How to calculate volt drop

Written by will charpentier
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A primary concern when installing long runs of wire is voltage drop. Voltage drop is a reduction in the voltage on a circuit between the power source, like a stereo, and the load, like a speaker. Voltage drop over a long run of wire can result in the underperformance of electrical or electronic equipment. You can calculate the voltage drop provided you know the current load in amperes (amps) and the AWG size of the wire. AWG is the wire gauge, expressed in terms of the American Wire Gauge standard, used in the United States since 1857.

Skill level:

Things you need

  • Standard AWG wire gauge
  • Multimeter
  • Calculator

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  1. 1

    Confirm the wire size (AWG, also known as American Wire Gauge) and the current load expressed in amperes (amps). The both of these factors can be determined by direct measurement (by wire gauge and multimeter) or from specifications.

  2. 2

    Multiply the current load in amps by 0.2. If, for example, the current load on the wires is 16 amps, then 16 x 0.2 = 3.2.

    The wire size component of the formula has a mantissa of 1.26 and an exponent calculated by subtracting 10 from the AWG of the wire. If the wire in the above example is 8AWG, then 8 - 10 = -2. The result, 1.26 E (-2), is 0.0126 in decimal notation.

    Multiply these two results to obtain the voltage drop over 100ft.: 3.2 x 0.0126 = 0.04032

  3. 3

    Divide the total wire run length by 100; if the wire run in the example is 300 feet, then 300 / 100 = 3. Multiply this result by the voltage drop over 100 feet. If you continue to use numbers given so far in the example, this would result in the following calculation: 3 x 0.04032 = 0.12096, which would equal the total voltage drop over the hypothetical 300-foot wire run.

Tips and warnings

  • The AWG standard is based on solid copper wire; however, for the purposes of determining voltage drop, the characteristics of stranded copper wire and solid copper wire are the same.

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