In electrical engineering, voltage drop is a critical factor, especially for long cable runs. Excessive voltage drop can lead to equipment failure, overheating, and inefficient energy use. CEC Rule 8-102 requires, and NEC 210.19(A) Informational Note 4 recommends, a maximum voltage drop of 3% for branch circuits and 5% total for feeder plus branch circuits.
When designing distribution systems for large commercial buildings or industrial plants, simply checking ampacity isn't enough. We must account for the resistance of the conductor over distance. Take a 20 A load 50 m from the panel on a 120 V circuit. Ampacity alone allows 12 AWG copper. The voltage drop calculator on this site returns 6 AWG to stay under 3 percent; 8 AWG lands at 4.27 percent. Two sizes larger than ampacity suggests, and that is a normal result on long runs.
Using the Voltage Drop Calculator on this site can help quickly estimate these requirements, but detailed engineering analysis should always consider factors like harmonics, temperature ratings, and conduit fill.