Ampacity of Wires

["WM" <molux@xxxxxxxxxxxxxxxx>]

The chart given in a recent digest post was a little generous in stating the
current carrying ampacity of various gauges of wiring.  Practically
speaking, most ratings are related directly to the operating temperature and
the insulation material's ability to withstand high temperature.  All wire
has resistance to the flow of current, and hence heats up on the square of
the flow.  At the limits, the wire is poised for physical failure.  The
automobile environment is one of the harshest possible for the average wire,
much more than what residential wiring goes through.  Ambient temperatures
are likely to be way above or below the 30 deg. Celsius quoted in the chart,
and thus with less current flow the "red line" of the insulation will be
surpassed.  This is especially true in the engine compartment.  Look at the
National Electrical Code's several tables relating the type of wire and
insulation to the current carrying capacity. Look for the wire you plan to
use in the charts and then you'll know how much juice you can run through
it, and what temperature range it is rated for.  I always recommend using
heavier wire than you think you need, since there isn't really any down side
to it.  The cost factor is negligible, and the result is a physically and
electrically more robust component.  Avoiding fires and meltdowns is why the
charts are written, and if you look at them, you shouldn't have any
problems.

And when you make connections, remember where they'll be and what they'll be
exposed to.  Temperature extremes up and down, salt and water vapor or
submersion, etc.  Add to the problem of current flow and voltage to rev up
the electrolytic demise of all the exposed components of the system.  Bigger
is better.

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