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Power Move
A Look at Transformers and Wiring

by Mike Dahl, LC/DBM



Some transformers such as those in the Kichler PRO series, and the VOLT(R) Clamp-Connect LED series have taps that can output at a variety of voltages to compensate for voltage drop that can occur in longer wire runs where a higher voltage terminal might be needed to ensure 12 volts of power to the fixtures at the end of the runs.

Besides cost savings on electricity, many homeowners are sold by the increased aesthetic values, according to Best Pro Lighting, of low volt lighting systems, which require a transformer to operate with the American Standard 120V 60Hz power. These values include lamps that are smaller and less obtrusive, and come with different beam angles that direct light more precisely for better artistic effects. The higher watt lamps of 120-volt systems create unwanted glare and reduce the ability for subtle landscape lighting.


For installation layouts, the straight run is the simplest but least efficient as each fixture down the line will receive less voltage due to voltage drop. This method is adequate for short runs where voltage difference is not more than 1.5 volts from first to last fixture according to Focus Industries. The split load method center feeds the cable run, minimizing voltage drop because it reduces only between the first and last fixture on either side of the run. With the loop method, the cable connects back to the transformer, providing voltage from both sides, which produces more uniform voltage to the fixtures.
Credit: Focus Industries

Transformer Installation
The Association of Outdoor Lighting Professionals advises to keep the transformer at least 10 feet away from open water and at least 12 inches above grade - more if the area is subject to flooding or even deep snow. To avoid fuse tripping mount it 15 feet away from the primary breaker. And when choosing a wall of the house to mount it on, be mindful of a transformer's low resonant hum it can occasionally produce.


Evening Star advises using 12/2 landscape lighting cable (#6) when connecting fixtures using Power Tap Connectors, which are double-sided and will also accommodate 10/3 dual circuit cable; offering greater design flexibility. When splicing cable, it is recommended to use UL listed wire nuts suited for direct burial.
Credit: Evening Star

Here are some transformer installation guidelines from VOLT(R) Lighting:
o It must be plugged into a ground fault interrupter circuit (either a GFIC receptacle or GFIC breaker at the circuit panel).
o Do not use extension cords with transformers.
o Transformer may become hot; do not mount on vinyl.

The National Electric Code specifies that the actual working capacity of a transformer should only be 80% of its rated capacity. Additionally, to account for loss of current through wire, reduce the 80% by another 10%.

Sollos Landscape Lighting reminds that if the transformer has a photocell to automatically turn the system off and on based on the amount of available daylight, it should be installed in an outdoor area that receives sufficient natural light in the photocell's sightline, such as on the side of a structure. Locations such as under a porch are not ideal. Focus Industries offers these additional installation tips:

In some cases, 120-volt power must be pulled into or placed in a specific location for the best results. A weatherproof receptacle (GFIC) cover must be used in all exterior applications.

If the transformer has to be mounted in areas with poor access to daylight or inside the home or garage, a remote photocell can be used. If necessary or if preferred, customers can choose manual or time clock operation. Time clocks require adjustments throughout the year as daylight hours change.

If a residence has a motion control security lighting system and landscape lighting system, it is recommended to dedicate an entire transformer to each separate system.

Voltage drop is inherent in all lighting systems. When possible, center the transformer and minimize cable distance from it. Make mutiple cable runs to transformer.

Plan ahead for future growth of the system by using a more powerful transformer than currently needed.


When calculating the wire runs, Sollos Landscape Lighting suggests adding up fixture wattages per run. Then measure the approximate distance from the transformer to the first fixture on each run and use the chart to determine which voltage tap can be used for each run.
Credit: Sollos Landscape Lighting

Garden Light LED recommends that all their LED luminaires be connected to a 15-volt AC tap on the transformer. While all of their fixtures can operate at voltages as low as 12 volts AC, and some at even lower voltages, using the 15-volt AC tap results in the luminaire consuming less current. This is because the constant current driver, which is the device that converts the available input power into the power required to drive the LEDs, uses less current to do so when the voltage is higher. And less current also reduces the stresses seen by the wiring, the connections, and the driver.

After wiring a system, Unique Lighting Systems advises to perform what they refer to as "the critical three." The first step is to check primary amperage -the amperage of the 120-volt power line - to make sure that the system is not exceeding the maximum allowed on the transformer. Next, the secondary amperage, that on the low-voltage side, should be checked to ensure that amount on each home run does not exceed the maximum allowed for the gauge of wire used (again using the 80 percent rule of the National Electrical Code). The third step is to check voltage at the point of connection, such as a hub, to make sure proper voltage is being sent.

As seen in LC/DBM magazine, April 2017.

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April 30, 2017, 5:33 pm PDT

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Last Updated 04-24-17