Voltage Drop - Landscape Lighting Tutorials & Low Voltage Lighting System Design Help

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How to Calculate and Use Voltage Drop to Determine Your Wire Size:

What is Voltage Drop anyway?

Voltage Drop is a decrease in the voltage created by internal resistance to the flow of electricity though the wire.
It is the difference between the voltage supplied at the beginning of a wire run (at the transformer) and that at the end of the wire run.
If the Voltage Drop is to big, you will be able to see a decrease in brightness of the lamps as they get further away from the transformer.

The maximum acceptable Voltage Drop is 1.3 to 1.5 volts.

Voltage Drop Formula

Voltage Drop = [Total Watts on Cable x Cable Length]/Cable Constant
Cable Constants: 16 gauge = 2200; 14 gauge = 3500; 12 gauge = 7500; 10 gauge = 11920

Effect of Voltage on Lamp Life & Light Output
Voltage at Lamp	Lamp Life Expectancy	% of Rated Candlepower
13.2	2/3 rated life	350
12.6	3/4 rated life	180
12.0	As Rated	100
11.5	2 x rated life	80
11.0	3 x rated life	75
10.75	4 x rated life	70
10.5	5 x rated life	65
10.0	9 x rated life	50

Recommended Voltage supply for low voltage landscape lights (measured at each fixture)

Ideal voltage for lamps is 10.5 to 12.0 volts
Ideal voltage for halogen lamps is 10.75 to 12.0 volts
Halogen lamps supplied with less than 10.75 volts will have a significantly reduced lamp life.

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