Wire Size of 12V Landscape Lighting

Voltage Drop

• Voltage drop occurs because the wire carrying the electrical current from the transformer to the lights is too thin. This condition causes the output of lights along the run to decrease. The electricity loses some of its voltage the further it has to travel. The length of the run, the number of fixtures and the wire gauge are all factors that affect voltage drop. To calculate voltage drop, multiply the total wattage of lights on the run by the length of the run and divide by the cable size constant.

Total Wattage

• Calculating the total wattage on a run of lights is straight forward. Simply add up all the individual bulb watts that you plan to run on each wire. This information can usually be found on the bulb itself and on the packaging. The bulb should say “100W” or other number followed by a “W” for Watts.

Wire Length

• Wire length varies depending on the desired set up. You may set up your lighting system as a series, also known as a “snake” or “daisy chain.” Or you may set up a split configuration with two or more directions coming from the transformer. Whatever you choose will depend on your lighting needs. Measure the complete distance of the wire from transformer to the last lighting fixture. In the case of split configurations, add up the lengths of both wires.

Cable Size Constant

• Cable size constants change depending on the gauge of the cable. Gauge is measured by the National Electrical Code’s AWG number system. The smaller the AWG number, the thicker the wire and the greater its constant. AWG No. 16, No. 12, No. 10 and No. 8 have wire constants of 2200, 7490, 11920 and 18960 respectively. Use this number when calculating the voltage drop.

Choosing Wire Size

• When choosing wire size, your goal is to minimize voltage drop so that all lights produce roughly the same wattage. Depending on your layout, you may need to use different size wires for each run. If your total wattage load is 250, for example, and the distance is more than 40 feet, you can expect to experience significant voltage drop using an AWG No. 12 wire. However, the voltage drop can be significantly reduced by using an AWG No. 8 wire. When in doubt, use a thicker, lower AWG number wire to avoid voltage drop.