Voltage Drop Calculator
Volts lost, percent drop, and load volts from wire size and run.
What it does
Wire resists current, so long runs arrive at the load with fewer volts than the panel sends. Motors run hot, lights dim, and heaters weaken when the loss grows past a few percent.
The tool above takes source volts, load amps, one way length, wire size, and metal. It returns volts lost with percent drop and the volts left at the load, flagging the 3% branch and 5% combined guidance used across the trade.
The defaults describe 12 A on 100 ft of 12 AWG copper at 120 V: 4.74 V lost, 3.95% drop, 115.3 V at the load. The metric mirror (30.5 m, 3.3 sq mm) gives 4.76 V, 3.96%, and 115.2 V.
The formula with K constants 12.9 for copper and 21.2 for aluminum comes from NEC Chapter 9 Table 8 at 75 C, as printed on NCCER and IAEI formula sheets. Both name the same single phase line with circular mil area in the denominator.
How to use
- Pick imperial (feet, AWG) or metric (meters, sq mm).
- Enter source volts and load amps.
- Pick copper or aluminum.
- Enter one way length, not round trip.
- Read volts lost and percent drop.
- Upsize wire or shorten the run past 5%.
Formula
- K
- 12.9 copper, 21.2 aluminum, ohm cmil per ft at 75 C.
- I
- Load current in amps.
- L
- One way length in feet (meters convert inside).
- CM
- Conductor area in circular mils (12 AWG = 6,530).
Single phase two wire carries the factor 2 for out and back; three phase uses root 3 instead. Metric inputs convert meters to feet and sq mm to circular mils at 1973.52 per sq mm before the same line runs.
Worked example
Example: 120 V, 12 A, 100 ft, 12 AWG copper
Defaults: imperial, 120 V, 12 A, 100 ft one way, 12 AWG, copper.
- K = 12.9, CM = 6,530.
- Top: 2 x 12.9 x 12 x 100 = 30,960.
- Drop: 30,960 / 6,530 = 4.74 V.
- Percent: 4.74 / 120 x 100 = 3.95%.
- Load: 120 - 4.74 = 115.3 V.
- Tool shows 4.74 V, 3.95 %, 115.3 V. Metric mirror gives 4.76 V, 3.96 %, 115.2 V.
Reference table
| AWG | CM | sq mm |
|---|---|---|
| 14 | 4,110 | 2.08 |
| 12 | 6,530 | 3.31 |
| 10 | 10,380 | 5.26 |
| 8 | 16,510 | 8.37 |
| 6 | 26,240 | 13.3 |
Limits
- Single phase two wire only; three phase needs root 3.
- K values assume 75 C operating temperature.
- Reactance and power factor sit outside this line.
- Ampacity and terminal ratings need their own checks.
Common mistakes
- Entering round trip length and doubling the answer.
- Using copper K for aluminum runs.
- Figuring percent on the wrong voltage.
- Ignoring temperature above 75 C.
- Chasing exactly 3% where 5% combined is fine.
Try it yourself
Quick checks. Answers sit tucked under each question.
Same run in aluminum?
7.79 V lost, 6.49%, 112.2 V at load.
240 V, 40 A, 150 ft, 6 AWG copper?
5.9 V lost, 2.46%, 234.1 V at load.
Why not exactly 3%?
3% branch and 5% combined are guidance notes, not strict code.
Questions
What is the voltage drop formula?
Single phase: 2 x K x I x L / CM, with K 12.9 copper or 21.2 aluminum.
One way or round trip length?
One way; the 2 in the formula already covers the return path.
What does the NEC require?
3% branch and 5% combined are recommendations, required only in special cases.
Why does aluminum drop more?
Higher resistivity; it needs larger sizes for equal drop.
How far can 12 AWG go at 20 A?
About 45 ft one way at 120 V before 3%; upsize past that.
Does temperature matter?
Yes; hotter wire resists more, so long hot runs drop extra.