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changing wire gauge

Discussion in 'Electronic Basics' started by Matt Warnock, Nov 8, 2004.

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  1. Matt Warnock

    Matt Warnock Guest

    I have a question about wire gauges. I've seen a plethora of information
    about gauges of wire and the current that should pass through them. What if
    the wire thickness changes inline, such as for a very short time? I'm
    thinking about what happens at connectors or fuse boxes where the cross
    sectional area changes. Is it the smallest thickness that's the most
    important or the overall average thickness because that affects the overall
    resistance?

    thanks for any incite!

    -matt
     
  2. If you are only worried about total wiring voltage drop, a small
    length of undersized wire often makes little difference. But if you
    are worried about the temperature of the wire, a necked down section
    will certainly get hotter than the larger wire, and with high enough
    current may melt its insulation or even melt the wire, acting as a
    fuse. You should calculate both effects to make sure you will not
    have problems with either voltage drop or peak temperature.
     
  3. Matt Warnock

    Matt Warnock Guest

    how much "headroom" is there in the rules about wire thickness?

    http://www.powerstream.com/Wire_Size.htm

    has some rules. 6 gauge wire will run 101 amps in a chassis wiring
    configuration. 8 gauge will do 73 amps and 10 gauge can do 55. When does
    it really start redlining? The description says its a "conservative
    rating".
     
  4. These ratings are based on some packing factor. I don't know what
    that is without a bit of digging, but a wire can carry a lot more
    current without overheating when it is suspended in air than it can
    when it is tightly bundled with lots of other wires that are also
    carrying the same current density (amperes per cross sectional area).
    More conservative ratings apply when the wire is wound tightly around
    a hot magnetic core. The rating also implies some maximum temperature
    based on the insulation temperature rating, so vinyl coated wire will
    have a lower maximum ampacity than, say, teflon coated wire, all other
    things being equal. You might also have to worry about other
    materials against the wire that either melt, degrade or catch fire
    easily.

    None of this addresses the question of acceptable voltage drop, which
    is a system question, not strictly a wire question.

    The rule of thumb is, if you don't know how to calculate temperature
    rise and voltage drop, you use conservative wire sizing rules to cover
    your ass.
     
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