Low Voltage Landscape Lighting Transformers - Criteria for Selection
A Review of Landscape Lighting Transformers - Features and Functions
Author: Steve Parrott
|Criteria for the Selection of Low Voltage Transformers for Landscape Lighting Systems|
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 carries the exiting low voltage current (the secondary). The proportional 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.
Toroidal core - preferred
EI Laminated Core - not preferred
Advantages of Toroidal vs. Laminated (EI) Cores
|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 sufficient 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 burned, melted and/or shorted. A transformer damaged in this way cannot be repaired.
All CAST transformers are internally wired with tin-plated, 125° C rated wire compared to 105° C rated all-copper wire from some other manufacturers.
Select a Voltage Range Appropriate for the Job
The length of wire runs, the lamp load on each run and the wire type determine the extent of voltage loss. Using calculations and direct voltage measurement at splice points, the correct voltage taps needed to deliver the correct voltage to the lamp are determined. Since the final tap decisions 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 yourself 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.
CAST Master Series Transformers have taps of 12v, 13v, 14v, 15v, 16v, 17v, and 18v.
CAST also makes the Power Pro Series with taps that go up to 22v. (Yes, we did just say that you're wasting energy going above 18v!) These higher voltages may sometimes make sense with heavy lamp loads and extremely long runs, so we make the Power Pro Series to cover you on those exceptional occaisions.
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).
You can also calculate voltage loss, total watts consumed, energy cost and other values using our online CAST Landscape Lighting System Calculator.
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.
- 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 voltage 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.
- 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.
Surprisingly, some transformers on the market limit the load on a single voltage tap to 600W (even for their 1,120W transformers). With all CAST transformers the entire load can be carried on any one voltage tap.
Commons Configuration Should Allow 100% Use of the Transformer’s Capacity
In theory, if you buy a 1,200 watt transformer, you should be able to load it with 1,200 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 will have four common taps – each with a 300W capacity
(4 x 300 = 1200). To fully use all 1200 watts you will need to have wire runs that add up to exactly 300 watts for each of the four taps – this never happens. The more likely scenario is that the combination of wire runs will leave unused wattage on each tap. The load of your initial wire runs may be only 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 1,200 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 of this feature, for example, a 900W CAST transformer can be selected where a 1,200W from another manufacturer would be required.
CAST's Full Load Configuration
6 x 300w = 1800w
Of course you wouldn't load up to 1800w but the extra common gives you an extra tap to distribute your wire runs, so you can load up to the full capacity of 1500w.
A magnetic circuit breaker on the primary prevents you from accidentally overloading the core.
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 manufactures have circuit breakers on all secondary common taps, but CAST is the only manufacturer to include a breaker 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 often 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 (well within the safety range of halogen lamps).
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 wire 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 quality 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 (Master and Power Pro Series) 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 caused 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 ensure a system that performs flawlessly for year after year.