Use the formula: Voltage Drop = Current × Resistance
For AC or long cable runs, use: Voltage Drop = 2 × Length × Current × Resistance per unit length
Determine the load current in amperes
Determine the conductor resistance from wire size and material
Measure or calculate the one-way cable length
Multiply current by total circuit resistance
For three-phase circuits, use the appropriate three-phase voltage drop formula
Compare the calculated voltage drop to the allowable limit
Verify the result with a multimeter if needed