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Minimum ANL size of 35A vs. power feed AWG

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Apr 11, 2020 9:22 am Post subject: Minimum ANL size of 35A vs. power feed AWG

At 06:30 PM 4/10/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Colyn Case" <colyncase(at)earthlink.net>



 I can see the logic of that.   However, if you abide that logic you

 probably wouldn't put any current limiters in your airplane at all, right? 	


   It's the same logic that specified sizes

   and materials of bolts that hold the wings

   on, optimizes engine oil for best service

   life and strives for failure tolerant

   electrical system architecture.



  	  Quote: 	
		  I think the guys that made my Nissan Stanza thought something

 like that.    One day for no reason that I could find, the

 electrical system fried itself, burned through a fender and

 destroyed the entire wiring harness in the car. 	


   The arcing burned through sheet steel

   while partitioning off enough energy

   to burn every wired in the car?



  	  Quote: 	
		  Fortunately, shortly into the episode, 	


   'Shortly'? The events described above

   take quite a bit of time . . . how

   long was 'shortly'?



  	  Quote: 	
		  someone had the presence of mind to grab one of

 the cables  connected to the battery and simply pull it off. 	


   Loose enough to be disconnected with

   bare hands?  I'm having difficulty

   wrapping my gray matter around the

   physics of this event.



 

  	  Quote: 	
		  Not something I would enjoy doing at 20,000' 	


   Building a pressurized airplane . . . or

   are you planning an oxygen system? What

   kind of cabin heat do you anticipate?



 

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Apr 11, 2020 10:35 am Post subject: Minimum ANL size of 35A vs. power feed AWG

At 07:45 AM 4/11/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Colyn Case" <colyncase(at)earthlink.net>



 Thinking about this a little differently:   When the wires are sized, they are sized for voltage drop vs. current and distance.   Tables for calculating that are widely available.   Typically we apply a fuse or breaker to allow the current used in that calculation.



 That breaker size is way way lower than the current required to compromise the insulating properties of the insulation. 	


   Circuit protection will operate wwwayyyyy before the insulation

   is at-risk.  A 22AWG wire will carry 20 amps in room air and

   still be far short of damaging insulation . . . 5A circuit

   protection makes risk-to-insulation still more remote.



   The vast majority of bad days in the cockpit

   are the result of poor piloting. Only a

   few are the result of mechanical failure

   and fewer still involve electrical failure.



   Of ALL electrical failures, death by 'unprotected'

   wires are a vanishingly small proportion.

   SwissAir 111 comes to mind . . . that WAS

   a compounded series of catastrophic design snafus.



   But for your personal airborne recreational vehicle,

   I suggest you're over thinking the problem.



 

   Bob . . .

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Colyn Case

Joined: 12 Oct 2009
Posts: 13
Location: Thetford Center

Posted: Thu Apr 16, 2020 8:44 pm Post subject: Re: Minimum ANL size of 35A vs. power feed AWG

Hi Bob,



I apologize for my somewhat wise-ass answer to Joe's post.   Clearly I have a lot to learn.  



We always fight the last war and my last war was smoke coming from behind the panel that someone else wired.    I could barely even get a look in there let alone identify the offending part.      My response is I am now rewiring the airplane.   Hence lots of questioning.   



It is a Lancair IV-P, usually overheated by the sun but manmade heat is from the output of the turbos.



What prompted this question is that I ended up with a co-pilot side CB panel.   All the feeds from the firewall go there.   ...and then I have a console CB panel where all the important stuff is.   The main bus comes through the firewall as a 6 AWG wire and goes to the side CB panel.   From there it goes to the center CB panel.   On the latter, I can conjure up 67 continuous amps.    But that assumes hydraulic pump is at max pressure and stays there (I've never seen it there for more than 2 seconds), pitot heat is on, TKS pump is on, my ancient strobes, incandescent nav lights, HID landing lights, incandesent taxi lights, the COM is transmitting.   ...a situation that never happens.



It's about an 8' run over to the center CB, but even at that imaginary 67 amp load, on this 28V airplane, I could in theory handle it with an 8 AWG wire from the side CB panel to the center CB panel.  ...but that would burn through before the ANL interrupted.



So you are saying, don't worry about the ANL.   Can I also not worry about maintaining 6 AWG all the way to the center CB panel?



While we're at it, I have a 27 amp "E-bus". (Yes I know that is extreme but really that's my whole g3xtouch system with all its canbus friends and relatives includeing the eis and auto pilot)   Today I broke one of the diodes that feeds it and am looking for some replqcements.   How do I identify those at digikey or wherever?

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_________________
Colyn Case
colyncase@earthlink.net

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Apr 17, 2020 8:31 am Post subject: Minimum ANL size of 35A vs. power feed AWG

 	  Quote: 	
		  It's about an 8' run over to the center CB, but even at that imaginary 67 amp load, on this 28V airplane, I could in theory handle it with an 8 AWG wire from the side CB panel to the center CB panel.  ...but that would burn through before the ANL interrupted.



 So you are saying, don't worry about the ANL.   Can I also not worry about maintaining 6 AWG all the way to the center CB panel? 	


  You're over-worrying it. Drop the wire gage for the

  last leg of the trip to center panel. Ditch the

  ANL.



  	  Quote: 	
		  While we're at it, I have a 27 amp "E-bus". (Yes I know that is extreme but really that's my whole g3xtouch system with all its canbus friends and relatives includeing the eis and auto pilot)   Today I broke one of the diodes that feeds it and am looking for some replqcements.   How do I identify those at digikey or wherever? 	


  Is this a bridge rectifier type? Just

  about anything that LOOKS like it

  will do. Voltage rating is not

  terribly significant in the 28v

  system. Current rating is more significant.



  HOWEVER, with this much

  current through the device, I'd

  plan on some heat sinking . . . say

  36 square inches of aluminum sheet

  or equivalent.



  You can 'kill' a 50A rated diode

  with 30A flowing if it's not

  heat sinked.



  Here's a likely candidate:

 

  https://tinyurl.com/yahl74yy



  



 

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Apr 17, 2020 2:38 pm Post subject: Minimum ANL size of 35A vs. power feed AWG

 	  Quote: 	
		  It's about an 8' run over to the center CB, but even at that imaginary 67 amp load, on this 28V airplane 	




   Really? Have you conducted a load analysis?

   This isn't a 'conjuring' activity, it's

   a numerical analysis of your architecture,

   component sizing, failure modes and crafting

   of plan-B etc.



   You can do this on a spread-sheet . . . there

   are several examples in Excel to be had

   at



  https://tinyurl.com/9rt6ymn



   but my personal favorite is the hand-entered

   and edited chart



  https://tinyurl.com/7jqypwj



   One sheet per bus. Each line depicts

   a accessory feeder. You pick the protection

   size (either fuse or breaker . . . don't

   fuss over the symbol, just get a number

   on it). Pick the wire size. Then fill in

   the boxes as to the current draw for

   each accessory in the flight conditions

   called out across the top.



   When I do a wirebook, the architecture

   drawing comes first then these pages

   follow. I had a column to call out the

   page on which that accessory's wiring

   details are called out.

  

   Hence, the architecture and loads

   pages become the foundation and index

   to the whole system which gets added

   on subsequent pages.



   I'm a bit perplexed by that 67A assertion

   above . . . I'd like to know how

   that was arrived at . . . you really

   NEED to know that the number(s)

   are golden.



 

   Bob . . .

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