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From: Dale Rogers <dale.r@cox.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Fuel Pump Load
Date: Mon, 24 Jan 2005 0:15:00 -0500
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Jim, 

   I'm more than a little puzzled at your purpose in cross-
posting your response to the COZY_builders ML.  What sort 
of grudge do you have that you want a dispute to spill over 
into that venue?  

   Specious?  Did you actually comprehend what I wrote - rather 
than just skim-read a few key words?

   What part of "up to" equated in YOUR mind that such a long 
run was required or even advocated?  

   Moreover, 
> 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

   In the case of the load on the B+ lead to the alternator, 
(the fusing of which was the original topic under discussion) 
the precise load is well known.  It's the maximum output of 
the alternator being used.  

   And *why* wouldn't you know the "precise" values for the 
load(s) of the device(s) you are connecting to the circuit(s)?  
Every device has a rating for maximum current draw.  That load, 
or the sum of the several loads connected to a particular 
branch, are what the circuit should be designed to safely 
carry.

Dale R.
COZY MkIV #1254

> 
> From: Jim Sower <canarder@frontiernet.net>
> Date: 2005/01/23 Sun AM 01:45:54 EST
> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> Subject: [FlyRotary] Re: Fuel Pump Load
> 
> <... 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.