As you can see, a lamp with a design voltage of 12 Volts will deliver 80% of its light output and its life will double by reducing the voltage to 11.5 Volts. The light loss is hardly noticeable to the naked eye while lamp life is generously extended. Therefore, you want to experience a slight voltage drop to all lamps in your low voltage lighting system. The recommended feed to each lamp is between 10.8 Volts and 11.5 Volts.

Voltage drop is determined by multiplying the total watts on the cable by the total length of cable and dividing by the cable constant listed below. A smaller number gauge equals a larger sized wire.

Cable length - length of cable used from the transformer to the fixture on which you are measuring the voltage drop (in feet).

Total watts - sum the wattages of every lamp along the cable length.

Cable constant - indicates the thickness of copper wire; a thicker wire results in less voltage drop.

(Cable length X Total watts) / Cable constant = Voltage Drop

IMPORTANT! When most of your lights are at the far end of the run, multiply your answer by 1.5 (i.e., multiply the voltage drop by 150%). Click here to view some example calculations. Always start your calculations with 12-gauge wire. The voltage at a fixture is the actual voltage supplied (12 Volts) minus the voltage drop. The voltage supplied may be increased by use of a multi-tap transformer. These multi-taps have alternate voltage sources to counteract a long cable run with too high of a voltage drop. Another way to counteract a high voltage drop is change to a thicker wire, such as 10-guage. If you need more voltage drop than you are getting, you may change to a thinner wire, such as 14-gauge, but a thinner wire supports less wattage. Here is a chart to determine the maximum wattage allowed for each buried wire.

Gauge #18 GA #16 GA #14 GA #12 GA #10 GA #8 GA

Max Wattage 120 W 156 W 180 W 240 W 300 W 480 W

Transformer Guidelines

A low-voltage transformer is an electrical device that changes voltage by inversely adjusting the current. A transformer contains a positive terminal and a common terminal. Your supply cable is connected to one of each terminal thus making a circuit as soon as the first quick connector is attached. The transformer is critical to your low-voltage landscape lighting.

The low voltage transformer is selected by first determining the total wattage being used in your plan. Select a transformer that has a higher wattage capacity than the total wattage being used; this will give you potential to expand or alter your design. You may need multiple transformers to achieve your landscape look. No single circuit may have more than 300 Watts of power, but one transformer can handle multiple runs of cable. The transformer will be placed close to the power source, and some are even rated for indoor installation. Your transformer must be at least one foot off the ground to avoid potentially rising water. All transformers must dissipate heat to cool; you should leave a 3-inch clearance around the transformer housing for indoor transformers.