Calculate voltage drop and find a cable length or cross-section that meets your selected target under the stated assumptions.

Voltage drop, maximum cable length and minimum area

Enter the one-way route length. Use a decimal point or comma; a single separator is decimal, so write 1000 for one thousand. Power means electrical input kW.

Known value: Load current (A): System: Source voltage: Power factor (PF): One-way length:

One-way distance. The calculator applies the correct path factor automatically.

Length unit: Conductor material: Conductor temperature (°C): Preset size (AWG/kcmil & mm²): Cross-section (mm²): Reactance model (Ω/km): Target voltage drop (%):

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Compare cable scenarios

Calculate an option, save it, then change the inputs and save another. Compare up to three snapshots with their assumptions.

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Plan one circuit across ampacity and voltage drop — enter shared inputs once and open both checks with those values.

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.

Voltage dropCheck DC, single-phase or balanced three-phase voltage drop from cable area and current.Wire ampacityCheck temperature, conductor grouping and terminal limits. Voltage drop is a separate check.

Cable length and area — current toolCompare cable areas and estimate the maximum distance for your voltage-drop target.

Three-phase cable impedanceUse electrical input kW or line current, with reference or custom cable resistance and reactance.

Find the maximum cable length for a voltage-drop target

This page answers two design questions: how far a cable can run for a selected percentage drop, and what theoretical conductor area would meet that target at a known distance. Enter current or electrical input kW, voltage, material, area, temperature, power factor and reactance. The tool also compares the selected area with nearby metric sizes.

Use source-to-load distance once. When selecting feet, the entered number is interpreted as feet; the result gives the maximum length in feet and identifies the metric conversion. For balanced three-phase circuits, use line-to-line voltage and line current. The single-phase and DC models assume equal outgoing and return conductors.

Formulas and units

Let I be current in A, L one-way length in km, R resistance in Ω/km at operating temperature, X reactance in Ω/km, and PF the lagging power factor. The estimated drop is ΔV = k × I × L × (R × PF + X × √(1 − PF²)). Here k = 2 for a two-wire circuit and √3 for balanced three-phase. DC uses PF = 1 and X = 0.

The AC expression is a longitudinal approximation for a balanced sinusoidal load. It follows the model described in the ELEK voltage-drop guide. It does not solve unbalanced circuits or motor-starting behavior.

With target drop ΔVtarget = source voltage × target percentage / 100, the maximum one-way length is Lmax = ΔVtarget / [k × I × (R × PF + X × √(1 − PF²))], in km. The calculator converts that length to metres or feet for display.

How minimum cross-section is calculated

The area model uses R = K(T) / A, with A in mm². K(T) = K20 × [1 + α × (T − 20)]. This page uses approximate material constants K20 = 17.241 for copper and 28.264 for aluminum, with α = 0.00393/°C and 0.00403/°C respectively.

Rearranging the voltage-drop equation gives Amin = K(T) × PF / [ΔVtarget / (k × I × L) − X × √(1 − PF²)]. In DC this reduces to K(T) × k × I × L / ΔVtarget.

Reactance is held constant in this rearrangement. If the denominator is zero or negative, no finite area can meet the requested target under that assumption. The calculator states this explicitly. Changing the physical cable arrangement or choosing another cable can change X, so use actual manufacturer data when selecting a final cable.

Six examples: length, temperature and area

All rows below use 230 V, single-phase, 20 A, copper, PF 1 and a 3% target. These examples demonstrate the model; they do not approve a conductor for an installation.

One-way length / conductor temperature / area	Drop	Maximum length	Minimum theoretical area
25 m / 20°C / 10 mm²	1.724 V (0.75%)	100.05 m	2.499 mm²
50 m / 20°C / 10 mm²	3.448 V (1.50%)	100.05 m	4.997 mm²
100 m / 20°C / 10 mm²	6.896 V (3.00%)	100.05 m	9.995 mm²
150 m / 20°C / 10 mm²	10.345 V (4.50%)	100.05 m	14.992 mm²
50 m / 75°C / 10 mm²	4.194 V (1.82%)	82.27 m	6.078 mm²
50 m / 20°C / 6 mm²	5.747 V (2.50%)	60.03 m	4.997 mm²

A calculated minimum of 4.997 mm² does not mean that a nominal 5 mm² cable is a verified choice. Select an available cable with adequate actual resistance and check ampacity, terminal limits, protection, installation method and the applicable code. The nearby-size comparison is a voltage-drop comparison only.

Frequently asked questions

Why is conductor temperature different from ambient temperature?

The metal can operate hotter than the surrounding air or soil. This tool changes conductor resistance using the entered operating temperature. It does not calculate that temperature from ampacity or installation conditions.

Is 3% always the required limit?

No. It is the initial comparison target in this tool. Set the target for your equipment, project and applicable requirements. Meeting that percentage does not establish compliance with all installation rules.

Can I enter motor shaft power?

The power input is electrical kW. Convert shaft output to electrical input using efficiency, or use actual line current. The current estimate uses the entered source voltage and does not iterate a constant-power load after voltage drop.

Why might another calculator give a different answer?

Compare material constants, conductor temperature, cable resistance, reactance, one-way versus loop length, and the meaning of line voltage. A tabulated maximum cable resistance can differ from the ideal area model used here.