Voltage Drop Calculator
Work out how many volts a copper wire run loses for its gauge, length and current, for DC, single-phase or three-phase circuits.
Voltage drop is the current times the resistance of the wire along its path, out and back for DC and single-phase. A 100 ft run of 12 AWG copper carrying 15 A on a 120 V circuit drops 5.80 V, or 4.83%, leaving 114.20 V at the load, with the wire at 75 °C. For DC and single-phase, multiply the one-way length by 2; for three-phase, by 1.732.
This calculator is an estimate for planning and is not a substitute for your electrical code or a licensed electrician. It covers copper conductors only, uses resistance alone without reactance, and does not check whether a wire size is rated for the current.
Voltage drop
5.80 V (4.83%)
- Voltage at the load
- 114.20 V
- Wire resistance at this temperature
- 1.9337 ohms per 1,000 ft
The same run in other sizes
| 4/0 AWG | 0.18 V | 0.15% |
| 3/0 AWG | 0.23 V | 0.19% |
| 2/0 AWG | 0.28 V | 0.24% |
| 1/0 AWG | 0.36 V | 0.30% |
| 1 AWG | 0.45 V | 0.38% |
| 2 AWG | 0.57 V | 0.48% |
| 3 AWG | 0.72 V | 0.60% |
| 4 AWG | 0.91 V | 0.76% |
| 6 AWG | 1.44 V | 1.20% |
| 8 AWG | 2.29 V | 1.91% |
| 10 AWG | 3.64 V | 3.04% |
| 12 AWG | 5.80 V | 4.83% |
| 14 AWG | 9.19 V | 7.66% |
| 16 AWG | 14.67 V | 12.22% |
| 18 AWG | 23.31 V | 19.43% |
Copper, resistance only (no reactance), from the NBS copper wire tables. Check the size against your electrical code and the conductor's ampacity.
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About the Voltage Drop Calculator
Every wire has some resistance, so some of the supply voltage is used up pushing current along it, and less reaches the load. The drop is the current times the resistance of the conductors the current passes through. In a DC or single-phase circuit the current goes out on one wire and back on another, so the one-way length counts twice; in a balanced three-phase circuit the factor is the square root of 3, about 1.732. The resistances here are the National Bureau of Standards copper wire tables of 1966, for standard annealed copper at 20 °C: 12 AWG, for example, is 1.59 ohms per 1,000 ft. Copper's resistance rises as it warms, by 0.00393, about 0.39%, of its 20 °C value for each degree, so a loaded wire at 75 °C has about a fifth more resistance and a fifth more drop: the default run drops 4.77 V at 20 °C and 5.80 V at 75 °C. The table under the result shows the same run in every size, which helps when comparing gauges.
Frequently asked questions
What is the voltage drop formula?+
For DC and single-phase: drop = 2 x one-way length x current x resistance per foot. For balanced three-phase, line to line: drop = 1.732 x length x current x resistance per foot. With 12 AWG at 75 °C, 100 ft and 15 A, that is 2 x 100 x 15 x 0.00193 ohms per foot, or 5.80 V.
Why does a 12 V system lose so much more?+
The drop in volts depends on the wire and the current, not the supply, so the same drop is a much bigger share of a low voltage. 20 ft of 14 AWG carrying 10 A at 12 V DC loses 1.23 V, which is 10.22% of the supply.
Does a higher voltage reduce voltage drop?+
The drop in volts follows the current, and the same power at a higher voltage draws less current. Over 100 ft of 12 AWG at 75 °C, 2,400 W drops 7.73 V (6.45%) at 120 V and 20 A, but 3.87 V (1.61%) at 240 V and 10 A. A 480 V three-phase run of 2 AWG carrying 100 A for 200 ft drops 6.58 V, or 1.37%.
Does this work for aluminum wire?+
No. The figures are for copper only, and aluminum needs its own resistance figures, so do not use these for aluminum conductors.
Resistance per 1,000 feet of standard annealed copper by AWG size at 20 °C, the resistivity of 10.371 ohm-circular mil per foot and the temperature coefficient of 0.00393: U.S. National Bureau of Standards, Handbook 100, Copper Wire Tables (1966), Table 5.
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