Calculate voltage drop in volts and percent for DC, single-phase and balanced three-phase circuits. Enter cable area, current, one-way length and conductor temperature. The result is an engineering estimate under the assumptions below.
Calculate voltage drop
Use a decimal point or comma. A single separator is treated as decimal: write 1000 for one thousand.
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Cable planning: choose your next check
Use ampacity and voltage drop as separate checks. Choose the tool for the information you have; review each model before using its result.
How to calculate voltage drop
- Select DC, single-phase AC or balanced three-phase AC. For three-phase, enter line-to-line voltage and line current.
- Enter the distance from the source to the load once. Choose metres or feet; do not add the return length.
- Select copper or aluminum and the area of one conductor. AWG entries are approximate geometric areas, not current ratings.
- Enter the conductor’s operating temperature. Ambient air temperature alone does not determine it.
- For AC, enter a lagging power factor and reactance for the cable arrangement and frequency. Zero reactance explicitly requests a resistance-only approximation.
- Set your project voltage-drop target and calculate. Read both the percentage and the assumptions.
Voltage drop formulas
With current I in amperes, one-way length L in kilometres, and per-conductor resistance R and reactance X in Ω/km:
| Circuit | Approximate voltage drop ΔV |
|---|---|
| DC, two conductors | 2 × I × L × R |
| Single-phase AC, two conductors | 2 × I × L × (R × PF + X × √(1 − PF²)) |
| Balanced three-phase AC | √3 × I × L × (R × PF + X × √(1 − PF²)) |
Percentage drop = 100 × ΔV / source voltage. The AC expressions estimate the longitudinal voltage change for a sinusoidal, lagging load. They are not the magnitude of the complete complex impedance drop. The DC and single-phase factors assume equal outgoing and return conductors. See the ELEK voltage-drop calculation guide for the AC model, and TxDOT’s explanation of one-way distance and loop resistance for the return-path distinction.
Resistance and temperature assumptions
This tool estimates resistance at 20°C as R20 = 17.24 / A for copper or 28.26 / A for aluminum, with A in mm² and R20 in Ω/km. It then uses R(T) = R20 × [1 + α × (T − 20)], with approximate coefficients α = 0.00393/°C and 0.00403/°C respectively. These ideal material estimates do not certify a particular cable; stranding, conductor class, alloy and AC effects can change the actual resistance.
The default X = 0.08 Ω/km is an illustrative input, not a value verified for your installation. Use cable manufacturer impedance data where available. For a three-phase calculation with your own R20, use the three-phase cable impedance calculator. Its copper reference presets can differ from the ideal-area estimates here.
Worked examples
These are arithmetic examples using the stated model; the conductor sizes are not installation recommendations.
| Inputs | Calculation | Result |
|---|---|---|
| 24 V DC; 10 A; 10 m one-way; 10 mm² Cu; 20°C | 2 × 10 × 0.01 × 1.724 | 0.3448 V; 1.44%; load voltage ≈ 23.66 V |
| 230 V single-phase; 20 A; 50 m; 10 mm² Cu; 20°C; PF 1; X 0 | 2 × 20 × 0.05 × 1.724 | 3.448 V; 1.50%; load voltage ≈ 226.55 V |
| Same single-phase inputs, conductor at 75°C | 3.448 × [1 + 0.00393 × 55] | 4.1932852 V; 1.82% |
How to interpret the result
“Within target” refers only to the percentage you entered. The default 3% is a comparison setting, not a universal code limit. Check the equipment voltage range and the requirements applicable to your circuit. A small voltage drop does not establish ampacity, breaker suitability, terminal temperature rating, grounding or short-circuit protection.
For the same current and cable properties, twice the distance gives twice the calculated drop. Increasing conductor area reduces the resistive component; it does not automatically remove a reactive component. If the tool reports a large drop, fixed current and the AC approximation may no longer describe the actual operating point adequately.
Frequently asked questions
Should I double the cable length?
No. Enter source-to-load distance. This calculator applies the two-conductor factor or the balanced three-phase factor itself.
Why does the power factor affect the result?
When current is already known, the AC expression projects resistance and reactance using PF and √(1 − PF²). It does not divide the entire drop by PF. Calculating current from a known electrical kW input is a separate step.
Can I use a motor’s shaft kW?
First convert shaft output to electrical input using efficiency, or enter measured line current. The dedicated three-phase calculator explains the kW input convention.
Related: kW to amps · mm² to AWG.