Voltage drop calculator
Volts lost over a run, the smallest conductor that keeps you under a target, and the arithmetic that produced both. Copper or aluminum, single or three phase.
Over a 100 ft run carrying 20 A at 240 V single phase, 10 AWG copper drops 4.96 V, which is 2.07% of supply.
What decided this: the run length and the conductor's own resistance. Nothing else here is binding — the 3% figure people quote is guidance, not a requirement.
Worked as 2 × 100 × 20 × 1.2395 ÷ 1000. The last figure is that conductor's resistance in ohms per 1,000 ft, computed rather than looked up — how, and from what.
The 3% figure, and what it actually is
Almost every chart you will find quotes 3% for a branch circuit and 5% overall. Those numbers come from informational notes in the National Electrical Code, and an informational note is explanatory text. It is not an enforceable requirement, and the code does not set a general voltage-drop limit for ordinary branch circuits. Some jurisdictions, and some provisions for particular equipment, do impose limits of their own.
So the honest reading of a result above 3% is that the run is losing more than the figure most designers aim at, not that it fails anything. What it costs is heat in the conductor and volts at the load.
Common runs at 20 A, 240 V, single phase, copper
| Size | 50 ft | 100 ft | 150 ft | 200 ft | 300 ft |
|---|---|---|---|---|---|
| 12 AWG | 1.64% | 3.28% | 4.93% | 6.57% | 9.85% |
| 10 AWG | 1.03% | 2.07% | 3.10% | 4.13% | 6.20% |
| 8 AWG | 0.65% | 1.30% | 1.95% | 2.60% | 3.90% |
| 6 AWG | 0.41% | 0.82% | 1.23% | 1.63% | 2.45% |
| 4 AWG | 0.26% | 0.51% | 0.77% | 1.03% | 1.54% |
| 2 AWG | 0.16% | 0.32% | 0.48% | 0.65% | 0.97% |
Where the numbers come from
- Conductor area
- Not a table. AWG is a geometric definition, ASTM B258, the American Wire Gauge geometric series: d(n) = 0.005 * 92 ** ((36 - n) / 39) inches; circular mils = (d in mils) ** 2
- Resistivity
- Electrical Tech Note 212, Conductor Properties, Biosystems & Agricultural Engineering Department, Michigan State University, © 2009/24, read 2026-09-03. Used for resistivity K and the temperature coefficient alpha only.
- Arithmetic
- R = K * L / A, Eq. 212.1, A in circular mils. Single phase multiplies the one-way length by two because the current returns; three phase multiplies by √3.
- Checked
- The computation runs against that document's own resistance table at build time and the build stops if it stops matching: n=23 published resistances and n=10 circular-mil areas, all within 1%.
Not affiliated with, endorsed by, or a publication of any standards body. NFPA, NEC and National Electrical Code are marks of the National Fire Protection Association, named here only to identify the document being discussed.