Calculate voltage drop for AWG or kcmil copper and aluminum conductors. Compare wire sizes at the same load, power factor and distance, or enter a cable manufacturer’s resistance in ohms per 1000 feet.
AWG and kcmil voltage drop calculator
Enter one-way distance. Presets estimate resistance from conductor area; they are not ampacity ratings.
Which voltage-drop tool should I use?
This page works directly with AWG/kcmil presets and custom resistance in Ω/1000 ft. For an area in mm², use the general voltage drop calculator. For a balanced load specified in electrical input kW, or a datasheet in Ω/km, use the three-phase impedance calculator. Check wire ampacity separately.
Formulas used by this calculator
With line current I in amperes, one-way length L in feet, operating resistance R and reactance X in Ω/1000 ft, and lagging power factor PF:
| Circuit | Approximate voltage drop |
|---|---|
| Single-phase, equal outgoing and return conductors | ΔV = 2 × I × L / 1000 × (R × PF + X × √(1 − PF²)) |
| Balanced three-phase, line-to-line | ΔV = √3 × I × L / 1000 × (R × PF + X × √(1 − PF²)) |
Percentage drop is 100 × ΔV / source voltage. These AC equations estimate the longitudinal voltage change for a steady, sinusoidal lagging load, as explained in ELEK’s voltage-drop method. They do not subtract the magnitude I|Z| from the supply voltage for every power factor.
Resistance presets and custom cable data
AWG presets calculate the geometric area from d = 0.127 × 92(36 − AWG)/39 millimetres and A = πd²/4. For 1/0 through 4/0, the AWG values are 0, −1, −2 and −3. One kcmil is approximately 0.506707479 mm². Use the mm² and AWG converter to compare areas.
The preset model uses R20 = 17.24 × 0.3048 / A for copper and 28.26 × 0.3048 / A for aluminum, giving Ω/1000 ft. These are ideal material estimates. They do not include a particular cable’s stranding, alloy, skin effect, proximity effect or installation geometry. For large AC conductors, actual cable data can materially change the result.
Custom R20 replaces the preset resistance completely. Enter the resistance of one conductor of the selected material at 20°C. Selecting aluminum changes the temperature coefficient; it does not multiply custom resistance by a second material factor. Do not enter a 75°C resistance as R20 and then correct it again.
Temperature correction is R(T) = R20 × [1 + α × (T − 20)], using approximate α = 0.00393/°C for copper and 0.00403/°C for aluminum. The advanced X/R input refers to the entered operating temperature: X = R(T) × X/R. Supply a ratio appropriate to the cable arrangement and frequency; zero is an explicit resistance-only assumption.
Worked examples you can reproduce
These examples use custom resistance so the inputs can be checked without a conductor-table assumption. They demonstrate arithmetic, not approved cable selections.
| Inputs | Result |
|---|---|
| Single-phase, 230 V, 20 A, 100 ft, custom R20 0.5 Ω/1000 ft, Cu, 20°C, PF 1, X/R 0 | 2 V; 0.87%; approximate load voltage 228 V |
| Same circuit with PF 0.8 and operating X/R 0.2 | 2 × 20 × 100 / 1000 × (0.5 × 0.8 + 0.1 × 0.6) = 1.84 V; 0.80% |
| Balanced three-phase, 480 V, 100 A, 200 ft, custom R20 0.1 Ω/1000 ft, Cu, 20°C, PF 0.8, X/R 0.6 | 4.01836 V; 0.84% |
| First example with conductor at 75°C | R = 0.608075 Ω/1000 ft; drop 2.4323 V; 1.06% |
| First example with aluminum custom R20 still 0.5 Ω/1000 ft at 20°C | 2 V; no additional material multiplier |
| First example entered as 30.48 metres | Same 100 ft route; 2 V |
Using the wire-size comparison
The table changes with your inputs and compares 17 AWG/kcmil sizes. It shows each estimated area and reference resistance alongside the calculated drop. It also solves the same equation for the maximum one-way distance at your chosen target. At zero current there is no finite voltage-drop distance limit in this model, so the table shows a dash.
For the first worked example, a 3% target on 230 V permits 6.9 V of drop. The voltage-drop distance is 345 ft. That distance says nothing about conductor heating or a protective device. Compare it with an independent ampacity check and the equipment’s permitted voltage range.
Frequently asked questions
Do I enter the round-trip cable length?
No. Enter the distance from source to load once. The single-phase equation already includes the outgoing and return conductors; the balanced three-phase equation uses √3.
Does “within target” mean the installation is compliant?
It means only that the calculated drop is no greater than your chosen percentage. Conductor ampacity, terminal ratings, overcurrent protection, grounding and equipment requirements need separate assessment. The default 3% is editable.
Can I use this for motor starting, harmonics or unbalanced loads?
The model assumes a balanced steady load, equal conductor paths and sinusoidal current. Starting behavior, source impedance, harmonics, neutral current and unequal phase loads can require a fuller circuit model. A large calculated drop is also a reason to use a more complete analysis.
Continue with all voltage-drop tools, three-phase current or the cable calculation directory.