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Date: Sun, 23 Jan 2005 00:45:54 -0600
From: Jim Sower <canarder@frontiernet.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>,
	Cozy Builders Mailing List <Cozy_Builders@mailman.qth.net>
Subject: Re: [FlyRotary] Re: Fuel Pump Load
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<... fuse/breaker is there to protect the wire ...>
Agreed

<... wire MUST be large enough to handle all the loads connected to it - plus a "fudge" factor of about 10 - 30% ...>
MUST is key here.  10% - 30% "fudge" factor becomes 30% - 50% if we are unsure of *precise* loads (which is almost always the case).  Also, using the higher safety factor will reduce line losses to a manageable level

<...If fuse is too big for the wire, the wire will protect the fuse ...>
But only the wire is too small for the circuit - which can happen only if you select wire that is so small that although it will [juuust] carry the load, you will incur unacceptable line losses.

<... A #6 wire will carry about 54A - up to *33 feet* - without an excessive rise in temperature (35*C over ambient).  A #4 wire will carry up to 72A under the same conditions ...>

My B+ is under FOUR feet.  IIRC we are supposed to size wire to reduce 
line losses.  Your 54A for 33 ft example would involve an unacceptable 
voltage drop in the line.

I think your example is a little specious ... Jim S.


Dale Rogers wrote:

>Wendell Voto <jwvoto@itlnet.net> wrote:
> 
>  
>
>>Okay,  I'm conviced, I'll go for 100% over rated current draw.
>>    
>>
>
>
>Wendell,
>
>   You are on the verge of being a victim of over-simplification.  
>I'll try to be as succinct as possible and still cover the 
>important considerations.
>
>   Rule number 1: The fuse/breaker is there to protect the 
>wire.  If too much current is pushed through the wire, it 
>will melt.  You don't want that to happen.
>
>   Rule number 2: The wire MUST be large enough to handle 
>all the loads connected to it - plus a "fudge" factor of 
>about 10 - 30%.  Remember: larger wire == more weight, so 
>you want to keep your wire sizes pretty close to the actual 
>loads they'll be carrying. Over on one of the canard lists, 
>someone put up a chart of how much current each wire size 
>will carry.  Probably Aeroelectric has a similar chart. 
>
>   Rule number 3: If your fuse is too big for the wire, the 
>wire will protect the fuse.  You don't want that to happen.
>
>   Here's an example of how to pick your wire/fuse combination:
>
>The maximum output of the alternator is 65 Amps.  A #6 wire 
>will carry about 54A - up to 33 feet - without an excessive 
>rise in temperature (35*C over ambient).  A #4 wire will 
>carry up to 72A under the same conditions.
>
>Since your worst-case normal operation only passes 65A - and 
>is usually a lot less - a 70A breaker should never trip 
>unless something is seriously awry, yet still trip before 
>your wire gets too hot from a short. 
>
>Hope this is some help.
>Dale R.
>COZY MkIV #1254
>
>
>
>  
>
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