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Alternator output wire size

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michele.delsol(at)microsi
Guest

Posted: Tue Jan 24, 2006 11:00 pm Post subject: Alternator output wire size

Hi all =96 I have been following this list for some time mainly to educate

myself as to putting together the electrics on my RV8, now at the canoe

stage. I purchased Van=92s harness, figured it would give me a head start but

I find that I question some of the choices made, namely the size of the

Alternator output. Am planning on a 40amp alternator, aft mounted battery.



Vans has chosen #8 awg =96 if I look at BK=92s figure 8-4 on temp rise vs wire

size, I read 21 amps for a 10=B0C rise and 40amps for 35=B0C rise. Since I shall

be running a 40amp alternator I figure that 35=B0C is too much since the wire

will be bundled with others =96 I=92d be better of with a larger wire. If I put

a #6, temperature rise at 40amps is somewhere around 20=B0C.



My question to you guys who have been there : should I go for #8 or # 6?



Thanks,



Mich=E8le

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nuckollsr(at)cox.net
Guest

Posted: Wed Jan 25, 2006 5:37 am Post subject: Alternator output wire size

At 07:59 AM 1/25/2006 +0100, you wrote:



 	  Quote: 	
		  

<michele.delsol(at)microsigma.fr>



Hi all =96 I have been following this list for some time mainly to educate

myself as to putting together the electrics on my RV8, now at the canoe

stage. I purchased Van=92s harness, figured it would give me a head start but

I find that I question some of the choices made, namely the size of the

Alternator output. Am planning on a 40amp alternator, aft mounted battery.



Vans has chosen #8 awg =96 if I look at BK=92s figure 8-4 on temp rise vs wire

size, I read 21 amps for a 10=B0C rise and 40amps for 35=B0C rise. Since I 

shall

be running a 40amp alternator I figure that 35=B0C is too much since the wire

will be bundled with others =96 I=92d be better of with a larger wire. If 

I put

a #6, temperature rise at 40amps is somewhere around 20=B0C.



My question to you guys who have been there : should I go for #8 or # 6?


	


   If you're worried, go #6. The operating temperature limit on Tefzel

   is 150C.  See:



http://www.aeroelectric.com/Reference_Docs/Mil-W-22759/22759slash16.pdf



   The time you're alternator will spend at 40+ amps is measured in



 	  Quote: 	
		  Thanks,



Mich=E8le





--




	




        Bob . . .

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

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nuckollsr(at)cox.net
Guest

Posted: Wed Jan 25, 2006 5:41 am Post subject: Alternator output wire size

At 07:59 AM 1/25/2006 +0100, you wrote:



 	  Quote: 	
		  

<michele.delsol(at)microsigma.fr>



Hi all =96 I have been following this list for some time mainly to educate

myself as to putting together the electrics on my RV8, now at the canoe

stage. I purchased Van=92s harness, figured it would give me a head start but

I find that I question some of the choices made, namely the size of the

Alternator output. Am planning on a 40amp alternator, aft mounted battery.



Vans has chosen #8 awg =96 if I look at BK=92s figure 8-4 on temp rise vs wire

size, I read 21 amps for a 10=B0C rise and 40amps for 35=B0C rise. Since I 

shall

be running a 40amp alternator I figure that 35=B0C is too much since the wire

will be bundled with others =96 I=92d be better of with a larger wire. If 

I put

a #6, temperature rise at 40amps is somewhere around 20=B0C.



My question to you guys who have been there : should I go for #8 or # 6?


	


   If you're worried, go #6. The operating temperature limit on Tefzel

   is 150C.  See:



http://www.aeroelectric.com/Reference_Docs/Mil-W-22759/22759slash16.pdf



   The time your alternator will spend at 40+ amps is measured in

   minutes per flight cycle and usually happens in the first few minutes

   of flight while recharging the battery. What's the hottest environment

   that the wire will traverse? For a 35C rise, you can operate in a

   150-35 or 115C environment indefinitely. For the vast majority of

   the time, your alternator will be running much less load.



   I don't think the #8 wire selection is a really big concern.



        Bob . . .

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

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klehman(at)albedo.net
Guest

Posted: Wed Jan 25, 2006 5:50 am Post subject: Alternator output wire size

Hi Mich



I used 8awg for that. In practice the 40 amps is seldom there for very 

long although I guess it could be considered continuous in some 

circumstances as you have observed. I've noticed that several folks have 

used 8awg with even larger alternators although I would not.



I think you've got html turned on in your mail program which puts those 

numbers in your posts here (at least on my computer) and makes them 

somewhat difficult to read.



Ken

Michle Delsol wrote:



 	  Quote: 	
		  



Hi all =96 I have been following this list for some time mainly to educate

myself as to putting together the electrics on my RV8, now at the canoe

stage. I purchased Van=92s harness, figured it would give me a head start but

I find that I question some of the choices made, namely the size of the

Alternator output. Am planning on a 40amp alternator, aft mounted battery.



Vans has chosen #8 awg =96 if I look at BK=92s figure 8-4 on temp rise vs wire

size, I read 21 amps for a 10=B0C rise and 40amps for 35=B0C rise. Since I shall

be running a 40amp alternator I figure that 35=B0C is too much since the wire

will be bundled with others =96 I=92d be better of with a larger wire. If I put

a #6, temperature rise at 40amps is somewhere around 20=B0C.



My question to you guys who have been there : should I go for #8 or # 6?



Thanks,



Mich=E8le

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Dan.Beadle(at)hq.inclines
Guest

Posted: Wed Jan 25, 2006 9:42 am Post subject: Alternator output wire size

The SIGNIFICANT current from the battery to the front end of the

aircraft is the starting load - this is normally 5-10x the alternator

sizing.



Once the aircraft has started, most of that 40A alternator output is

going to the main buss and all of the loads (lights, radios, etc.).

Recharging the battery is only one of the loads.  If you look at an

aircraft with a load meter, a discharged battery only gets about 1/3 of

the total alternator output.  Once the battery is re-charged from the

startup procedure, there is very little current flowing back there.



So, I wouldn't worry so much about steady state currents to the rear of

the aircraft.  I would worry more about the starting currents over that

wire and the voltage drop.  If you pull 80Amps during startup (cold

engine), there can be significant voltage drop through the wiring,

causing less voltage to the starter for slower RPM and harder starts....



Dan



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michele.delsol(at)microsi
Guest

Posted: Wed Jan 25, 2006 11:39 pm Post subject: Alternator output wire size

Thanks Ken and Bob - 8awg wins hands down - and I'll turn off HTML.



Michele



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