Date: Tue, 25 Jan 2000 12:09:27 -0500
From: Lynda Frantz <LFrantz@compuserve.com>
Subject: Sizing of circuit breakers or fuses
Sender: Lynda Frantz <LFrantz@compuserve.com>
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>This particular schematic shows a 70A fuse for the alternator, but that
is obviously to be determined by the size of the alternator being used.<

Circuit breakers and fuses generally  have one purpose in life and that is
to protect the wiring not the component that the wiring feeds.  The wiring
needs to be of adequate size to deliver the required amps without excess
voltage drop.  Once you properly size the wiring to deliver the amps only
then can you determine the value of the circuit breaker.  

To make a point let's say you want to power an electrical device that draws
only  1/100 amp continuous.  You have a 7 1/2 amp circuit breaker
available. Could you run a #20 wire or smaller that can easily handle the
1/100 amp?  No!!!!!  The smallest wire than can be run is #20 AWG because
according to AC43.13-1A Figure 11.1 a #20 wire can be protected with up to
either a 7.5 amp C/B or a 5 amp fuse.

You size wire using AC 43.13-1A knowing the wire length, acceptable voltage
drop, circuit voltage and continuous or intermittant current load rating. 
Once the wire size has been established then you determine the circuit
breaker or fuse rating.

In the case of the alternator, the circuit breaker protects the wiring
only.  On my aircraft, I used a current limiter instead of the breaker.  A
current limiter protects the wiring as well as limits the current.  It is
less expensive as compared to a circuit breaker, would need to be replaced
if it blows and is firewall mounted.  I am a believer that if the big guy
blows in flight, I would not want to reset it anyway.  Turning off the
alternator in flight is done by popping the field to the alternator not by
poping the main big guy. 

Regarding the discussions on what sized fuse is used to power what devices.
 This discussion is technically in error too.  The discussion should be
"what load is typical for each device".  Your load analysis, wire sizing
and C/B specifications could look something like this.

Device  current draw    wire size required      biggest C/B allowed
                        (length and V drop)     (to protect the wire)
Pitot heat      7.5 amps                #18 AWG         10
Vision  1/10 amp                #22             5
Position        3.5 amp         #22             5

Typical load charts and excellent discussions of wiring sizing and C/Bs
have been in past issues of the Lancair Network News.  Buy AC 43.13-1A at
your local aviation publications store.

Jim Frantz   
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