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From: "Wendell Voto" <jwvoto@itlnet.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-617962@logan.com>
Subject: Re: [FlyRotary] Re: Fuel Pump Load
Date: Sun, 23 Jan 2005 18:25:18 -0600
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Thanks Dale,
I was really referring to the fuel pump breaker, didn't make that clear. I
believe Tracy said to go to the higher breaker on this service.
I have not seen as yet any real world wire sizing tables, were did you get
the info for the 30 feet?  This is a more practical footage for aircraft
wiring.  I have  table from powerstream.com giving 18ga. as capable of 16
amp for chassis wiring but 2.3 amp for transmission wiring.  Would aircraft
fall between these or is the transmission closer to what we need?  I don't
like calculating from resistance per K feet and such.  Quick look-up tables
is what I'd prefer if they exist for practical vehicle/aircraft wiring.
Make any sense?
Wendell
> 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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