelectric.com/Pictures/Grounding/AVG_RA.jpg



http://aeroelectric.com/Pictures/Grounding/Avionics_Bus_3.jpg

    Figure Z15 suggests that a parallel bundle of 5,

    20AWG wires be run from the avionics ground to the

    tab on the end of your copper tube. Landing

    light and position lights could also ground to the

    bolt in the copper tube tab.



    If you're fabricating your own d-sub panel ground

    bus, you could solder a couple of 14AWG wires to

    the commoning conductor at the back of the

    connector. This is described in



https://matronics.com/aeroelectric/Catalog/AEC/9031/IM9031-700A.pdf

   Bob . . .

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MikeDunlop

Joined: 02 Jan 2012
Posts: 15

Posted: Wed Jan 25, 2012 2:07 pm Post subject: Re: Instrument Earth Bus cable size

Many thanks to everyone.  Very informative and helpful replies, I will be studying the resources.



MikeD (U.K.)

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu Jan 26, 2012 6:26 am Post subject: Re: Instrument Earth Bus cable size

(partial repost) I sell lots of Copper-Clad Aluminum cable to 300 mph fuel dragster and racecar builders, and weird groups doing strange things where the weight is important. I even sell the stuff to the companies building remote-controlled drones. Predator drones and the more advance secret stuff... 



Airbus and Boeing use similar CCA configurations, and they won't sell you any. 



I sell Super-2-CCA, Super-4-CCA, AWG-6-CCA (same dimensions as AWG-6 but CCA with Tefzel insulation), and Super-CCA RG+142 



See my website. CCA is 60% the weight of copper for the same conductivity. 



If you have the battery on the firewall, you are okay. Otherwise, use CCA. 



Also see Bob's most excellent article, http://www.aeroelectric.com/articles/economics_of_weight_reduction.html 



and on my website: http://www.periheliondesign.com/fatwires_files/Coppercables.pdf

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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

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