Voltage Drop Calculator
Check whether a circuit run will hold acceptable voltage before you pull wire — enter gauge, distance, and load.
This calculator gives an estimate for planning purposes only. Verify quantities and code requirements with a qualified professional before ordering materials or performing work.
How it's calculated
Voltage drop (single-phase)
VD = (2 × K × I × D) / CM
K is the resistivity constant for the conductor material (12.9 for copper, 21.2 for aluminum), I is load current in amps, D is one-way distance in feet, and CM is the wire's circular mil area. The factor of 2 accounts for the round-trip path.
Voltage drop (three-phase)
VD = (√3 × K × I × D) / CM
Three-phase circuits use √3 (≈1.732) instead of 2, since the return path differs from single-phase.
Percent drop
% drop = (VD / source voltage) × 100
Most designers target 3% or less on a branch circuit and 5% or less total (feeder + branch) as common guidance — not a hard code requirement in every jurisdiction, but a widely used rule of thumb.
Max one-way distance for ≤3% drop (120V, 20A, copper, single-phase)
| Wire gauge | Max distance |
|---|---|
| 14 AWG | 29 ft |
| 12 AWG | 46 ft |
| 10 AWG | 72 ft |
| 8 AWG | 115 ft |
| 6 AWG | 183 ft |
Skip the manual math on your next takeoff
Struvia reads your plans and pulls quantities like this automatically — upload a plan and see it measure a real takeoff.
Voltage Drop Calculator FAQ
What voltage drop percentage is acceptable?
A common rule of thumb (from NEC informational notes, not a strict requirement everywhere) targets 3% or less for a branch circuit and 5% or less combined feeder + branch. Check your local code and engineering requirements for the specific project.
Why does wire gauge matter for voltage drop?
A larger wire (lower AWG number) has more circular mils of copper or aluminum, which lowers resistance and reduces voltage drop over the same distance and current.
Does distance mean one-way or round-trip?
Enter the one-way distance from source to load — the formula already accounts for the return path via the multiplier (2 for single-phase, √3 for three-phase).