Catalog
Holiday Lighting
Specialty Lighting
Landscape Lighting
Patriotic Lighting
Displays
Banners

Holiday Lighting Manual

Calculating Voltage Drop

Voltage drop on your system will affect lamp life and the amount of light the lamps will deliver. Too much voltage drop lowers the light output and changes the color of the light from white to yellow or orange. Too little voltage drop burns the lamps too hot and shortens lamp life.

The following table shows the effect of voltage drop for standard non-halogen lamps:

Voltage	Light Output	Lamp Life
12.5	170%	80%
12.0	100%	100%
11.5	80%	200%
11.0	75%	300%
10.5	65%	500%
10.0	50%	900%

As you can see, a lamp with a design voltage of 12 volts will deliver 80% of its light output and it's life will double by reducing the voltage 0.5 volts. The light loss is hardly noticeable to the naked eye while the lamp life is greatly extended. Therefore, in the outdoor environment, we want to experience a slight voltage drop to all lamps. The recommended feed to each lamp is 10.8 - 11.5 volts.

Keep in mind that with halogen lamps, such as the MR16, a voltage feed of less than 10.8 volts may adversely affect lamp life.

How to determine voltage drop

Voltage drop is determined by multiplying the total watts on the cable by the total length of cable and dividing by the cable constant. (These constants are listed below.)

total watts x cable length -------------------------------------------- = cable constant (see below)	voltage drop

Important! When most of the load is at the far end of the run, multiply voltage drop by 2.

Total Watts - Sum of wattage for every lamp on one piece of cable.
Cable Length - Length of cable used (in feet) from the transformer to the fixture for which you are calculating voltage drop.
Cable Constant - Indicates thickness of copper wire. The thicker the wire, the lower the conduction resistance and the lower the voltage drop.

Wire Size	Cable Constant
#18	1380
#16	2200
#14	3500
#12	7500
#10	11,920
#8	18,960

How is the voltage determined at each lamp?

Voltage at each lamp is determined by subtracting the voltage drop at each lamp from the voltage at the transformer. The voltage at the transformer will depend on:

Which voltage tap is being used (11 volt, 12 volt or 13 volt).
The total load on the transformer.
the voltage feed to the transformer.

Try to locate the transformer in the most central location possible. Voltage drop can be controlled by:

Using the proper voltage tap on the transformer.
Center feeding to the load of each run.
Using proper cable size.
Using fewer fixtures on a cable run.
Using lower wattage lamps.

Conclusion

Any of these methods can be used in conjunction with each other. Since there are so many variables involved, it is suggested to use the voltage drop calculation to get you in the ball park. It is always recommended to check the actual voltage readings at each fixture with a volt meter before burying the cable. This allows for much easier wiring connections.

In short, the simplest and easiest way to lay out your wire runs is to connect all wires and fixtures, leaving the cable above ground. Then take a voltage meter reading at the first fixture. It should read 11 volts to 11.5 volts. If you have followed all of the recommendations above and on the other pages, all other fixtures should be receiving a minimum of 10.8 volts, but check them to make sure.

For more information on Landscape Lighting, please call Nida Lighting at 800.761.6432 or email us at info@nidagroup.com.