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Voltage Drop Calculator
Work out how many volts a cable loses between the source and the load. Choose copper or aluminum, an AWG or metric mm² size, the one-way length, current and voltage, and see the drop in volts and percent, the voltage at the load and the smallest wire that stays within your limit.
Conductor
System
V
A
Distance from the source to the load (not the round trip).
°C
75 °C matches NEC Chapter 9 Table 8 values.
%
Enter your values to see the result.
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How to use the Voltage Drop Calculator
- Pick the conductor (copper or aluminum) and the system: single-phase/DC or three-phase.
- Choose the size standard (AWG or mm²) and the wire size.
- Enter the source voltage, load current and one-way length from the panel or battery to the load.
- Press Calculate voltage drop to see the drop in volts and percent and the voltage left at the load.
- Check the table to compare all sizes and find the smallest wire within your maximum drop.
Voltage drop formula
- Single-phase / DC: VD = 2 × K × I × L ÷ CM
- Three-phase (line-to-line): VD = √3 × K × I × L ÷ CM
- Percent drop = VD ÷ source voltage × 100; voltage at load = source − VD
- K = 12.9 Ω·cmil/ft (copper) or 21.2 (aluminum) at 75 °C; L = one-way length in feet; CM = area in circular mils (1 mm² = 1,973.5 cmil)
- AWG area: diameter (mils) = 5 × 92(36 − AWG)/39, CM = diameter²
Example: 12 AWG copper (6,530 cmil), 20 A, 100 ft one way on 120 V: VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V, which is 6.6% — above the 3% guideline, so 8 AWG (16,510 cmil, 3.13 V, 2.6%) would be the choice for voltage drop.
Wire resistance reference (75 °C)
| Size | Area (mm²) | Circular mils | Copper Ω/1000 ft | Aluminum Ω/1000 ft |
|---|---|---|---|---|
| 14 AWG | 2.08 | 4,110 | 3.14 | 5.16 |
| 12 AWG | 3.31 | 6,530 | 1.98 | 3.25 |
| 10 AWG | 5.26 | 10,380 | 1.24 | 2.04 |
| 8 AWG | 8.37 | 16,510 | 0.781 | 1.28 |
| 6 AWG | 13.3 | 26,240 | 0.492 | 0.808 |
| 4 AWG | 21.2 | 41,740 | 0.309 | 0.508 |
| 2 AWG | 33.6 | 66,360 | 0.194 | 0.319 |
| 1/0 AWG | 53.5 | 105,600 | 0.122 | 0.201 |
| 4/0 AWG | 107 | 211,600 | 0.061 | 0.100 |
Values are calculated from K × 1000 ÷ CM and agree closely with NEC Chapter 9 Table 8 for stranded, uncoated conductors.
Tips to reduce voltage drop
- Use a larger conductor: each step of three AWG sizes roughly halves the resistance.
- Shorten the run or move the panel, inverter or battery closer to the load.
- Use a higher system voltage (for example 24 V or 48 V instead of 12 V for solar), which lowers the current for the same power.
- Choose copper instead of aluminum where size or space is limited.
Frequently asked questions
What is the voltage drop formula?
For single-phase and DC circuits: VD = 2 × K × I × L ÷ CM. For balanced three-phase: VD = √3 × K × I × L ÷ CM. K is the conductor resistivity (about 12.9 Ω·cmil/ft for copper and 21.2 for aluminum at 75 °C), I is current in amps, L is one-way length in feet and CM is the conductor area in circular mils.
What is an acceptable voltage drop?
A common guideline, from the informational notes in the US National Electrical Code, is no more than 3% on a branch circuit and 5% in total for feeder plus branch circuit. Many installers use the same targets elsewhere; low-voltage DC systems such as 12 V solar or automotive wiring often aim for 2–3% or less.
Should I enter one-way or round-trip length?
Enter the one-way distance from the source to the load. The formula already doubles it for single-phase and DC circuits (current goes out and back), and uses √3 for three-phase.
Why does a 12 V system need much thicker wire?
Voltage drop in volts depends only on current, length and wire resistance, but the percentage is relative to the supply voltage. The same 1 V drop is under 1% on 120 V but over 8% on 12 V, and low-voltage loads also draw more current for the same power.
Does wire temperature change the voltage drop?
Yes. Copper resistance rises about 0.39% per °C, so a conductor at 75 °C has roughly 22% more resistance than at 20 °C. This calculator uses 75 °C by default, matching NEC Chapter 9 Table 8, and you can change it.
Does this calculator replace wire ampacity sizing?
No. Voltage drop is one check. The wire must also be rated for the current (ampacity), match the breaker or fuse, and meet insulation, installation and local code requirements. Use it as an estimate and follow your electrical code or a licensed electrician.
How is aluminum wire different from copper?
Aluminum has about 61% of the conductivity of copper, so for the same voltage drop it needs roughly two AWG sizes larger. Its K-factor is about 21.2 versus 12.9 for copper.
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