Voltage Drop
As electrical energy passes along a wire it looses a little "electrical pressure" or voltage the further that it travels. This is called voltage drop. Lamp life, light output, and color temperature are affected by voltage as it is measured at the fixture. The values for a given lamp are based on a voltage measured at the lamp of 12 volts (11.8 volts for some models). Operating the lamp at any other voltage will change those values. The table illustrates the effects on life, output, and color temperature for various voltages as a percentage of the values at 12 volts.

As can be seen from the chart, small changes in the voltage can have a big impact on the performance of the bulb.

 The factors that affect voltage drop are the load (wattage), the length of the wiring run, the voltage at the transformer, and the size of the wire. Use the table below as a starting point for determining the wire size for various lighting loads while maintaining an acceptable voltage.

* These values assume an equal spacing of the load. If the load is at or near the end of the run, the maximum wattage may have to be reduced. If there are no halogen lamps in the circuit then you may increase the maximum wattage by 10%. Always confirm the actual voltage with a digital volt meter. 

Methods for Reducing Voltage Drop
 To reduce the voltage drop we must change one of these factors: 

• The load in watts can be reduced by changing the lamps to ones with a lower wattage. You can also eliminate a fixture or two or move them to another circuit. Obviously this will have an impact on the intensity of the light and or size of the area that will be lit. 

• The length of the run can be changed several ways. It may be possible to use multiple transformers so that they may be moved closer to the fixtures. By looping back to the transformer from the last fixture you effectively cut the length of the run in half.

• The size of the wire can be changed by using a larger gauge of wire or by doubling the same size wire. You may also use a large wire such as #8 to get to the area where the fixtures are located and then switch to a smaller gauge for the connection to the fixtures. 

• The first thing to do with the transformer is determine the actual output voltage. Remember the transformer simply reduces the primary voltage by a fixed factor of 10 to produce the output or secondary voltage. If the primary voltage is 130 volts, the output will be 13 volts and voltage drop will be desirable.  

• If the output voltage is 12 volts or less and the length of the run results in excessive voltage drop, you may want to use a multi-tap transformer that has outputs of more than 12 volts. These transformers have terminals that are marked with the higher (and lower) voltages or have a switch used to select the alternative voltages. The type with several terminals allow you to have shorter runs that are not affected by voltage drop receive only 12 volts while the longer runs can be connected to the higher voltage terminals. 

Voltage drop can work in your favor as well. Operating the lamp at a lower voltage reduces light output, but it also increases lamp life, particularly with non-halogen lamps. This can be used to your advantage when there are fixtures in the system that are located in difficult to service places such as in tall trees. By selecting a lamp with a higher wattage than required and reducing the voltage to 10.5, the lamp could last 5 times longer than it would if operated at 12 volts.

Written by Ernie Sears
© 2003 Backyard America