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icubob(at)newnorth.net
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 Posted: Mon Jul 21, 2008 2:13 am Post subject: fuses, wire sizes |
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hi all,
i am following a schematic for an ov mod and am stumped at the 10 ga wire with 30 amp fuse in the diagram. the fuseholder from b and c for a 30 amp fuse will only allow a 12 ga wire be fitted inside the crimp connection. a fuseholder from NAPA labeled for a 30 amp fuse uses 12 ga wire. this 10 ga wire is used with the 20 amp breaker from 12 volt power to the relay. this is on a 3300 jabiru. what do i do? any help appreciated.
bob noffs
[quote][b]
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nuckolls.bob(at)cox.net
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| Posted: Mon Jul 21, 2008 5:10 am Post subject: fuses, wire sizes |
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At 05:10 AM 7/21/2008 -0500, you wrote:
| Quote: | hi all,
i am following a schematic for an ov mod and am stumped at the 10 ga
wire with 30 amp fuse in the diagram. the fuseholder from b and c for a
30 amp fuse will only allow a 12 ga wire be fitted inside the crimp
connection. a fuseholder from NAPA labeled for a 30 amp fuse uses 12 ga
wire. this 10 ga wire is used with the 20 amp breaker from 12 volt power
to the relay. this is on a 3300 jabiru. what do i do? any help appreciated.
|
Who's schematic?
Wire sizes versus published recommended protection levels
is a VERY broad brush. The "conventions" adopted by most
of the aircraft industry are exceedingly conservative too.
There are TWO considerations for selecting a size of wire
and the protection for that wire. (1) TEMPERATURE rise over
ambient as it relates to that particular wire, where it's
to be installed and the rating of it's INSULATION for
maximum operating temperature. (2) Voltage drop in a wire
might degrade performance of the appliance at the other
end.
To make our jobs easier in the TC aircraft world where
the problems to be solved are much greater and more
frustrating than wire selection, conventions were adopted
that assured the designer of good performance in 99.9% of
all instances where wire is used aboard the airplane.
Chapter 8 of the 'Connection speaks to these issues. You
can download a copy of this chapter published at:
http://aeroelectric.com/articles/Rev11/Ch8_R12.pdf
where you will find two figures that speak to these
conventions where we said that 10C rise is a good
target for conservative application of wiring in
aircraft and that we choose wire gages to limit
voltage drop in wiring to 5% of system voltage.
Folks are often worried about burning up wires
should they be protected by the wrong size of
fuse. I'll refer you to this photo . . .
http://aeroelectric.com/articles/Rev11/Ch8_R12.pdf
where a 22AWG wire has been loaded to 20 amps for
about 15 minutes. The insulation temperature of
the wire has settled out at 112C (the wire is
rated at 150C). However, at 16 milliohms per
foot and a 5% wire drop value of 0.7 volts in
a 14v system, the maximum length of a 22AWG
conductor in a 20A circuit would be (0.7/20x0.016)=
2.2 feet long!
Obviously, nobody would WANT to use a 22AWG
wire in such a demanding way . . . the point
is that it COULD be used without creating
an issue with respect to fire hazard.
So the short answer to your question is use the
12AWG wire. If this wire in in series with your
alternator (rated at 20A I think) then a 30A
fuse is appropriate protection for the purpose
of preventing nuisance trips . . . and this in
no way places your 12AWG wires at risk for
immolation.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
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icubob(at)newnorth.net
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| Posted: Mon Jul 21, 2008 6:35 pm Post subject: fuses, wire sizes |
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bob, thanks for the good reply to my question. the schematic is the one supplied by b and c with the ov mod.
bob noffs
[quote][b]
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