Use the formula: Voltage Drop = Current × Resistance
For DC circuits: Vdrop = I × R
For AC circuits: Vdrop = I × Z
Find the circuit current in amperes
Find the total conductor resistance or impedance in ohms
Multiply current by resistance or impedance
For wire runs, use: Vdrop = 2 × Length × Current × Resistance per unit length
Use the round-trip length for single-phase DC or AC circuits
Use the line-to-line length for three-phase circuits as applicable
Convert wire size and material into resistance per unit length
Calculate percentage voltage drop: (Voltage Drop ÷ Source Voltage) × 100
Compare the result to the allowable voltage drop limit
Reduce wire length, increase wire size, or reduce current if voltage drop is too high