Low Voltage Landscape Lighting Transformers - Criteria for Selection

 A Review of Landscape Lighting Transformers - Features and Functions

The singe most important component of a landscape lighting system is the multi-tap transformer. This unit converts house current (115V - 120V AC) to a range of voltages from 12V and above. This range is needed to compensate for power loss through varied-length (and load) wire runs to the fixture. Halogen lamps used in the fixtures now have acceptable operation range of between 10.8V and 12V, with the optimal operating range of 11V to 11.5V.

The design and construction of the transformer is critical to the proper functioning and safety of the system. The following are the characteristics of a quality transformer:

  • A unit that efficiently transforms house current to a range of low voltages appropriate for the job.
  • A unit that does not generate excess heat and that safetly dissipates the heat that is generated.
  • A unit that withstands corrosion, water damage, and other physical damage.
  • A unit that is simple to install resulting in solid wire connections.
  • A unit with quality circuit protection
  • A unit that operates efficiently and trouble-free for year after year

The Transformer Core - Choose Toroidal, not Laminated
The core of the transformer is basically two lengths of wire wound on a core in a configuration that creates a magnetic field. This magnetic field transfers the current from one wire to the other. One wire carries the incoming high voltage current (the primary). The other wire carriesw the exiting low voltage current (the secondary). The porportional number of windings on these two wires determines the amount of voltage drop. There are two main core designs used in landscape lighting transformers, the laminated type (EI) and toroidal. The toroidal core has several advantages over the EI type. All CAST Transformers have toroidal cores.

Advantages of Toroidal vs. Laminated (EI) Cores

  1. Higher efficiency (approx. 95% vs 85%)
    This makes for less energy expenditure (approx. a 10% difference in the homeowner's electric bill). For example, for a 1,000 watt system, running 12 hours per day, at $0.12/kwh the cost savings would be around $50.00 per year. 
    It also reduces the difference between voltage at the taps under full load and partial load. With a highly efficient transformer, you can use seconday breakers to switch off some of the wire runs without greatly affecting the voltage on the other taps. With the lower efficiency (laminated) cores, switching off wire runs will cause an unacceptable rise in voltage to the other taps. For example, a fully loaded toroidal transformer will deliver about 11.4V at the 12V tap. Cut the load in half and the voltage at the tap will increase only about 0.3V (well within +/- 0.5V safety range of MR-16 lamps.) On the other hand, a fully loaded EI transformer will deliver about 10.8V at the 12V tap. Cut the load in half and the voltage at the tap will increase to about 11.7V. This rise of nearly one volt is enough to burn out or reduce the life of your MR-16 lamps. 
  2. Runs Cooler
    Since the electricity is so efficiently transformed, far less heat is produced.
  3. Less Weight
    Toroidal cores are about half the weight of laminated cores.
  4. Runs Quieter with No Vibration
    Toroidal cores shield themselves so they emit about 1/10 of the magnetic field hum that may be present with laminated cores. There is also no vibration - important because vibration can loosen connections over time and lead to burned out terminals and wires.

Internal Wiring - Ensuring wires to the terminal blocks and other components are well shielded and designed to withstand prolonged use.

All transformer manufacturers conform to UL specs in the design and sizing of internal wire. But these specs may not be sifficient if there are shorts in the field, if a loose connection at the tap causes high heat, or if the transformer is subject to a high heat environment. It's not uncommon for a distributor to receive a burned-out transformer with internal wires burns, melted, and/or shorted. A transformer damaged in this way cannot be repaired.

All CAST transformers are internally wired with tin-plated, 125ºC rated wire compared to 105ºC rated all-copper wire from some other manufacturers.

Select a Voltage Range Appropriate for the Job
The length of the wire runs, the lamp load on each run, and the wire type determine the extent of voltage loss. Using the calculations and direct voltage measurements at splice points, the correct voltage taps needed to deliver the correct voltage to the lamp are determined. Sine the final tap decesions are made in the field based on voltage measurements, you want to have the full range of voltages available. Big jobs with long runs may require up to 18V. There is almost never a situation where greater than 18V is needed - if you find yourslef wanting a higher voltage than 18V you should know that you are burdening the homeowner with a higher electric bill. The better practice is to increase the gauge of the wire or to double up on the wire to reduce voltage loss.

Formulas and worksheets for calculating voltage taps, wire sizing, and transformer selection are included with every CAST Transformer. These are also detailed in the CAST Landscape Lighting Training Manual (available through CAST Distributors).

