The Fundamentals of a Successful Low-Voltage Lighting Design - EC&M Apr 29, 2003

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.

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.

A minimum light output of 50% for the last fixture on a given run is normally an acceptable limit. Table 1 shows the maximum wattage allowed at various distances from a 12V transformer for various wire sizes.

You can use the information in Table 2 to identify the required wire size, length of straight run, and wattage load on a preselected value of light output at the end of any given cable.

Many transformers have multiple taps on the low-voltage side of the unit that provide 12V, 13V, and 14V output. Using a higher voltage tap to offset the expected voltage drop helps maintain the desired light output by delivering a closer-to-rated voltage at the remote lamps. This technique is used primarily where the distance to the first fixture is a long way from the transformer. Take care to avoid providing an over-voltage situation to the first grouping of fixtures, as this would shorten the lamps’ life.

You can also minimize voltage drop by altering your cable layout design. There are several options you can use other than a straight run:

• Multiple straight runs can be made in several directions.

• Tee connections reduce voltage drop by using heavier gauge cable for the primary feed.

• A loop design reduces voltage drop and allows the lighting units to give off a more uniform light output. It’s important to match the wire polarity at the transformer connection in a loop layout:

• Locating several smaller transformers closer to the end of your cable runs can also help limit voltage drop. If at all possible, you should center the transformer in the run. If the light fixtures are located too far from the 120V source, consider running a 120V feed to a transformer located closer to the low-voltage lighting fixtures.
• If you’re only trying to serve a few limited fixtures, consider using direct burial, 12V transformers for each fixture, which can be fed with a 120V source.
• Another way to minimize voltage drop is to use lower voltage lamps in your design. For example, 18W lamps are the preferred choice for spread lights, but you can also use 12W lamps. If you do decide to use lower wattage lamps, however, recheck your photometric levels so you don’t wind up with an inferior lighting design. 

Link to Original