A cable size calculator based on voltage drop
This cable size calculator finds the smallest standard conductor cross-section (in mm²) that keeps the voltage drop within your limit. Give it the load in kW or amps, the supply voltage, single- or three-phase, the one-way length and the allowed drop, and it returns the minimum area, the next standard size and the exact drop you would get with it.
Long runs and motors are where undersized cable hurts: a motor that sees a few volts too few draws more current and runs hotter. The tool works out the current for you (with power factor and efficiency for a motor) and shows how every standard size performs, so you can see the margin rather than a single number.
How to use the Cable Size Calculator
- Choose whether the load is in kW or amps, then enter the load, voltage and phases.
- For kW loads, set the power factor and efficiency (use 100 % efficiency for heaters).
- Enter the one-way length, the conductor metal, the allowed voltage drop and the conductor temperature.
- Read the smallest standard size, the drop it gives and the table of all sizes.
- Open your cable maker’s datasheet, apply the grouping and ambient factors, enter the combined derating factor and the rated current of the size you are considering, and read the check.
Cable Size Calculator features
Kilowatts to amps
Three-phase I = P ÷ (√3 × V × PF × efficiency); single-phase I = P ÷ (V × PF × efficiency).
Resistivity at temperature
Copper 0.017241 and aluminium 0.028264 Ω·mm²/m at 20 °C, corrected with the IEC 60228 temperature coefficients to your chosen conductor temperature.
Every standard size
Metric sizes from 1.5 to 630 mm² with the drop in volts and per cent, the chosen size highlighted.
Datasheet rating check
You supply the manufacturer’s current rating; the tool does not embed copyrighted standards tables.
How the minimum cross-section is calculated
For a cable of resistivity ρ (Ω·mm²/m), area S (mm²) and one-way length L (m), carrying current I, the voltage drop is ΔV = k × ρ × L × I ÷ S, where k = 2 for single-phase (out and back) and k = √3 for three-phase. Rearranged, S = k × ρ × L × I ÷ ΔV, with ΔV the allowed drop in volts (supply voltage × drop %).
Worked example: a 15 kW three-phase motor at 415 V, power factor 0.85 and efficiency 90 % draws 27.3 A. Copper at 70 °C has ρ = 0.017241 × (1 + 0.00393 × 50) = 0.02063. Over 50 m with a 3 % limit (12.45 V), S = 1.732 × 0.02063 × 50 × 27.3 ÷ 12.45 = 3.9 mm², so the next standard size is 4 mm². Compare with the motor full-load current calculator, voltage and voltage drop calculator and wire gauge calculator.
What this tool does not check
- Current rating. The size that satisfies voltage drop may still overheat. The cable must also carry the design current after installation method, ambient temperature and grouping factors. Those tables vary by standard and cable type and are copyrighted, so take the rating from the manufacturer’s datasheet and use the check box above.
- Reactance. The calculation is resistive, which is conservative for small cables and optimistic for large ones (95 mm² and up).
- Starting current. A motor started direct-on-line draws several times full-load current for seconds; the drop during starting may need a separate check.
- Fault protection. Short-circuit withstand and breaker or fuse coordination are outside this estimate.
Typical voltage-drop limits
Limits differ by country and circuit type. Many codes use roughly 3 % for lighting and 5 % for other circuits from the origin of the installation, and some recommend 3 % for branch circuits. Treat these as starting points and check the rule that applies to you — IEC 60364, BS 7671, NEC guidance, AS/NZS 3008 and IS 732 all address this differently.
Cable Size Calculator FAQ
What size cable do I need for my load?
Enter the load, voltage, length and allowed drop. The tool returns the smallest standard size for voltage drop; then check that size’s current rating in the manufacturer’s datasheet with your installation factors.
How do I size a cable for a motor?
Enter the motor’s kW, power factor and efficiency, or its full-load amps from the nameplate. Consider a design multiplier if your code requires a margin, and check starting drop separately.
Is this valid in the UK, India, US or Australia?
The voltage-drop maths is the same everywhere, but each country’s code sets its own limits and current-carrying tables (BS 7671, IS 732, NEC, AS/NZS 3008). Use this as a planning aid and follow your local code.
Why is my answer a standard size and not the exact number?
Cables are made in fixed sizes, so the tool rounds up to the next IEC 60228 metric size and shows the actual drop it gives.
Copper or aluminium?
Aluminium has about 64 % higher resistivity than copper, so it needs roughly 1.6 times the area for the same drop. It is lighter and cheaper but needs suitable terminations.
Is this professional electrical advice?
No. It is an estimate for planning. Consult a licensed electrician and your local code before choosing or installing a cable.