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Sizing Requirement Guidelines for Outdoor 12 Volt Lighting Cable 


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
__________________________________         = 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



Keep the voltage drop less than or equal to 1.5 volts on the last fixture in each run.

Many new transformers have various Multi-Tap positions which offer a range from 11 volts to 15 volts of output. Use the higher voltage for longer runs and the lower voltage for shorter ones. (Always ensure that the voltage on the fist fixture never exceeds more than 12 volts.)

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