With the rising popularity of residential and commercial landscape lighting, end-users and homeowners have begun looking for systems and components that combine easy installation and adequate safety considerations in one package. Standard 120V systems are unable to meet these requirements, so the industry’s landscape lighting manufacturers have responded by adopting 12V low-voltage systems as the standard for outdoor applications.
Low-voltage systems use smaller light sources, are easy to modify to accommodate changes in landscape layout, and are safer to operate and maintain than their 120V counterparts.
In standardizing the 12V low-voltage system, manufacturers have made available various sizes of step-down transformers to convert a standard 120V source to a 12V supply. The components of the system may be easier to work with, but voltage drop must be considered and understood to effectively service the customer’s landscape lighting needs.
Depending on its size and length, the conductor serving the fixtures of a low-voltage lighting system acts as a resistor. As current runs through the conductor, a voltage drop occurs: the voltage at the end of the conductor is lower than at the source. Smaller wires and higher currents will increase the voltage drop by raising resistance and increasing the fixture load respectively.
Why worry about it?
Voltage drop on a lighting circuit in a 120V system isn’t considered a major issue. The branch circuit currents are relatively low—usually 20A or below—and the standard wire sizes are usually large enough to minimize resistance problems. When working with 12V systems, however, the line current for any given load increases by a factor of 10. For example, a 100W 120V incandescent lamp draws .83A, but an equivalent load of two 50W MR16 12V lamps draws 8.3A. If you use the same size and length wire in both systems, the voltage drop in the 12V system will be 10 times greater than in the 120V system. In this case, voltage drop becomes a significant consideration.
When the actual voltage delivered to a given incandescent lamp is lower than the lamp’s rated voltage, the light output will be reduced. This relationship isn’t linear. When the voltage decreases to about 85% of its rated value, the visible light output is only about 50%. It continues to drop quickly from this point forward.
How to minimize voltage drop
Once you’ve finalized the lighting layout, you can control voltage drop by selecting the most effective gauge wire. The smaller the gauge, the less the voltage drop.
Low-voltage lighting installations require a great deal of care during installation due to the high currents in the system. Placing too many fixtures on the circuit or using the wrong wire size can lead to a system overload. Therefore, it’s important to troubleshoot the system after installation.
The most common problems encountered in low-voltage landscape lighting installations are poor wire connections, too many fixtures on one transformer (overloading), and cables that are too small for the load.
By understanding voltage drop, you can address these problems and implement a successful low-voltage lighting design. Low-voltage lighting will allow the end-user to change the landscape layout and ensure safe outdoor lighting.