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Trucks Of The Month
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BY
Will Wills
Whether
you have a trail rig with powerful add-on lighting and a winch, or a
street truck with a huge audio system, you need some way to calculate
what size wiring you need to make your equipment perform, and avoid a
fire. There are several rating standards that can be used for selecting
proper wire gauge. The two I am familiar with were produced by the
American Bureau of Shipping (ABS) and the Federal Aviation
Administration (FAA). For the purpose of this article, I chose the FAA
as a source because their material is public information and can be
posted here without copyright infringement. The book I used is the FAA's
Acceptable Methods and Practices.
The
FAA stipulates two types of loads: Intermittent (no more than two
minutes) and Continuous. An example of an Intermittent load is a winch
or a starter motor. The charts are fairly easy to use. We'll use a
hypothetical 8,000# winch drawing 400amps at it's maximum pull rating
and run an example.
The
chart to the left of the graph is labeled "Intermittent Circuit
Voltage." The voltages listed are 200/115/28/14. We will use
14volts and follow the "Wire Length in Feet" down to our
estimated wire run. For the sake of simplicity, use the next longest run
if the chart doesn't list yours exactly. For the sake of example, we
want to run a 13 foot wire from the front battery to the rear receiver
hitch. Our chart lists 12 feet and 14 feet, we will use 14 feet.
Next,
we trace our "14 feet" line to the right across the graph
until we hit our Amperes rating. The Amperes are represented by the
diagonal lines running from top right to bottom left. As we trace
though, we run into the heavy curve labeled "Curve 3". ONCE
YOU HIT THAT CURVE, FOLLOW THE CURVE! You can not select a wire gauge
that falls below that curve. In our case, the curve intersects 400 amps
just before 3/0. If you study the chart, you will find that no winch
should have less than 3/0 even for a 4' run from battery to front mount.
Makes you wonder about that 2-gauge cable the winch manufacturer gave
you.
Now,
let's look at the Continuous Rating chart. Continuous Ratings are given
'in free air' and 'in a bundle'. there are simply two 'do not fall
below' charts instead of one. For our example, let's assume we are
installing four 55 watt H3 halogen lights on a ten foot run that are
drawing approximately 16amps (55w*4/14V=15.7Amps). We will use the
'bundle' rating on our example, just in case the wiring for our off-road
lights share a space with some other wiring somewhere in the truck.
We
follow the 14 volt chart down until we get to 10 feet. Then we run the
ten feet over to the 20 amp diagonal line (remember, always use the next
longest run, the next heights amperage and the next largest wire size),
and then run vertically down to find with wire gauge. In this case, we
fall between 14 and 12 gauges, so we will use 12. If we would have hit
'Curve 1', we would have followed it as we did in the previous example.
Now,
some people may look at these charts and say, "Hey, aren't those a
little conservative?" I will agree that they are, but airplanes
can't really afford fires and failures. And, well, neither can we.
Especially when you may be depending on your winch to run for four hours
in one or two minute intervals rescuing a dozen or so other trucks in
zero-degree temps. Like that's ever going to happen.
TRS
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