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You are here: Home > Lighting School™ > Halogen/Incandescent > Voltage Drop
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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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