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rvg8tor

Joined: 06 Apr 2009
Posts: 50
Location: Olympia, WA

Posted: Fri Oct 23, 2009 5:25 am Post subject: Need help with desicsion

I am running a 60 amp alternator in my system and can't decide which way to go with the wire size to the main bus.  #8 is good to 57 amps, my max continuous load is 45 amps.  If I get right up to the 60 amp load for some reason this will exceed the wire limits, I will also have a 60 am ANL.



Is the #8 wire a compromise or is it just fine, or should I go up to #6?  I do have the #6 wire, so it is not a matter of saving $, just the weight, the only thing I would need is some lugs.  I bought the #6 wire without really thinking this through.  I want to do this the best way possible, not just what will work.  So since materials are not the issue which way would you guys and gals go.



Thanks for the help, am I over thinking this!



Cheers

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_________________
Mike "Nemo" Elliott
RV-8A QB (Engine)
www.mykitlog.com/rvg8tor

rjquillin

Joined: 13 May 2007
Posts: 123
Location: KSEE

Posted: Fri Oct 23, 2009 6:43 am Post subject: Need help with desicsion

At 06:25 10/23/2009, you wrote:

  	  Quote: 	
		  I am running a 60 amp alternator in my system and can't decide which way to go with the wire size to the main bus.  #8 is good to 57 amps, my max continuous load is 45 amps.  If I get right up to the 60 amp load for some reason this will exceed the wire limits, I will also have a 60 am ANL.



 Is the #8 wire a compromise or is it just fine, or should I go up to #6?  I do have the #6 wire, so it is not a matter of saving $, just the weight, the only thing I would need is some lugs.  I bought the #6 wire without really thinking this through.  I want to do this the best way possible, not just what will work.  So since materials are not the issue which way would you guys and gals go.



 Thanks for the help, am I over thinking this!



 Cheers 	


 I'll wade into this only to suggest rather than just using the somewhat generic wire capacity tables, dig into Chapter 11 of AC43-13-1B and work it out with actual numbers.  What will the IR (voltage) drop be for your proposed run length based on the load, resistance, temperature, altitude, bundling and such of your chosen cable.  Is your calculated voltage drop acceptable?  



 AC43-13 has a suggestion:



 b. Voltage Drop in Wires. The voltage drop in the main power wires from the generation source or the battery to the bus should not exceed 2 percent of the regulated voltage when the generator is carrying rated current or the battery is being discharged at the 5-minute rate. The tabulation shown in table 11-6 defines the maximum acceptable voltage drop in the load circuits between the bus and the utilization equipment ground.



 Ron Q.



     [quote][b]

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rvg8tor

Joined: 06 Apr 2009
Posts: 50
Location: Olympia, WA

Posted: Fri Oct 23, 2009 6:55 am Post subject: Re: Need help with desicsion

Ron,



Good point, I have done the voltage drop with the table in AC43 but have not done the further study of altitude and such.  I will work though it, as far as voltage drop with the amps and the cable length #8 is the answer.  Thank you for pointing this out.



Mike



[quote="rjquillin"]At 06:25 10/23/2009, you wrote:

  	  Quote: 	
		  I am running a 60 amp alternator in my system and can't decide which way to go with the wire size to the main bus.  #8 is good to 57 amps, my max continuous load is 45 amps.  If I get right up to the 60 amp load for some reason this will exceed the wire limits, I will also have a 60 am ANL.



 Is the #8 wire a compromise or is it just fine, or should I go up to #6?  I do have the #6 wire, so it is not a matter of saving $, just the weight, the only thing I would need is some lugs.  I bought the #6 wire without really thinking this through.  I want to do this the best way possible, not just what will work.  So since materials are not the issue which way would you guys and gals go.



 Thanks for the help, am I over thinking this!



 Cheers 	


 I'll wade into this only to suggest rather than just using the somewhat generic wire capacity tables, dig into Chapter 11 of AC43-13-1B and work it out with actual numbers.  What will the IR (voltage) drop be for your proposed run length based on the load, resistance, temperature, altitude, bundling and such of your chosen cable.  Is your calculated voltage drop acceptable?  



 AC43-13 has a suggestion:



 b. Voltage Drop in Wires. The voltage drop in the main power wires from the generation source or the battery to the bus should not exceed 2 percent of the regulated voltage when the generator is carrying rated current or the battery is being discharged at the 5-minute rate. The tabulation shown in table 11-6 defines the maximum acceptable voltage drop in the load circuits between the bus and the utilization equipment ground.



 Ron Q.



      	  Quote: 	
		  [b]

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_________________
Mike "Nemo" Elliott
RV-8A QB (Engine)
www.mykitlog.com/rvg8tor

longg(at)pjm.com
Guest

Posted: Fri Oct 23, 2009 7:09 am Post subject: Need help with desicsion

Mike,



I guess the question is are you feeling lucky or conservative? No, the

wire won't melt down if you get to a 58 amp draw. Tefzel will run hot a

long time before it deteriorates and dissolves the connector and

destroys all your components. If you want to be safe and are not worried

about the few extra ounces, use the #6. 



If you want to be a nerd you can buy a micro processor, program the

imbedded code and attach it to a thermocouple sensor on the wire, thus

monitor the temperature for a while. Who's got that much time? We gotta

go fly.



