Received-SPF: none
 receiver=logan.com; client-ip=68.230.241.35; envelope-from=dale.r@cox.net
From: Dale Rogers <dale.r@cox.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: Fuel Pump Load
Date: Sat, 22 Jan 2005 22:05:46 -0500
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Message-Id: <20050123030546.PSXC23615.fed1rmmtao04.cox.net@smtp.west.cox.net>

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