How to Calculate the Voltage Drop In Lighting Fixture

Definition

A voltage drop is the reduced electric potential when the current passes along the cable, so the voltage at the end of a cable is lower than at the start. The level of voltage drop correlates with the resistance or impedance of the power source and cables, contacts, and connectors that lead to loss of energy. A voltage drop correlates positively with the length of the wire and negatively with the cross-section, that is, the longer the wire, the greater the voltage drop; the larger the cross-section, the smaller the voltage drop. An excessive voltage drop may lead to unsatisfactory performance such as lights being dimmed and motors running slowly. To compensate for voltage drop larger cross-sectional sized cables may be used which offer less resistance/impedance to current flow.

Voltage drop has an important impact on Low voltage lighting systems. Voltage drop is a function of cable length, cable size, and total fixture wattage. Voltage drop can be minimized in several different ways:

• Use heavier gauge cable
• Shorten cable length or runs
• Reduce wattage of each fixture
• Use multiple transformers
• Reduce the total number of fixtures on a run

How to Calculate the Voltage Drop in LED Driver

Use the following formula to calculate voltage drop using the distance of the run (lighting transformer to last
fixture), the total wattage of the system (load), and the cable constant of the wire:

Voltage Drop = Length of the Run x Load x 2 ÷ Cable Constant

Example: A 75 ft. run with 100W load using 12/2 wire

Voltage Drop = (75 x 100 x 2) ÷ 7500 = 2V. This means that a 14V tap is needed to deliver 12V to the final fixture on the run.

How to Calculate the Max. Cable Length

Use the following formula to calculate the maximum cable length using the total wattage of the system (load), and the cable constant of the wire listed above:

Max. Cable Length = Cable Size Constant ÷ Load x 2

Example: using cable size 12/2 to run with 100W, max. cable length = 7500 ÷ 100 x 2 = 150ft