If the #8 suits your load pattern and length calculations, then use it.

I think a common misconception is that if you have a 60 amp alternator,

that the alternator is constantly trying to burn up your 10 amp load

with 60 amps of pressure. The alternator works on demand, so it won't

produce more than you need. Think of it as a throttle. If you demand

more, you'll get more. You could put a 150 amp alternator on there and

run a #8 wire to it. Just depends on demand and the design of your

system.



Remember, it's the voltage that kills you not the amperage. Remove the

VARs and you've got nothing.

Glenn



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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Oct 23, 2009 7:38 am Post subject: Need help with desicsion

At 08:25 AM 10/23/2009, you wrote:

 	  Quote: 	
		  



I am running a 60 amp alternator in my system and can't decide which 

way to go with the wire size to the main bus.  #8 is good to 57 

amps, my max continuous load is 45 amps.  If I get right up to the 

60 amp load for some reason this will exceed the wire limits, I will 

also have a 60 am ANL.



Is the #8 wire a compromise or is it just fine, or should I go up to 

#6?  I do have the #6 wire, so it is not a matter of saving $, just 

the weight, the only thing I would need is some lugs.  I bought the 

#6 wire without really thinking this through.  I want to do this the 

best way possible, not just what will work.  So since materials are 

not the issue which way would you guys and gals go.



Thanks for the help, am I over thinking this!


	


   Assuming that you're alternator b-lead is 4' long,

   a 6AWG at 4 mOhms/Ft will drop 60 x .004 x 4 = .96

   volts at full load. If your alternator is an externally

   regulated device then the regulator senses BUS voltage

   instead of B-terminal voltage and it will compensate

   for the voltage drop. However, if internally regulated,

   the bus will run about 1.0 volts low during full load

   operations. In practice, you'd never be able to realize

   that loading because loads on the alternator are a

   combination of system requirements + battery recharge

   currents. For a 14.2v setpoint, a 13.2v bus wouldn't

   significantly stimulate the battery to soak up the

   joules it would like to have if the bus were actually

   higher.



   If your alternator is externally regulated, then

   voltage drops in the b-lead only limit your minimum

   speed for full alternator output  . . . which at

   take-off or cruise settings is not a limiting

   factor.



   The trends are moving toward internally

   regulated (b-lead sense) and small b-lead feeders

   COMBINED with all other wires between the alternator

   and battery can stack up voltage drops that will

   impact battery recharge performance. This is why

   the Z-figures generally show 4AWG or larger for

   b-lead feeders . . . and all other fat-wires.



   Having said that, unless your battery is seriously

   discharged, these simple-ideas in physics have

   little perceivable effect on your operations.

   Going to the wire tables is not particularly

   useful for fat-wire calculations since their

   recommendations are based on temperature rises

   that generally fall WELL BELOW that which the

   wire will tolerate in your airplane. The hard-

   hat consideration is for voltage drop under the

   worst case conditions. There are additional

   voltage drops to be considered for the full-

   up system.



   Know that automobiles tend to use fat-wires that

   are a lot smaller than the aviation community

   would find acceptable for meeting design goals.

   At the same time, except for a severely discharge

   battery, automobiles almost never encounter conditions

   that exploit the full capability of the

   alternator. I've often suggested that if it

   were my airplane and the batteries were up front,

   I'd craft all the fat-wire paths with 4AWG welding

   cable for reasons that are not readily revealed by

   praying over the wire table . . .

   Bob . . .

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rnewman(at)tcwtech.com
Guest

Posted: Sun Oct 25, 2009 3:16 pm Post subject: Need help with desicsion

I believe you slipped a zero.   6 awg is 0.4 mOhms per foot  so the voltage 

drop on 4 feet is just under 0.1 volts.



An easy rule of thumb to remember is 10 awg wire is 1 milliohm per 1 foot, 

every three wire sizes doubles the resistance.  (so 7 awg would be about 2 

milliohms per foot.)



-Bob Newman

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mrspudandcompany(at)veriz
Guest

Posted: Sun Oct 25, 2009 4:47 pm Post subject: Need help with desicsion

Bob N.,



I believe you slipped in the wrong direction. #7 AWG is larger than #10 AWG

therefore the resistance should be less.



Roger



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rnewman(at)tcwtech.com
Guest

Posted: Sun Oct 25, 2009 5:23 pm Post subject: Need help with desicsion

oops.   I sure did.   ,#7 awg would be 1/2 mohm..#13 is 2 mohms     .  my 

bad

-Bob Newman

---

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Oct 26, 2009 12:43 pm Post subject: Need help with desicsion

At 06:13 PM 10/25/2009, you wrote:

 	  Quote: 	
		  



I believe you slipped a zero.   6 awg is 0.4 mOhms per foot  so the 

voltage drop on 4 feet is just under 0.1 volts.


	


   Oops!  Good eye. You're correct.

 	  Quote: 	
		  An easy rule of thumb to remember is 10 awg wire is 1 milliohm per 1 

foot, every three wire sizes doubles the resistance.  (so 7 awg 

would be about 2 milliohms per foot.)


	


    Yup, wrote about that in several places . . . but

    drop to 7 gets you 0.5 mOhm/ft and 6 awg would necessarily

    be lower. See:



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



    Thanks for the heads-up

        Bob . . .



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

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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

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