A conductor is a resistor. Current through it eats volts before the load ever sees them. This calculator uses the circular-mil K-method for copper or aluminum, on single-phase, DC, or three-phase. Enter load amps, one-way feet, and AWG. You get volts lost and percent of source voltage. It does not pick a wire size.
A conductor is a resistor.
Use it when running a circuit to a detached shop, a long run to a welder or compressor, or any branch long enough that the far end may see low voltage. Enter the circuit type, conductor metal and size, the one-way length and the load current; the page gives the drop in volts and as a percent. A common design target is about 3% on a branch circuit.
Circular mils measure the cross-section of a round conductor. One circular mil is the area of a circle 0.001 in across. Resistance of a given metal is then K × length / CM. Voltage drop is current times that resistance. On a two-wire run the current goes out and comes back, so length is counted twice. On a balanced three-phase run the three line currents sit 120° apart and the factor is √3 instead of 2. L is one-way feet in both formulas.
Single-phase / DC: VD = (2 × K × I × L) / CM
Three-phase: VD = (√3 × K × I × L) / CM
Percent: % = (VD / V) × 100
KCu = 12.9 · KAl = 21.2
√3 used here = 1.7320508
I in amps, L in one-way feet, CM from the AWG table, V = source volts.
Those K numbers are the usual 75 °C ohm-circular-mil-per-foot constants for uncoated copper and for aluminum. They already bake in the metal and that temperature. This page does not apply a further temperature factor, skin effect, or reactance. For 14–10 AWG branch work, resistance dominates. On large feeders in steel conduit, AC impedance from Chapter 9 Table 9 is a better model than this K-method — this calculator will understate drop there.
NEC 210.19(A) and 215.2 carry informational notes that mention 3% drop on a branch (or on a feeder) and 5% on the feeder and branch combined, at the farthest outlet. Informational notes are not enforceable Code. This page flags those two percentages so you can see the arithmetic against the note. It does not inspect the installation, and it does not decide whether a run “passes.”
| Case | Arithmetic | VD / % |
|---|---|---|
| 20 A, 100 ft, 12 AWG Cu, 120 V 1φ | (2×12.9×20×100) / 6530 | 7.90 V · 6.58% |
| 20 A, 100 ft, 10 AWG Cu, 120 V 1φ | (2×12.9×20×100) / 10380 | 4.97 V · 4.14% |
| 20 A, 100 ft, 8 AWG Cu, 120 V 1φ | (2×12.9×20×100) / 16510 | 3.13 V · 2.60% |
| 20 A, 100 ft, 12 AWG Cu, 240 V 1φ | same 7.90 V over 240 | 7.90 V · 3.29% |
| 20 A, 100 ft, 12 AWG Cu, 208 V 3φ | (1.7320508×12.9×20×100) / 6530 | 6.84 V · 3.29% |
| 20 A, 100 ft, 12 AWG Al, 120 V 1φ | (2×21.2×20×100) / 6530 | 12.99 V · 10.82% |
Those six rows use the same math as the calculator (12 AWG = 6530 CM, KCu = 12.9, KAl = 21.2, √3 = 1.7320508). The default example is the first row: a 20 A, 100 ft, 12 AWG copper, 120 V single-phase run drops 7.90 V (6.58%). That is over both note figures. Same amps and feet at 240 V is the same 7.90 V but half the percent. Stepping to 10 AWG copper on the 120 V calculation lands at 4.14% — over the 3% branch note, under the 5% combined note. 8 AWG copper on that same 20 A / 100 ft / 120 V calculation is 2.60%, under both notes. None of those comparisons is a Code verdict.
The denominator is circular mils, not “wire size” as a recommendation. These areas match the usual NEC Chapter 9 Table 8 uncoated-copper column for the sizes this calculator offers. 12 AWG is 6530 CM — the sanity-check value. kcmil sizes are the kcmil number times 1,000.
| Size | CM | Size | CM |
|---|---|---|---|
| 14 AWG | 4,110 | 1 AWG | 83,690 |
| 12 AWG | 6,530 | 1/0 | 105,600 |
| 10 AWG | 10,380 | 2/0 | 133,100 |
| 8 AWG | 16,510 | 3/0 | 167,800 |
| 6 AWG | 26,240 | 4/0 | 211,600 |
| 4 AWG | 41,740 | 250 kcmil | 250,000 |
| 3 AWG | 52,620 | 350 kcmil | 350,000 |
| 2 AWG | 66,360 | 500 kcmil | 500,000 |
Percent drop is (drop volts ÷ source volts) × 100. Many shop rules of thumb aim near 3% for branch circuits and 5% for feeder-plus-branch combined — those are planning targets, not a substitute for the adopted electrical code or a licensed electrician.
One-way length vs round-trip: this calculator follows the formula printed on the page. If your inputs are one-way run length, confirm the formula’s length basis before you upsize wire. Copper and aluminum have different resistivities; pick the conductor material that matches the cable you will pull.
Units: Amps, volts, feet (or meters if the page mode says so), and AWG or mm² as labeled. Do not mix AWG tables with metric cross-sections without converting.
Long runs to outbuildings, welders, and well pumps are common reasons to check drop before buying cable. The calculator does not size breakers, conduit fill, or temperature derates.
One-way. Tape from the source to the load, in feet. The formula already puts a 2 in the numerator for single-phase and DC (out and back on the same size conductors). Three-phase puts √3 there instead. If you double the length yourself, the drop comes out twice as large as the K-method intends.
Because the arithmetic is 7.90 V, which is 6.58% of 120. That is the default example on this page, computed as (2 × 12.9 × 20 × 100) / 6530. A 20 A, 120 V branch that long is a classic “it works on ampacity, it sags on volts” case. Raising the source to 240 V does not change the 7.90 V; it only halves the percent. This page still does not tell you which size to pull.
No. They appear as informational notes under NEC 210.19(A) (branch circuits) and 215.2 (feeders): 3% at the farthest outlet of the branch, or of the feeder, and 5% on the feeder and branch together. Informational notes are not enforceable. Some jobs, some manufacturers, and some inspectors still want the number. This calculator only compares your percent to those two figures and labels them as notes. It does not enforce anything.
Two-wire DC and single-phase AC share VD = (2 × K × I × L) / CM. Balanced three-phase uses VD = (√3 × K × I × L) / CM with √3 = 1.7320508, and the voltage in the percent is line-to-line. A 208 Y / 120 V wye feeder is 208 V on the three-phase calculation, not 120. A 120 V line-to-neutral load on that same system is a single-phase 120 V calculation.
No. You bring the size; the page returns the drop. Ampacity, termination temperature, grouping, ambient, continuous-load multipliers, and the overcurrent device are a different problem. A conductor can be legal on ampacity and still drop more volts than you want, or the other way around. This is not a wire-size schedule and not a substitute for Chapter 3 or Annex C.
K is the resistance of one circular mil of that metal, one foot long, at the temperature baked into the constant. This page uses 12.9 for copper and 21.2 for aluminum. Aluminum’s higher K is why the same 12 AWG, 20 A, 100 ft, 120 V calculation jumps from 7.90 V (copper) to 12.99 V (aluminum). Compact, coated, or odd-strand conductors can differ slightly from Table 8; this calculator uses the table CM for the size you picked.
No. K-method is DC resistance scaled to 75 °C. It ignores inductive reactance. Fine for small copper branches. Weak for 4/0 and up, long 60 Hz feeders, and anything in magnetic conduit. Chapter 9 Table 9 is the impedance model for those runs. If you need Table 9, this is the wrong page.