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Most problems arising in landscape lighting are due to incorrect calculations
of voltage drop. These errors in calculation, could prove very costly to the
customer. Although in some cases the error can be fixed, in other cases
it might be impossible to correct, thus leading to a very dissatisfied and
unhappy homeowner.
Using
the proper selection of wire sizing will ensure that sufficient voltage is
available to produce the desired effect that we want to achieve from each
fixture. Voltage affects not only the intensity of light, but also the useful
life of the lamp. A reduction of only 1/2 volt to a fixture can double the
useful life of the lamp. Consequently, this drop in voltage can also reduce
the lamp's candlepower by as much as 20%.
Many transformers available on the market today may incorporate a High-Low
switch.
Placing the switch on Low, will increase the lamp life considerably without
necessarily being visible to the naked eye.
Another error made by homeowners stems from the incorrect calculation of the
wattage to be used by each fixture. Always consider using the maximum wattage
permissible for the fixture. Most up-lighting fixtures which illuminate trees,
can utilize various lamp wattages such as a 20 watt, a 35 watt or a 50 watt
lamp. Small trees might require a 20 watt lamp to generate enough light
illumination in their first years of growth, however with time, you will
probably need to upgrade the lamp to 35watts or 50 watts to achieve the same
effect.
The basic formula for calculating wire sizing when using #12-2 gauge wire is
to follow the
following rule:
"Runs
of wire more than 100 feet in any direction, should not exceed a load in
excess of 100 watts." This means that
shorter runs might use more wattage while longer runs should use less.
Three factors should be taken into account when determining wire sizing. (1)
Total watts to be consumed on each run, (2) The length of the run, and (3) The
size of the wire.
Once we know these three factors, the formula below will help to determine the
voltage drop as well as the size of wire to be used.
Total watts on cable x Length of run
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= Voltage Drop
Cable size constant (see table below)
12 Volt Lighting Cable
Chart |
Cable Size |
Constant |
#18
|
1,380 |
#16
|
2,200 |
#14
|
3,500 |
#12
|
7,500 |
#10
|
11,920 |
#8
|
18,960 |
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