How to use the voltage drop calculator
- Select the wire gauge from the dropdown menu.
- Enter the one-way length of the wire run in feet.
- Type the expected load current in amperes.
- Set the source voltage and choose copper or aluminum.
- Read the voltage drop, percentage loss and voltage at the load below.
How the voltage drop formula works
The voltage drop calculator uses Ohm's law applied to the total round-trip resistance of the wire. For a single-phase or DC circuit the formula is:
V_drop = 2 x L x I x R
Where L is the one-way length in feet, I is the current in amperes and R is the wire resistance per foot in ohms. The factor of 2 accounts for both the supply and return conductors.
For a balanced three-phase system the multiplier changes because the line-to-line geometry reduces the effective path length. The formula becomes V_drop = sqrt(3) x L x I x R, which is roughly 1.732 instead of 2.
As a worked example, consider 100 feet of 12 AWG copper carrying 15 A on a 120 V single-phase circuit. The resistance of 12 AWG copper is 0.001585 ohms per foot. The calculation is 2 x 100 x 15 x 0.001585 = 4.755 V. Dividing by the source voltage gives a 3.96 percent drop, leaving about 115.2 V at the load.
AWG resistance reference table
The table below lists the DC resistance at 20 degrees Celsius for common American Wire Gauge sizes. These values are used internally by the tool for both copper and aluminum calculations.
| AWG size | Copper (ohms/ft) | Aluminum (ohms/ft) |
|---|---|---|
| 18 | 0.006385 | 0.01037 |
| 14 | 0.002525 | 0.004099 |
| 12 | 0.001588 | 0.002579 |
| 10 | 0.0009989 | 0.001622 |
| 8 | 0.0006282 | 0.001020 |
| 6 | 0.0003951 | 0.0006416 |
| 4 | 0.0002485 | 0.0004035 |
| 2 | 0.0001563 | 0.0002538 |
Resistance increases as the gauge number goes up because the cross-sectional area shrinks. Aluminum has roughly 61 percent of the conductivity of copper, so its resistance per foot is about 1.62 times higher for the same gauge.
Acceptable voltage drop limits
The National Electrical Code does not enforce a strict maximum voltage drop for most installations, but it recommends keeping the combined drop under 5 percent for reasonable efficiency. Fine Print Note 2 in NEC 210.19 suggests no more than 3 percent on branch circuits and 5 percent total from service entrance to the farthest outlet.
For sensitive electronics, motors or low-voltage systems like 12 V lighting, even a 3 percent drop can cause noticeable problems. A 12 V system losing 3 percent delivers only 11.64 V, which may be below the minimum operating threshold for some LED drivers or relays. Always check equipment specifications when designing low-voltage runs.
If this voltage drop calculator shows a percentage above your target, increase the wire gauge (use a lower AWG number), shorten the run or split the load across multiple circuits.
Common mistakes when calculating wire voltage drop
A frequent error is entering the total round-trip length instead of the one-way distance. Because the formula already multiplies by 2 for single-phase circuits, doubling the length input will produce a result that is twice the actual cable voltage drop.
Another mistake is confusing aluminum with copper. Using copper resistance values for an aluminum wire underestimates the drop by about 38 percent. Always select the correct conductor material.
Temperature also matters. The resistance values used here assume 20 degrees Celsius (68 degrees Fahrenheit). In hot environments or when wires carry near their ampacity limit, conductor temperature rises and resistance increases. A rule of thumb is that copper resistance grows by about 0.393 percent per degree Celsius above 20 C. For critical applications in high ambient heat, apply a temperature correction factor to the results shown by this tool.
When to use this voltage drop calculator
Electricians and engineers use voltage drop calculations during the design phase to verify that a chosen wire size will deliver adequate voltage to the load. It is especially important for long feeder runs, such as power to a detached garage, well pump or solar array.
Homeowners planning DIY projects like landscape lighting, RV wiring or shed subpanels can use this tool to avoid undersized cables. If you are estimating materials for a build, pair these results with a concrete calculator for conduit trenches or other construction math tools available on Xerobit.
For three-phase industrial panels, switch the system type to three phase. The tool automatically applies the square root of three multiplier so you do not need to adjust the math manually.
Worked examples
Each result below was produced by the same code the tool runs, so you can check your own numbers against them.
| Example | Input | Result |
|---|---|---|
| 120 V branch circuit, 12 AWG copper, 100 ft, 15 A | Wire gauge (AWG): 12; One-way wire length (ft): 100; Current (A): 15; Source voltage (V): 120; Conductor material: cu; System type: 1 | Voltage drop: 4.75 V; Drop percentage: 3.96%; Voltage at load: 115.25 V; Wire resistance: 1.5831 mΩ/ft; Power loss in wire: 71.24 W |
| 240 V feeder, 6 AWG copper, 50 ft, 40 A | Wire gauge (AWG): 6; One-way wire length (ft): 50; Current (A): 40; Source voltage (V): 240; Conductor material: cu; System type: 1 | Voltage drop: 1.59 V; Drop percentage: 0.66%; Voltage at load: 238.41 V; Wire resistance: 0.3977 mΩ/ft; Power loss in wire: 63.63 W |
| 12 V automotive, 18 AWG copper, 10 ft, 5 A | Wire gauge (AWG): 18; One-way wire length (ft): 10; Current (A): 5; Source voltage (V): 12; Conductor material: cu; System type: 1 | Voltage drop: 0.63 V; Drop percentage: 5.25%; Voltage at load: 11.37 V; Wire resistance: 6.3026 mΩ/ft; Power loss in wire: 3.15 W |
| 480 V three-phase, 4 AWG aluminum, 200 ft, 60 A | Wire gauge (AWG): 4; One-way wire length (ft): 200; Current (A): 60; Source voltage (V): 480; Conductor material: al; System type: 3 | Voltage drop: 8.47 V; Drop percentage: 1.76%; Voltage at load: 471.53 V; Wire resistance: 0.4074 mΩ/ft; Power loss in wire: 508.09 W |
FAQ
What is voltage drop?
Voltage drop is the reduction in electrical potential along a conductor due to its resistance. It means the load receives slightly less voltage than the source supplies.
How much voltage drop is acceptable?
The NEC recommends a maximum of 3 percent on branch circuits and 5 percent total from the service to the farthest outlet. Sensitive equipment may require tighter tolerances.
Does this tool handle three-phase power?
Yes. Select "Three phase" in the system type field. The calculator applies the sqrt(3) multiplier instead of 2 for the round-trip path.
Why is aluminum voltage drop higher than copper?
Aluminum has lower electrical conductivity than copper. For the same AWG size, aluminum wire has about 1.62 times the resistance, resulting in a proportionally larger voltage loss.
Should I enter the total wire length or one-way distance?
Enter the one-way distance from source to load. The formula automatically doubles it for single-phase circuits to account for the return path.
Does temperature affect the results?
Yes. The resistance values used here are for 20 degrees Celsius. Higher temperatures increase resistance and therefore increase the actual voltage drop.
Can I use this for DC circuits?
Absolutely. Select "Single phase / DC" as the system type. The math for DC voltage drop is identical to single-phase AC when using resistance rather than impedance.