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Voltage Drop Examples

These worked examples apply the voltage drop formula (VD = I × R) and its single-phase and three-phase variants to find voltage drop, percentage drop, and maximum run length.

  1. Step 1

    A circuit carries 20 A through a conductor with 0.5 Ω of total resistance. Find the voltage drop.

    VD = I × R = 20 A × 0.5 Ω = 10 V

  2. Step 2

    A 120 V circuit has a 4.5 V drop. Find the voltage drop as a percentage of source voltage.

    %VD = (VD ÷ V) × 100 = (4.5 ÷ 120) × 100 = 3.75%

  3. Step 3

    A single-phase circuit carries 20 A over conductor with 2.5 Ω per 1000 ft, run 100 ft one-way. Find the voltage drop.

    VD = (2 × I × R × L) ÷ 1000 = (2 × 20 × 2.5 × 100) ÷ 1000 = 10 V

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