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Percent Voltage Drop

%Vd=100 VdVs\%V_{d} = \frac{100 \, V_{d}}{V_{s}}
sig figs

Worked example: 6.9 V lost on 230 V → 3%. Press Try an example to run it live, then adjust anything.

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See also

Voltage Drop, Single Phase: Both use voltage drop.Voltage Drop, Three Phase: Both use voltage drop.Kirchhoff's Voltage Law (Three-Element Loop): Both use voltage drop.LED Series Resistor: Both use supply voltage.RC Capacitor Charging: Both use supply voltage.
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Percent Voltage Drop explained

VdVs%Vd

Volts lost only mean something relative to volts supplied: 6.9 V is a trivial loss on 4160 V and a serious one on 230 V, where it is 3%. The familiar limits (the NEC's informational 3% on a branch circuit and 5% overall, and similar figures in IEC and CEC practice) are recommendations aimed at equipment performance, not safety minimums, but designers treat them as hard rules because the consequences are real.

Undervoltage is unkind to motors in particular: torque falls with the square of voltage, so a motor at 90% voltage makes only 81% of its torque and pulls extra current to compensate, running hotter for it. Incandescent lamps dim visibly at a few percent, and electronic supplies simply draw more amps as voltage sags, deepening the drop. Note this solver's percent fields also accept a plain fraction.

Percent Voltage Drop formula

%Vd=100 VdVs\%V_{d} = \frac{100 \, V_{d}}{V_{s}}
Where
  • %Vd\%V_{d}= Percent voltage drop (%)
  • VdV_{d}= Voltage drop (V)
  • VsV_{s}= Supply voltage (V)
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Voltage drop

4 formulasSingle-phase and three-phase voltage drop, percent drop and conductor resistance at temperature — sizing a conductor for a long run.

Revised 16 October 2026
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formula.expert. (2026, October 16). Percent Voltage Drop Formula & Calculator. https://formula.expert/solvers/percent-voltage-drop

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