|
Vocabulary
Fixture - a housing and the electrical components contained in
that housing, a luminaire.
Lamp - a light bulb.
Transformer - an electrical device that changes voltage by
inversely adjusting the current; this device is required to
operate any low voltage fixture, in this case 12-volts.
Design Voltage - the amount of voltage a fixture is designed to
operate on; the actual voltage that you supply may be more or less
depending on the wiring method.
Fixture and Lamp Selection
The fixtures and lamps are selected after determining which
features of your landscaping are to be utilized. Please contact
our professional landscape lighting consultants for help in making
your selection.
Mounting Method
The mounting method is determined by the location of the fixture.
Metal canopies allow you to mount fixtures to a deck or soffit,
while plastic stakes are for mounting into the ground. There is an
extensive variety of selection in mounting hardware, so check to
see which kind is offered with your light fixture.
Power Cable
It is important to note that the cable referred to in this
document is two-pronged; i.e., there are two independent wires
inside.
The cable lengths in your design will determine many further
aspects of your lighting system. You want a cable wire that will
deliver the best voltage to each individual fixture, and the best
method for selecting this cable begins with a layout of your
project. Be sure to include all landscape features, buildings,
individual light fixtures, and power supplies in your drawing.
Group your light fixtures into clusters of 2 to 6; do not form a
group whose total wattages exceed 240 Watts. One cluster at a
time, select a fixture -or a point between two fixtures- that is
the most centralized and draw a line from the power supply to this
point (your center point). From the center point, draw a line to
the nearest fixture on the right, and do the same for the nearest
fixture to the left. Repeat this process from the two newly
connected fixtures until the entire cluster is connected. Click
here to view a sample layout.
To achieve this wiring method, you will need a common device known
as a T-connector. The T-connector allows you to place your cable
along the path of the lights, cut the cable at the last light, and
then connect the middle of the laid-out cable to a new piece of
cable that will run back to the power supply. Then you will use a
similar item known as a Quick Connector to fasten the laid-out
cable to each light fixture. These Quick Connectors are included
with most low-voltage outdoor landscape lights. If quick
connection devices are not available, you can use a common wire
nut. Place a small bead of household silicone seal inside the wire
nut to create a water-tight fit. Here is an example of a typical
layout. The "T" stands for the Transformer, and the "35"
represents a 35 Watt light fixture.
Now you must determine the amount of voltage being delivered to
each fixture of each cluster. Though you may start with 12 Volts,
there is a phenomenon called voltage drop that will reduce the
voltage being delivered to each fixture. Voltage drop on your
system will affect lamp life and the amount of light the lamps
will deliver. Too much voltage drop lowers the light output and
changes the color of the light. Too little voltage drop burns the
lamps too hot and shortens the lamp life. A good rule of thumb to
prevent voltage drop is no more than 100 Watts on 100 feet of
12/2
cable. Here is a chart expressing the acceptable lengths of cable
for a given amount of power in Watts.
| How long is too
long? |
Your lights total |
Your lights total |
Your lights total |
Your lights total |
| Wire Size |
100 Watts |
200 Watts |
300 Watts |
500 Watts |
| #16 gauge (thinnest) |
30 feet |
15 feet |
10 feet |
Unacceptable |
| #14 gauge |
45 feet |
24 feet |
15 feet |
Unacceptable |
| #12 gauge |
100 feet |
50 feet |
32 feet |
20 feet |
| #10 gauge |
160 feet |
80 feet |
50 feet |
32 feet |
| #8 gauge |
240 feet |
125 feet |
80 feet |
50 feet |
| #6 gauge (thickest) |
400 feet |
200 feet |
130 feet |
80 feet |
Another very effective way to eliminate voltage drop is to use continuous
loop wiring. This entails the same preparation as the T-connector method, except
you lay the cable from the transformer, along side each light fixture (one
cluster per cable), and back to the transformer. To achieve continuous loop,
simply connect both ends of one wire of the cable to the Positive lead and both
ends of the other wire of the cable to the Common lead. Be very careful to not
mix these connections! The two internal wires can be distinguished by their
markings. One will have a solid color and the other will have writing, a stripe,
or a different solid color.
Sometimes, a lighting designer or a home owner will want to take
advantage of voltage drop. Here is a table showing the effects
voltage drop has on standard non-halogen lamps. With a halogen
lamp, a voltage feed of less than 10.8 Volts may have a negative
effect.
| Voltage |
Light Output |
Lamp Life |
| 12.5 V |
170% |
80% |
| 12 V |
100% |
100% |
| 11.5 V |
80% |
200% |
| 11 V |
75% |
300% |
| 10.8 V |
70% |
400% |
| 10.5 V |
65% |
500% |
As you can see, a lamp with a design voltage of 12 Volts will deliver 80% of
its light output and its life will double by reducing the voltage to 11.5 Volts.
The light loss is hardly noticeable to the naked eye while lamp life is
generously extended. Therefore, you want to experience a slight voltage drop to
all lamps in your low voltage lighting system. The recommended feed to each lamp
is between 10.8 Volts and 11.5 Volts.
Voltage drop is determined by multiplying the total watts on the cable by the
total length of cable and dividing by the cable constant listed below. A smaller
number gauge equals a larger sized wire.
Cable length - length of cable used from the
transformer to the fixture on which you are measuring the voltage
drop (in feet).
Total watts - sum the wattages of every lamp along the cable
length.
Cable constant - indicates the thickness of copper wire; a thicker
wire results in less voltage drop.
| Cable length X Total watts |
= Voltage Drop |
| Cable constant
|
IMPORTANT! When most of your lights are at the far end of the run,
multiply your answer by 1.5 (i.e., multiply the voltage drop by
150%). Click here to view some example calculations.
Always start your calculations with
12-gauge wire. The voltage at
a fixture is the actual voltage supplied (12 Volts) minus the
voltage drop. The voltage supplied may be increased by use of a
multi-tap transformer. These multi-taps have alternate voltage
sources to counteract a long cable run with too high of a voltage
drop. Another way to counteract a high voltage drop is change to a
thicker wire, such as
10-guage. If you need more voltage drop than
you are getting, you may change to a thinner wire, such as
14-gauge, but a thinner wire supports less wattage. Here is a
chart to determine the maximum wattage allowed for each buried
wire.
| Gauge |
#18 GA |
#16 GA |
#14 GA |
#12 GA |
#10 GA |
#8 GA |
| Max Wattage |
120 W |
156 W |
180 W |
240 W |
300 W |
480 W |
Transformer Guidelines
The low voltage transformer is
selected by first determining the total wattage being used in your
plan. Select a transformer that has a higher wattage capacity than
the total wattage being used; this will give you potential to
expand or alter your design. You may need multiple transformers to
achieve your landscape look. No single circuit may have more than
300 Watts of power, but one transformer can handle multiple runs
of cable. The transformer will be placed close to the power
source, and some are even rated for indoor installation. Your
transformer must be at least one foot off the ground to avoid
potentially rising water. All transformers must dissipate heat to
cool; you should leave a 3-inch clearance around the transformer
housing. If you have read
this far and still have questions, now may be the time to call an
experienced lighting consultant at Residential Landscape Lighting
and Design 1-800-239-2939. |
