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Wire size dilemma

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aadamson(at)highrf.com
Guest

Posted: Thu Mar 22, 2007 5:44 pm Post subject: Wire size dilemma

Ok, so I'm planning on a dual alt/batt 24v setup for a Lancair legacy.

Batteries will be mounted behind the seats, a single 24v Concorde, and a

backup B&C.  I'm stuck on wire sizes to accommodate my goals.  Wonder if

anyone would care to comment.  I had planned to run #2 between the starter

contactor and the starter (both will be on the firewall).  Then #4 between

the Main alternator (a 100A 24v version) the starter contactor and the main

batt contactor (which will be at the batteries about 5ft behind the

firewall).  Because the standby alternator is only 20A and a backup, was

going to run #8 for it thru the firewall, to the battery contact/cross tie.

Mine you, the cross tie will be "strap" connected so it won't actually be

wire there.



But after seeing a few larger airplanes, I started to re-think this plan.

Perhaps I should use #2 for everything on the Main battery side, and stay

either with #8 or go to #6 or #4 on the standby side.  



Trying to avoid losses over length, etc.  I've looked thru everything I can

find on Bob's site, but most of them are for 60A alternators on the main

side and 12v, so I'm really not sure what to do.



Any takers?



To help with a little more placement info.  The firewall will have the

starter contactor only, the batteries are about 5-7 (wire feet) behind the

firewall, the battery and cross tie contactors will be at the batteries.



Thanks in advance,

Alan

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nuckollsr(at)cox.net
Guest

Posted: Thu Mar 22, 2007 7:23 pm Post subject: Wire size dilemma

At 09:42 PM 3/22/2007 -0400, you wrote:



 	  Quote: 	
		  

<aadamson(at)highrf.com>



Ok, so I'm planning on a dual alt/batt 24v setup for a Lancair legacy.

Batteries will be mounted behind the seats, a single 24v Concorde, and a

backup B&C.  I'm stuck on wire sizes to accommodate my goals.  Wonder if

anyone would care to comment.  I had planned to run #2 between the starter

contactor and the starter (both will be on the firewall).  Then #4 between

the Main alternator (a 100A 24v version) the starter contactor and the main

batt contactor (which will be at the batteries about 5ft behind the

firewall).  Because the standby alternator is only 20A and a backup, was

going to run #8 for it thru the firewall, to the battery contact/cross tie.

Mine you, the cross tie will be "strap" connected so it won't actually be

wire there.



But after seeing a few larger airplanes, I started to re-think this plan.

Perhaps I should use #2 for everything on the Main battery side, and stay

either with #8 or go to #6 or #4 on the standby side.



Trying to avoid losses over length, etc.  I've looked thru everything I can

find on Bob's site, but most of them are for 60A alternators on the main

side and 12v, so I'm really not sure what to do.



Any takers?



To help with a little more placement info.  The firewall will have the

starter contactor only, the batteries are about 5-7 (wire feet) behind the

firewall, the battery and cross tie contactors will be at the batteries.


	


    I'd run a 2AWG ground for both batteries. Put crosstie contactor on

    firewall to use as fwd distribution point for the two systems. Run 2AWG for

    all fat wires on the main battery(+) side and 4AWG for fat wires on the 

auxiliary

    battery(+) side.



        Bob . . .



        ----------------------------------------

        ( IF one aspires to be "world class",  )

        ( what ever you do must be exercised   )

        ( EVERY day . . .                      )

        (                  R. L. Nuckolls III  )

        ----------------------------------------

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nuckollsr(at)cox.net
Guest

Posted: Thu Mar 22, 2007 7:23 pm Post subject: Wire size dilemma

At 09:42 PM 3/22/2007 -0400, you wrote:



 	  Quote: 	
		  

<aadamson(at)highrf.com>



Ok, so I'm planning on a dual alt/batt 24v setup for a Lancair legacy.

Batteries will be mounted behind the seats, a single 24v Concorde, and a

backup B&C.  I'm stuck on wire sizes to accommodate my goals.  Wonder if

anyone would care to comment.  I had planned to run #2 between the starter

contactor and the starter (both will be on the firewall).  Then #4 between

the Main alternator (a 100A 24v version) the starter contactor and the main

batt contactor (which will be at the batteries about 5ft behind the

firewall).  Because the standby alternator is only 20A and a backup, was

going to run #8 for it thru the firewall, to the battery contact/cross tie.

Mine you, the cross tie will be "strap" connected so it won't actually be

wire there.



But after seeing a few larger airplanes, I started to re-think this plan.

Perhaps I should use #2 for everything on the Main battery side, and stay

either with #8 or go to #6 or #4 on the standby side.



Trying to avoid losses over length, etc.  I've looked thru everything I can

find on Bob's site, but most of them are for 60A alternators on the main

side and 12v, so I'm really not sure what to do.



Any takers?



To help with a little more placement info.  The firewall will have the

starter contactor only, the batteries are about 5-7 (wire feet) behind the

firewall, the battery and cross tie contactors will be at the batteries.


	


    I'd run a 2AWG ground for both batteries. Put crosstie contactor on

    firewall to use as fwd distribution point for the two systems. Run 2AWG for

    all fat wires on the main battery(+) side and 4AWG for fat wires on the 

auxiliary

    battery(+) side.



    P.S. Use welding cable for jumpers from battery(-) terminals to common

    gnd and battery(+) terminals to their respective contactors. Use what ever

    style wire you like for other conductors.



        Bob . . .



        ----------------------------------------

        ( IF one aspires to be "world class",  )

        ( what ever you do must be exercised   )

        ( EVERY day . . .                      )

        (                  R. L. Nuckolls III  )

        ----------------------------------------

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longg(at)pjm.com
Guest

Posted: Fri Mar 23, 2007 5:54 am Post subject: Wire size dilemma

Alan,

	Also, if you have questions about impedance and length, take a

look at and try one of the worksheets from Jim's engineering page. His

worksheet takes out a lot of the guesswork.



http://www.rst-engr.com/rst/jimsdata/index.html



 



--

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longg(at)pjm.com
Guest

Posted: Fri Mar 23, 2007 6:01 am Post subject: Wire size dilemma

Alan,

	Just thought of something else. As Bob indicated (contactors on

the firewall) Lancair sells a pre-built mountable package which includes

the contactors, fuses etc. I assume you are using the Lyc 540? I am

doing a similar thing with the Subaru engine except my two batteries

will be supported by a single alternator for the electrically dependent

engine. It still makes sense to keep the battery contactor near the

batteries. For these glass boats we need to run a fat 02 ground from the

engine back to the common ground behind the seat. Let me know what you

work out for your battery boxes; I've not designed mine yet.



--

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