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From: "Marc Wiese" <cardmarc@charter.net>
To: "'Rotary motors in aircraft'" <flyrotary@lancaironline.net>
Subject: Wire/fuse sizes in aircraft
Date: Sun, 23 Jan 2005 18:06:54 -0600
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You guys really need to get the FAA's AC 43.13 1A and B (latest ones are
greatly revised!). It is the bible for aircraft repair; and in it in the
electrical section it gives wire sizes depending on load, voltage and
length, and from that you can figure the fusing you need. It is required
reading for anybody who wants to get a repairman's certificate or has
gotten an A/P license. If your intend to build an aircraft, you need
these books. Also check those switches-don't use AC switches. Use
switches rated for DC.  Test will be at 8 am tomorrow.
Marc Wiese

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Dale Rogers
Sent: Saturday, January 22, 2005 9:06 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Fuel Pump Load

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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