Determine your total wattage for
each wire run by adding up all the individual bulb wattages. Do this
separately for each wire run from the transformer.
Measure the total length of each
wire run. Do this separately for each wire run from the transformer.
Multiply the Total Wattage x Total
Length of cable. Do this separately for each wire run from the transformer.
If the total is 10,000 or less:
Use 12 gauge cable
If the total is 10,001 to
15,000: Use 10 gauge cable
If the total is 15,001 to
25,000: Use 8 gauge cable
If the total exceeds 25,000:
split the run into 2 runs and recalculate.
This method is only a basic guide
to determine which wire size you should use. For a more accurate method of
determining which wire size you should use, you should actually calculate
the voltage drop for each wire run.
Landscape lighting cable looks like
heavy lamp cord with two stranded conductors. Larger stranded conductors
(designated by smaller gauge numbers) can carry more current.
You must understand the effect known as
"cable voltage drop". The longer the cable run is and the more lights
that are connected to it, the more the voltage falls in the cable. If you start
with 12-volts from a transformer, there is a limit to how far you can go and to
how many lights can be connected to one cable.
Below, we provide you with two ways to
select your wire size:
A Basic Guide (an easy general way
to select the wire you should use)
Using the calculation of the Voltage
Drop along the wire to more accurately select the wire sizes you should use.
16 Gauge Cable
14 Gauge Cable
12 Gauge Cable
Max. Cable Length (FT)
Max. Cable Length (FT)
Max. Cable Length (FT)
Tips You Should Know:
Voltage decreases as electricity
travels along low voltage cable.
The amount of voltage drop depends
on the gauge of the wire, length of the cable, and the load connected to it
(total wattage used by the bulbs).
As voltage drops, light output
decreases (and lamp life increases). If there is too much voltage drop,
fixtures at the end of the cable may not provide as much light as you want
and there will be less light and less color of light than that of fixtures
closer to the transformer (where the voltage drop is less).
Keep Voltage Drop less than 8 to 10%
(1 to 1.3 volts) for best results.
You can usually run 12 gauge cable
up to 100 ft without excessive voltage drop as long as you keep to total
load (total of all bulb wattage on run) to less than 100 watts.
Voltage Drop = [Total Watts on Cable
x Cable Length]/Cable Constant
To minimize voltage drop, divide the
lighting up among several separate cables and run them back to the
transformer. You can also loop a single cable back to the transformer,
connecting both ends to the terminal blocks, which halves its effective
12 gauge or 14 gauge cable typically
will maintain light output within 85% of the bulb's rated performance and
will maintain its color temperature within 5% of its rated temperature.
How to Choose your Cable Layout Runs
Important: Group fixtures
into distance zones from the transformer (examples: 0-50', 50-75' and 75-100').
Do not have a fixture that is 10' away from the transformer on the same cable
run as a fixture that is 100' away. Try to center load your runs as much as
possible to minimize the voltage differential between the first and last light
(one volt difference is optimum, two volts is too much). All the lights can be
in the 50-75' zone but you still need to run several separate cable runs to
reduce the load per cable, thereby minimizing voltage loss. Typically a 300-watt
transformer has three separate cable runs with about 80 to 120 watts on each
cable - a 600-watt transformer has about five separate cable runs, again, with
about 80 to 120 watts on each cable.
After you have determined the
location(s) where you will be installing your Transformer(s) and fixtures,
draw lines on your plan to indicate your cable runs.
Keep your runs as short as possible
by avoiding unnecessary backtracking.
There are at least five different
ways to layout your cable paths:
1. Series - Most common. The cable is ran as one long run with lights located along the run. Other words for this are "Snake" or "Daisy Chain" - e.g. you have one long snake of lights.
2.Split - Run up to the recommended maximum distance in two or more directions from the transformer.
3. Tee - Allows more even distribution of power to the center of a run or to a run some distance away (e.g. across a driveway or under a sidewalk). Heavier gauge cable or a double run of cable should be used to make the tee. All cable to cable connections should be soldered or very well protected.
4. Split Tee - Allows uniform distribution of power to both legs (e.g. to both sides of the yard). It's like doing a Split layout with Tee layout in each leg.
5. Loop - Allows for relatively uniform light output - However, you must be extremely careful to connect the same wire ends to the proper transformer terminals.
Remember: For maximum light
output and lamp life, the goal in low voltage systems is to provide each lamp
with between 10.5 - 11.5 volts. To stay within this 1 volt optimum differential
you must group the fixtures into distance zones from the transformer and not
overload the cable with excessive wattage. When it's possible, center feed the
zone as this will provide a much more even voltage to each fixture.
Use our links below to find detailed help and information from our Lighting School!