Voltage Drop Calculator
Estimate the voltage drop over a wire run with the circular-mils method and check it against the NEC-recommended 3% target — a planning estimate to confirm with a licensed electrician.
= 7.9 V = 6.58%
Voltage drop is the voltage a conductor loses to its own resistance on the way to the load. Over a long run it can leave motors humming, lights dim, and electronics unhappy, so it is worth checking before you pull wire. This calculator uses the circular-mils method, the same one in most electrical references: `VD = 2 × K × I × L ÷ CM`. Here `K` is the conductor constant — `12.9` for copper and `21.2` for aluminum — `I` is the load current, `L` is the one-way run length in feet, and `CM` is the wire's circular-mil area from NEC Chapter 9, Table 8. The results are planning estimates; confirm any final design with a licensed electrician.
The leading `2` accounts for the round trip: current flows out on one conductor and back on the other, so the electrons travel twice the run length. For a three-phase circuit the calculator swaps that `2` for `√3` (about `1.732`), the standard line-to-line factor. Enter your amperage, one-way length, gauge, material, and system voltage, and it returns the volts lost, the percent of source voltage, and the voltage that actually reaches the load.
The number to watch is the percentage. The NEC does not mandate a limit, but the Informational Notes to 210.19(A) and 215.2(A) recommend keeping a branch circuit or feeder at or below `3%`, and total drop at or below `5%`. If your run comes in over `3%`, the usual fix is to step up one wire size, which increases `CM` and cuts the drop.
VD = `2` × `12.9` × `20 A` × `100 ft` ÷ `6530` = `7.90 V`
percent = `7.90 V` ÷ `120 V` = `6.59%` (over the `3%` target)
end voltage = `120 V` − `7.90 V` = `112.1 V` — step up to `10 AWG` to fix it