Voltage Drop Calculator

How it works

1-phase: Vdrop = (2 x K x Amps x Length) / cmil. 3-phase: Vdrop = (1.732 x K x Amps x Length) / cmil. K = 12.9 for copper, 21.2 for aluminum (ohm-cmil per foot at 75C); cmil is the conductor's circular-mils area from the AWG size. Percent = Vdrop / source voltage x 100.

Worked example

A 12 AWG copper conductor (6530 cmil) carrying 20 Amps over a 100-foot single-phase 120-Volt run: Vdrop = (2 x 12.9 x 20 x 100) / 6530 = 7.90 Volts, which is 6.58% of 120 Volts, leaving about 112.1 Volts at the load. That is past the 3% branch-circuit rule of thumb, so you would step up a wire size.

When to use it

• A common design target keeps branch-circuit drop under 3% and total (feeder + branch) under 5%.
• Longer runs and higher current increase drop; a larger conductor (more circular mils) reduces it.
• Aluminum has higher resistance than copper (K 21.2 vs 12.9), so an aluminum run drops more voltage at the same size.
• This is a DC-resistance estimate at 75C. It ignores AC reactance, power factor, and temperature derating — verify against NEC Chapter 9 tables for design work.

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