Maximum Wattage:

12/2 has a maximum of 192 watts or 16 amps per run (line)

10/2 has a maximum of 288 watts or 24 amps per run (line)

8/2 has a maximum of 384 watts or 32 amps per run (line)

Illustration: Using the previous example of 800 watts

If you use 12/2 wire, you will require 5 runs

If you use 10/2 wire, you will require 3 runs

If you use 8/2 wire, you will require 2 runs

Measure the distance from the transformer to the very first fixture on each run. You do this to calculate the voltage loss. Use this formula:

Cable Voltage Drop = (Wire Run length x Total watts on the run/ Constant) x 2

12/2 wire cable has a constant of 7500

10/2 wire cable has a constant of 11920

8/2 wire cable has a constant of 18960

These guidelines are used for illustration purposes and manufacturer Installation & Instruction Manuals should be used for actual installation.

Illustration:

Your 900 watt transformer is mounted on the outside of your garage. You are installing 40 fixtures at 20 watts each (800 watts). 12/2 direct burial wire was easy to find at your local hardware store so you decide that is the way to go. You’ve done your homework prior to installing your low voltage landscape lighting system and have rightfully decided on 6 wire runs; making sure not to exceed 192 watts on any given run or 300 watts for each of the 3 COMs.

• Run 1 has 180 watts (9 lights) – courtyard
• Run 2 has  140 watts (7 lights) – front walkway/driveway
• Run 3 has  140 watts (7 lights) – front left side trees/house
• Run 4 has 140 watts (7 lights) – front right side trees/house
• Run 5 has 120 watts (6 lights) – plants beds, shrubs
• Run 6 has  80 watts (4 lights) – Steps

Run 1: First fixture is 25 feet away from the transformer. Use the cable voltage drop formula above.

25 x (180/7500) x 2

25 x (.024) x 2 = 1.2 v. drop

Run 5: First Fixture is 150 feet away from the transformer. Use the cable voltage drop formula above.

150 x (120/7500) x 2

150 x (.016) x 2 = 4.8 voltage drop

You’ll need this information when making connections to the appropriate tap on the transformer.

Voltage range is 10.8v to 12v at each fixture.

If you set Run 1 on the 12v tap, you will have a reading of 10.8 volts (12v – 1.2v drop). Great for the first fixture, however that last fixture on that run will fall below 10.8. Use a volt meter for a reading.

Run 2 on the other hand have a voltage drop of 4.8v. Set on 12v, you will have a reading of 7.2v at the first fixture. To get back to the 10.8 – 12v range, you will have to use voltage taps between 16v – 17v tap.

This is a great illustration for using multi-tap transformers versus the standard 12 volt single tap found in many hardware stores.

These guidelines are used for illustration purposes and manufacturer Installation & Instruction Manuals should be used for actual installation.