When designing a new low voltage outdoor lighting system, one of the most important
decisions you will make is selecting the right transformer for the job.
To do this, you’ll first need to know a few things about the project.
In addition to transformer power requirements, the information you collect will also help you
determine the most efficient wire gauge and the best wiring method to use for each run.
Start by creating a sketch of your design that includes the location
and type of fixtures you plan to use. To determine the total wattage required
to operate the system, you will need to know the volt-amp draw of each fixture.
Most manufacturers have already made this calculation for you and will have this value
in their catalog or on their website. Now, add up the values for your lights;
this is the total wattage your transformer will need to support.
It’s recommended that you should not exceed 80% of the transformer’s maximum wattage load.
For example, a 150-watt transformer should have no more than 120-watts
of total system load. To account for possible load miscalculations
as well as future expansion, it’s a good idea to purchase a transformer
that provides considerably more wattage than the minimum required for the initial design.
Next, measure the total length of wire you will need to deliver power from the
transformer’s location. Be sure to include an extra 3 to 5 feet at each fixture so you will
have some slack to allow for repositioning of the light as the landscape matures.
These measurements will help you determine the best wire gauge and wiring method to use, if you
need a single 12-volt tap or multi-tap transformer and will also help you plan for voltage drop.
Like pressure loss from friction in an irrigation system, Voltage Drop is
essentially electrical pressure loss caused by resistance as electricity moves down the wire.
This is important because today’s LED lamps typically require a minimum of 8-10 volts and
no more than 15-25 volts to operate properly. If the voltage at the fixture is too low or too high,
it can significantly reduce the life of the lamp.
Since voltage requirements vary by manufacturer,
Ewing recommends a target voltage range of 11-15 volts at each fixture. This means no more than 15
volts going into the first light on your run and no less than 11 volts at the last light.
The 11-volt minimum will give you some headroom to work with to reduce the potential
of dropping below the 10-volt minimum that most lamp manufacturers require.
The maximum of 15-volts is a result of UL 1838. This is the Underwriters Laboratories safety
standard for landscape lighting transformers which limits the maximum voltage output to 15 volts.
You can use the voltage drop calculator in the tools section of the Ewing app
to help you determine the maximum voltage drop for each run. It can
also help you decide what wire gauge to use. For example, here we’ve selected 16-gauge wire
with a load of 45 watts and a total run length of 60 feet. This tells us that we can expect to lose
a maximum of about 2 ½ volts by the time we get to the last light in the run. If our transformer
only has a 12-volt tap, then we would potentially only have 9.5 volts available at the last light.
This would not be enough power to keep above the 11-volt suggested minimum.
The solution would be to use a thicker gauge wire with less resistance.
If we switch to 12-gauge wire, the voltage drop would be less than 1-volt at the last light which,
would keep us above the suggested 11-volt minimum.
Another solution would be to use a Multi-tap transformer that supports higher voltage outputs.
If we were to use a 15-volt tap, we could then use the less expensive 16-gauge wire. Just take
into consideration that the wire you select should support future expansion of the lighting system.
For most situations however, we recommend using 12-gauge wire as it offers more flexibility for
future expansion and provides better protection against shorts and subsequent fire hazards.
The example we provided will give you a conservative estimate of the voltage drop
you can expect. For a more accurate result, you should calculate the voltage drop on each segment
of your run, from the transformer to the 1st light as well as each length of wire between fixtures.
To further reduce voltage drop, it’s a good idea to use a center-fed
or t-method of wiring rather than a daisy-chain design. With center-feeding, you’re dividing the
run in half reducing the distance between the first and last light in each direction.
This in turn reduces the voltage drop between the first and last light to a third.
Finally, when selecting a transformer, also consider how you would like to control it.
Options include mechanical and astronomical timers, photocells, wireless control over Wi-Fi or
Bluetooth or a combination of these options. Some transformers come with these controls built-in
while others can be equipped with optional control accessories to give you more flexibility.
Outdoor lighting as well as all your other landscape product needs can be found at
your local Ewing branch. For locations or to shop online, visit us at ewingirrigation.com.