Estimate the voltage drop of a copper or aluminum wire run from its size, length, load current and phase — in volts and percent.
Estimate the voltage lost along a copper or aluminum wire run. Choose a wire size, or enter your own resistance data.
| Result | Value |
|---|
Single-phase or DC: voltage drop = 2 × current × wire resistance per length × one-way length. Three-phase: √3 × current × resistance × length. Example: 12 AWG copper, 100 ft, 20 A, 120 V → about 6.35 V (5.3%).
The Voltage Drop Calculator estimates how much voltage is lost along a cable run between the supply and the load. Enter the wire material and size (or your own resistance data), the source voltage, phase, load current and one-way distance, and it returns the voltage drop in volts and as a percentage, the voltage left at the load, and the power wasted as heat in the wire.
Excessive voltage drop makes lights dim or flicker, heaters under-perform and motors run hot. It matters most on long runs, such as feeding a workshop, borewell pump, outbuilding or solar array, and on low-voltage (12 V and 24 V DC) systems where a small loss is a large percentage. A common design guideline is to keep drop under about 3% on a branch circuit and about 5% overall, but always follow the standard that applies to your installation.
Important: this tool estimates resistive drop only. It does not check the wire's current-carrying capacity (ampacity), insulation, grouping or protective devices, and it ignores reactance. Have a qualified electrician confirm the final design.
| Circuit | Voltage drop | Drop % |
|---|---|---|
| Single phase 230 V, 20 A, 30 m, copper 14 AWG (≈2.1 mm²) | 9.94 V | 4.32% |
| Single phase 230 V, 20 A, 30 m, copper 10 AWG | 3.93 V | 1.71% |
| Single phase 230 V, 20 A, 30 m, aluminum 10 AWG | 6.43 V | 2.80% |
| 3-phase 415 V, 30 A, 200 m, copper 4 AWG | 8.47 V | 2.04% |
| 3-phase 415 V, 30 A, 200 m, copper 1 AWG | 4.22 V | 1.02% |
| DC 12 V, 10 A, 5 m, copper 12 AWG | 0.52 V | 4.34% |
Estimated resistance of common wire sizes at 20 °C. Values for copper match published AWG tables to within about 0.1%.
| Wire size | Diameter (mm) | Area (mm²) | Copper (Ω/km) | Aluminum (Ω/km) |
|---|---|---|---|---|
| 14 AWG | 1.63 | 2.08 | 8.285 | 13.552 |
| 12 AWG | 2.05 | 3.31 | 5.210 | 8.523 |
| 10 AWG | 2.59 | 5.26 | 3.277 | 5.360 |
| 8 AWG | 3.26 | 8.37 | 2.061 | 3.371 |
| 6 AWG | 4.12 | 13.30 | 1.296 | 2.120 |
| 4 AWG | 5.19 | 21.15 | 0.815 | 1.333 |
| 3 AWG | 5.83 | 26.67 | 0.646 | 1.057 |
| 2 AWG | 6.54 | 33.63 | 0.513 | 0.839 |
| 1 AWG | 7.35 | 42.41 | 0.407 | 0.665 |
| 1/0 AWG | 8.25 | 53.48 | 0.322 | 0.527 |
| 2/0 AWG | 9.27 | 67.43 | 0.256 | 0.418 |
| 3/0 AWG | 10.40 | 85.03 | 0.203 | 0.332 |
| 4/0 AWG | 11.68 | 107.22 | 0.161 | 0.263 |
| 250 kcmil | 12.70 | 126.68 | 0.136 | 0.223 |
| 500 kcmil | 17.96 | 253.35 | 0.068 | 0.111 |
| 1000 kcmil | 25.40 | 506.71 | 0.034 | 0.056 |
| Circuit part | Commonly cited limit | Why it matters |
|---|---|---|
| Branch circuit (final circuit) | About 3% | Keeps lights, appliances and motors performing normally |
| Feeder + branch combined | About 5% | Overall limit from supply to the load |
| Low-voltage DC (12 V / 24 V) | About 2–3% | A 0.5 V loss is over 4% on a 12 V system |
Guidelines vary by country and application. Check the wiring regulations or standard that applies to your installation.
Voltage drop is the loss of electrical potential along a conductor because the wire has resistance. The voltage at the load ends up lower than the supply voltage by V = I × R.
For single-phase or DC: 2 × current × resistance per length × one-way length. For three-phase: √3 × current × resistance per length × one-way length. This calculator does the maths and also gives the percentage.
A common guideline is up to about 3% on a branch circuit and about 5% for the whole installation. Sensitive equipment and low-voltage DC systems often need less. Follow the standard that applies to you.
Current has to travel to the load and back, so the resistance that matters is that of two conductors of the one-way length.
Copper. For the same size and length, aluminum's resistance is about 64% higher, so it needs a larger cross-section to match copper.
Yes. Resistance rises with temperature — about 0.39% per °C for copper. A conductor at 75 °C has roughly 22% more resistance than at 20 °C. Enter the expected operating temperature for a conservative estimate.
Use a larger wire, shorten the run, use copper, raise the supply voltage or use parallel conductors.
No. It estimates voltage drop only. The wire must also have enough current-carrying capacity (ampacity) for the load and conditions, and be properly protected — consult a qualified electrician.
No. This tool uses resistance only, which is accurate for DC and for most small AC circuits. For large AC conductors, power factor and reactance can change the result.
American Wire Gauge is a standard for the diameter of round electrical wire. A smaller AWG number means a thicker wire; every 6 gauge steps doubles the diameter.
Large cables are often built from several smaller conductors run side by side per phase. The current is shared between them, so the drop is divided by their number.
