Know Your Voltage

By Jodie Carter, regional editor

The experts at FX Luminaire have translated Ohm’s law into field application with this model that demonstrates how the length of cable affects low-voltage circuit guidelines.

In planning a landscape lighting system, contractors must first anticipate the requirements of the most critical element in a healthy, efficient system?EUR??,,????'??+the voltage.

Low-voltage systems are perfect for lighting an average residential landscape, and are preferable to line voltage systems because they can easily retrofit into the existing landscape with minimal disruption of the surrounding environment. Ensure that your next low-voltage lighting project maximizes lamp life by planning proper voltage at each fixture.

Insufficient Voltage

A common problem in low-voltage lighting systems is insufficient voltage at light fixtures. Low voltage is often the result of an excessive voltage drop caused by using cable that is too small. This occurs when an installer runs too many lights on a single cable, overloading it and causing high resistance.

Symptoms of excessive voltage can include lights becoming very dim as they run further away from the transformer, with the last light reduced to a dim glow.

Any lighting installer who has tried to reinvent Ohm’s law through sheer force of will knows that running a 165-watt lighting circuit on a 12-gauge cable running 200 feet in length will result in a system of lights that dim as they run away from the transformer, with the last light reduced to a meager golden glow. To avoid this situation, landscape contractors should approach their lighting project by asking three questions. How many lights will be included in this circuit? How long will my cable run be? What gauge cable am I planning to use?

Lamp Life

For maximum performance, landscape contractors should adjust their lighting circuits to provide between 11.5 and 12 volts. This voltage should be measured with a voltmeter at the lamp terminals while all the lamps are operating, providing a full load of the circuit. Low-voltage, multi-tap transformers adjust voltage by changing tap settings, allowing a low-voltage transformer to distribute voltages between approximately 11 to 14 volts. These transformer taps can be used to compensate for voltage drop on close, medium and long cable runs. For example, if you discover that your circuit is running on just over 10 volts, you can change the transformer tap setting, raising the voltage toward 12 volts, increasing lamp life and lumen output.

Focus Fact

80% – The recommended percentage of transformer capacity you should not exceed in initial lighting design.

Clients usually add lights–they seldom delete lights. When planning transformer size, leave room for that eventuality.