Landscape Low Voltage Wire Gauge
The primary issue with low voltage wiring is the voltage drop down the line from the supply transformer or starting point. The longer the cable, the less voltage is available for the lights at the end of the cable that caused the far lights to be seen as dim. An analogy is the loss of water pressure when using a long garden hose with several 
sprinklers connected along the length of the hose. The hose is expected to supply a reasonable amount of water to each of the sprinklers but if the hose is too small, the sprinklers at the furthest end of the hose will receive less water pressure and less water than the first sprinkler.
Understanding Wire Gauge and Voltage Drop
Electrical wiring is similar, if the wire is too small and the cable is too long with several other lights on the same cable, the voltage at the furthest point from the starting point will be less than needed. The 12 volts at the starting point could be 8 volts at the end of the line. The light receiving the 8 volts of electrical power will have less light output, dimming, than the lights closer to the starting point. This voltage drop is due to both the number of lights you have attached to the cable wire, the cable wire size and to the cable length.
To avoid this issue, you need to install the correct size of electrical cable wire and limit the distance. When addressing the electrical wire size, you are talking about the wire gauge. The wire gauge will be numbers like 8, 10, 12, 14, 16, and 18 AWG. The standard size for outdoor low voltage wiring will be 14 gauge, have 2 conductor wires inside the cable and come in 50 ft and 100 ft lengths. The lower the number, the bigger the wire, so a size 10 gauge wire will be larger than a size 14 gauge wire and more expensive.
The type and size of lights connected to the low voltage cable will also help determine the maximum cable length and the wire gauge. If you are using the 1 – 2 watt LED lights, you will be able to have longer cable runs and more lights connected to the single cable than if you are using the 7 – 13 watt incandescent lights. The power or wattage of the lights does affect the required wire size since the wire size can limit the amount of power that can be supplied to the lighting system. This is a secondary but related issue to the voltage drop.
The low voltage cable run should be planned to keep the amount of cable used to a minimum. This can be accomplished by having a main line run fairly straight and connect branch cables to this main line for lights that are 10 ft or more off this “fairly straight” line.
Selecting the Cable Wire Gauge
Determine the length of cable needed for each line including any zigzaging along the way. If you have a secondary cable that branches off the main line, include this length. Also include an extra couple feet to reposition a light in the future if needed.
Low Voltage Cable Length
|
|
Recommended max length in ft per total fixture watts |
|||||
|
Cable Gauge |
50 watts |
100 watts |
150 watts |
200 watts |
250 watts |
300 watts |
|
14 |
180’ |
90’ |
60’ |
45’ |
NA |
NA |
|
12 |
270’ |
135’ |
90’ |
70’ |
55’ |
NA |
|
10 |
430’ |
215’ |
145’ |
110’ |
90’ |
80’ |
|
8 |
675’ |
340’ |
225’ |
170’ |
135’ |
115’ |
Even with the above recommended maximum cable lengths, you may notice the lights at the far end of the cable line to be a little dimmer then others.
Example one: you have 8 fixtures at 12 watts each or a total fixture of 96 watts. Using the chart above, you can use the #14 gauge wire and keep the maximum cable length to less than 90 feet. You will also need at minimum a single 100 watt transformer.
Example two: you have 10 fixtures at 15 watts each or a total fixture of 150 watts. Using the chart above, you have the following options:
- Use #14 gauge wire with max length of 60 ft
- Use #12 gauge wire with max length of 90 ft
- Use #10 gauge wire with max length of 145 ft
Keep in mind that the larger the wire, a smaller gauge number, the wire cost goes up dramatically. So if you need 140 ft of wire, I would consider using smaller wattage fixtures or same fixtures with lower wattage bulbs. Also for 150 watts you will need to have at least a 150 watt transformer, I would recommend a 200 watt transformer.
It is possible to get a transformer with multiple voltage taps like 12v, 14v and 16v. These can be handy when you need to run your low voltage cable a long distance from the high voltage electrical outlet just to get to where your garden lighting starts. The voltage drop to the first light fixture could be 2 volts. If this is the case, use the 14 volt tap on the transformer so the first light fixture will have the 12 volts required. If you have a DC volt meter, this can be tested easily. If you do not, use the trial and error method.
Deciding exactly the size and length of cable and the fixture wattage can be a balancing act. Add a little margin into the overall design to provide room for changes and additions later.
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