Transformer Design Should Allow the Full Transformer Load to be Carried on Each Voltage Tap
There are two cases where it is critical that each voltage tap be able to handle the entire transformer load.

  1. During Installation
    After all fixtures have been connected in the field, the next installation step is to connect all wire runs to the 12V tap. This allows you to check voltge loss at each splice location while the transformer is fully loaded. This method can only be done if the transformer has been designed to carry the full load on the 12V tap.
  2. After Installation
    You may have lighting jobs where all wire runs are nearly the same length or are carrying similar loads and require the same tap. 

Common Configuration Should Allow 100% Use of the Transformer's Capacity
In theory, if you buy a 1200 Watt transformer, you should be able to load it with 1200 Watts. However, in most transformers you cannot do this because of the configuration of the common taps.
In traditional transformer design, a 1200 Watt Transformer with have four common taps - each with a 300W capacity. To fully use all 1200W you will need to have wire runs that add up to exactly 300 wats for each of the four taps - this never happens. The more likely scenario is that the combination of wire runs will leave unused wattage o each tap. The load of your initial wire runs may only be 900 Watts, but if you wanted to add another run of 200 Watts (still within the transformer capacity) you may not be able to because there is not enough capacity left on any one common tap.

CAST solved this problem by adding an additional common tap in its Master Series Transformers. Instead of four commons for the 1200 Watt Transformer, with CAST you get five. This gives you the flexibility to connect wire runs up to the full capacity of the transformer without overloading any of the commons. Because fo this feature, for example, a 900W CAST Transformer can be selected where a 1200W from another would be required.

Both Primary and Secondary Circuits Should be Protected with Magnetic Circuit Breakers
Overload protection is critical for any power supply. Transformer manufacturers use three main types of protection - fuses, push-type breakers, and magnetic breakers. Of the three, magnetic breakers are far superior in performance and functionality.

Fuses are troublesome in that they must be replaced when blown. They are subject to the user putting in the wrong fuse type and they can be bypassed entirely with a nail, foil, or even a cigarette wrapper.

Push-type breakers perform well, but they cannot be used as a switch.

Magnetic breakers are the best choice because of their durability, ease of use, and can be used to switch on or off individual common taps.

Better manufacturers have circuit breakers on all secondary common taps, but CAST is the only manufacturer to include a breakers for the primary voltage as well. This added level of protection ensures that the entire transformer is protected, not just the secondary. It also has the dual use of acting as an on/off switch for the transformer. Without that switch, the only way to turn off a transformer is at the main breaker box (may not be easily accessed) or at the external GFI outlet (this outlet is sealed when properly installed). *Homeowners oftem request that they be able to manually switch off certain lighting zones. The magnetic circuit breakers on CAST transformers are ideal for this. Because of the high-efficiency toroidal core, switching off several secondaries will only result in a small increase in voltage on other taps (withing the safety rainge of halogen lamps). The primary circuit breker can be used as an on/off switch for the entire transformer.

Voltage and Common Terminals Should be Screw-type and Accommodate Multiple Wire Runs.
Older transformer designs have wires that dangle from the core. The installer must use wrie nuts to make the connections. This is problematic for many reasons. During the voltage adjustment phase of installation, wires need to be twisted in and out of wire nuts several times - each time the wire is damaged and must be re-stripped. This is also very hard on the fingers. In addition, wire nut connections with several large wires in each nut are susceptible to failure and may result in bad connections.

Better manufacturers, including CAST, now use heavy-duty terminals that allow multiple home-run wires to be screwed securely in place.

When more than 6 or 7 wires need to be connected to one tap, we recommend the use of a terminal lug (CAST Model# CTESTLUG) that can easily accommodate (12) #12/2 wires. This lug is attached to the transformer terminal with a single wire.

Time Clock and Photocell Outlets Should Not Carry the Full Transformer Load
All qaulity transformers have convenient outlets inside the transformer housing for connections to a photocell and time clock. But most transformer manufacturers wire these outlets so they share the full load of the transformer. This puts unnecessary stress on the photocell and timer and can lead to the burning out of these sensitive components.

Instead, CAST transformers utilize a power bypass relay that feeds a tiny current to the photocell and timer. When the photocell or timer cuts the power, it signals the relay to cut the power to the transformer.

For many years, faulty and poorly designed transformers casued the failure of many landscape lighting systems. The advances described in this article represent the state-of-the-art in transformer technology. CAST Lighting working with leading engineers in the field have developed a line of transformers that now qualify as professional highest quality components, that simplify installation and that insure a system that performs flawlessly for year after year.

Created: 07/12/08