How the Voltage Drop Calculator Works
Wire is not a perfect conductor. It has a little resistance, and when current flows through it, some voltage is lost along the way. This loss is called voltage drop. If it is too large, lights dim, motors run hot and struggle, and electronics can fail. The Voltage Drop Calculator finds the drop for a wire of a given size, material and length, and tells you the smallest wire that keeps it within your limit.
What you enter
- Current: the amps that flow in the wire.
- Wire material and size: copper or aluminum, and the AWG size, from 14 AWG (thin) to 4/0 (very thick). A smaller AWG number means a thicker wire.
- One-way length: the distance from the power source to the load, in feet or meters. Do not double it, as the calculator does this for you.
- Type of circuit: DC or single-phase AC, which use two wires, or three-phase AC.
- Supply voltage: for example 12, 24, 120, 240 or 480 volts.
- Highest drop you accept: as a percent. Many guides suggest 3% for a branch circuit, and 5% for the whole run from the panel to the load.
How the results are worked out
First the resistance of the wire. It depends on the material and on the size of the conductor. The area of an AWG wire follows a fixed rule, and the resistance is the resistivity of the metal divided by that area. It rises when the wire gets hot, so the calculator uses the resistance at 75 °C, a common working temperature for building wire.
resistance of the wire = resistivity / area, corrected for 75 °C voltage drop = factor x current x resistance per foot x one-way length percent drop = voltage drop / supply voltage x 100 power lost = current x current x resistance of the loop
The factor is 2 for two-wire circuits, because current goes out along one wire and comes back along the other, so the loop is twice the one-way length. For three-phase circuits the factor is the square root of 3, which is 1.732.
If you already know the total resistance, you can use the simple form of Ohm's law: voltage drop = current x resistance.
The resistance of common wires
Ohms per 1,000 feet, at 75 °C:
| Wire size | Copper | Aluminum |
|---|---|---|
| 12 AWG | 1.932 | 3.180 |
| 10 AWG | 1.215 | 2.000 |
| 8 AWG | 0.764 | 1.258 |
| 6 AWG | 0.480 | 0.791 |
| 4 AWG | 0.302 | 0.498 |
| 2 AWG | 0.190 | 0.313 |
| 1/0 AWG | 0.120 | 0.197 |
| 4/0 AWG | 0.060 | 0.098 |
These are worked out from the size and the metal, so they can differ by a few percent from the tables in wiring codes, which are for stranded wire.
A worked example
A 120 V circuit carries 15 A. It uses copper wire of 12 AWG, and the run is 100 feet one way.
| Step | Result |
|---|---|
| Resistance per foot (1.932 / 1,000) | 0.001932 ohms |
| Voltage drop (2 x 15 x 0.001932 x 100) | 5.79 V |
| Percent drop (5.79 / 120) | 4.83% |
| Voltage at the load (120 - 5.79) | 114.21 V |
| Power lost in the wire | 86.9 W |
The drop is 4.83%, above the 3% that is often suggested. The wire wastes about 87 watts as heat.
A thicker wire fixes it
The same circuit, with different wire sizes:
| Wire | Voltage drop | Percent drop | Within 3%? |
|---|---|---|---|
| 14 AWG | 9.21 V | 7.68% | No |
| 12 AWG | 5.79 V | 4.83% | No |
| 10 AWG | 3.64 V | 3.04% | No, just over |
| 8 AWG | 2.29 V | 1.91% | Yes |
| 6 AWG | 1.44 V | 1.20% | Yes |
| 4 AWG | 0.91 V | 0.76% | Yes |
Chart: the percent drop by wire size
Red bars are over 3%, and green bars are within it. Each block is about 0.19%, so 3% is about 16 blocks.
14 AWG ████████████████████████████████████████ 7.68% 12 AWG █████████████████████████ 4.83% 10 AWG ████████████████ 3.04% 8 AWG ██████████ 1.91% 6 AWG ██████ 1.20% 4 AWG ████ 0.76%
Length matters
The drop is proportional to the length. The same 12 AWG copper wire, 15 A and 120 V:
| One-way length | Voltage drop | Percent drop |
|---|---|---|
| 25 ft | 1.45 V | 1.21% |
| 50 ft | 2.90 V | 2.41% |
| 100 ft | 5.79 V | 4.83% |
| 150 ft | 8.69 V | 7.24% |
| 200 ft | 11.59 V | 9.66% |
25 ft █████ 1.21% 50 ft ██████████ 2.41% 100 ft ████████████████████ 4.83% 150 ft ██████████████████████████████ 7.24% 200 ft ████████████████████████████████████████ 9.66%
Low voltage is much more sensitive
A drop of 1.5 V is small on a 120 V circuit, but large on a 12 V one. Take a 12 V DC circuit with 10 A and 25 feet one way, for example for lights or a pump in a vehicle or a boat:
| Wire | Voltage drop | Percent drop |
|---|---|---|
| 14 AWG | 1.54 V | 12.80% |
| 12 AWG | 0.97 V | 8.05% |
| 10 AWG | 0.61 V | 5.06% |
| 8 AWG | 0.38 V | 3.18% |
14 AWG ████████████████████████████████████████ 12.80% 12 AWG █████████████████████████ 8.05% 10 AWG ████████████████ 5.06% 8 AWG ██████████ 3.18%
This is why 12 V and 24 V systems use thick wires, and why a higher voltage is used for long runs. The same 100 ft run of 12 AWG at 15 A drops 4.83% at 120 V, but only 2.41% at 240 V.
Copper and aluminum
Aluminum has about 1.64 times the resistance of copper for the same size. On the 120 V example with 8 AWG, copper drops 2.29 V (1.91%), and aluminum drops 3.77 V (3.14%). To match copper, aluminum wire is usually made one or two sizes thicker.
Three-phase circuits
A three-phase circuit shares the load between three wires, so the drop is 1.732 times the current, times the resistance, times the one-way length. For example, at 480 V, with 50 A on 6 AWG copper for 200 feet, the drop is 8.32 V, which is 1.73%.
Tips
- Measure the one-way length, and let the calculator double it. A common mistake is to use the round trip.
- Size the wire for both things. It must be thick enough to carry the current safely, and thick enough to keep the drop low. The larger of the two sizes wins.
- Keep sensitive loads short. Motors, pumps, LED lighting and electronics need a steady voltage.
- Use a higher voltage for a long run. Doubling the voltage halves the current for the same power, and cuts the percent drop by a factor of four.
- Check the connections. Loose or dirty joints add resistance that is not counted here.
What this calculator does not include
- The reactance of the wire in AC circuits. It matters for large wires, and for loads with a low power factor. The results are for resistive loads.
- Motor starting current, which can be several times the running current.
- The current rating of the wire, which the wiring code sets by the size, the insulation and the way the wire is run.
- Temperatures other than 75 °C, wire in a bundle, and special cable.
This tool gives an estimate for general information only. It is not an electrical design, and it does not check that a circuit meets the wiring code. Have a qualified electrician confirm the wire size, and follow your local rules.