Cable Size Calculator kW: How to Size a Cable

This cable size calculator for kW works out the minimum cross-sectional area of a copper or aluminium cable needed for a given power load in kilowatts, based on the system voltage, power factor, cable length, and allowable voltage drop. You enter the load in kW, the voltage (single-phase or three-phase), the cable length, and the maximum acceptable voltage drop, and it returns the required cable size in mm² (or AWG) along with the actual voltage drop and percentage loss. The calculator uses the standard voltage-drop formula from electrical engineering, so you get a conservative, code-friendly estimate for most residential and light commercial installations.

Formula

The calculator uses the fundamental voltage-drop formula for a cable carrying a known current. First, the current is derived from power:

I = (P × 1000) / (√3 × V × pf) for three-phase, or I = (P × 1000) / (V × pf) for single-phase.

Then the required cross-sectional area is:

A = (2 × L × I) / (σ × Vd)

Where:

• A = cable cross-sectional area in mm²
• L = one-way cable length in metres (the factor 2 accounts for the return path)
• I = load current in amperes
• σ = conductivity of the conductor material (for copper ≈ 56 S·m/mm², aluminium ≈ 35 S·m/mm²; the calculator uses resistivity, the reciprocal)
• Vd = allowable voltage drop in volts (calculated from the percentage you set)
• V = system voltage (line-to-line for three-phase, line-to-neutral for single-phase)
• pf = power factor (0.8–1.0 typical)

The result is the minimum area that keeps the voltage drop within your chosen limit. In practice, you round up to the nearest standard cable size (1.5, 2.5, 4, 6, 10, 16, 25, 35, 50 mm², etc.).

Worked Example

Using the default values: 7.5 kW, 230 V single-phase, power factor 0.9, cable length 20 m, max voltage drop 3%, copper resistivity 0.0175 Ω·mm²/m.

1. Current: I = (7.5 × 1000) / (1 × 230 × 0.9) = 7500 / 207 = 36.23 A
2. Allowable voltage drop: Vd = 3% of 230 V = 6.9 V
3. Area: A = (2 × 20 × 36.23) / (56 × 6.9) = 1449.2 / 386.4 = 3.75 mm²
4. Round up to the next standard size: 4 mm² (or 6 mm² if you prefer extra margin)

The calculator returns 3.75 mm², which matches. The actual drop with a 4 mm² cable would be (2 × 20 × 36.23) / (56 × 4) = 6.47 V, or 2.81% – just under your 3% limit.

When to Use This Calculator

Use this tool when you are sizing a cable for a fixed load like a motor, heater, oven, air conditioner, or EV charger. It is most accurate for continuous loads where the cable runs a significant distance – say over 10 metres. For short runs under 3 metres, the cable is usually limited by current-carrying capacity (ampacity) rather than voltage drop, so you should also check the cable’s rated ampacity for the installation method (clipped direct, in conduit, buried, etc.).

This calculator is a voltage-drop calculator first. It does not replace a full electrical design that considers ambient temperature, grouping of cables, insulation type, or protective device rating. Those factors can force a larger cable even if the voltage drop is fine.

How to Choose the Right Cable Size

Follow these steps in order:

• Determine the total connected load in kW (add all appliances that can run simultaneously).
• Identify the system voltage and phase – most homes use 230 V single-phase; larger workshops and commercial sites use 400 V three-phase.
• Estimate the cable route length – measure the actual path, not straight-line distance.
• Choose an acceptable voltage drop – 3% for lighting and general use, 5% for other circuits is common in many national codes.
• Run the calculator to get the minimum area, then round up to the nearest standard size.
• Check the cable’s ampacity table for your installation method – if the ampacity is lower than the load current, go up a size.
• Verify that the protective device (fuse or breaker) rating matches the cable’s ampacity.

For example, a 7.5 kW single-phase load at 230 V draws 32.6 A at unity power factor. A 6 mm² copper cable clipped direct typically carries about 47 A, so it would be acceptable for short runs, but at 20 m the voltage drop is 2.8% – still fine. At 50 m, the drop rises to 7%, so you would need 10 mm² or 16 mm².

Common Mistakes and Misconceptions

Mistake 1: Using the wrong voltage. For three-phase, the voltage in the formula is line-to-line (400 V), not line-to-neutral (230 V). If you mix them up, the current is overestimated by √3.

Mistake 2: Forgetting the power factor. Motors and fluorescent lights have a pf below 1. If you ignore it, you undersize the cable because the actual current is higher than the apparent power suggests.

Mistake 3: Confusing kW and kVA. kW is real power; kVA is apparent power. For a pf of 0.8, 10 kVA is only 8 kW. The cable carries the current based on kVA, not kW.

Mistake 4: Rounding down. If the calculator says 3.75 mm², you must round up to 4 mm², not down to 2.5 mm². Always round up to the next standard size.

Misconception: “Thicker is always better.� While a thicker cable reduces voltage drop, it costs more and can be harder to terminate. It also changes the fault current characteristics. Use the minimum size that meets both voltage drop and ampacity, unless you have a specific reason to oversize (e.g., future expansion).

Copper vs. Aluminium Cables

Copper has about 60% higher conductivity than aluminium (56 vs. 35 S·m/mm²), so a copper cable can be roughly two-thirds the cross-section of an aluminium cable for the same voltage drop. However, aluminium is lighter and cheaper, which is why it is common for large service entrances and overhead lines. For household circuits, copper is almost always used because it is easier to terminate and less prone to oxidation at connections.

If you switch the resistivity in the calculator to 0.028 (aluminium), the required area increases by about 60%. For the same 7.5 kW example, aluminium would need 6 mm² instead of 4 mm². Always use the correct resistivity for the conductor material.

Safety and Regulation Notes

This calculator is an educational tool, not a substitute for a qualified electrician. Electrical work is dangerous and regulated. In most countries, you must follow the national wiring code (such as the NEC in the US, BS 7671 in the UK, or AS/NZS 3000 in Australia) which specifies minimum cable sizes, derating factors, and installation methods. Always consult a licensed electrician before designing or modifying any electrical circuit.

Key safety points:

• Never exceed the cable’s rated ampacity – overheating can cause fires.
• Keep voltage drop below the recommended limits to avoid equipment malfunction.
• Use the correct cable type for the environment (wet, outdoor, underground, high temperature).
• Ensure all connections are tight and use appropriate terminals.
• Install a correctly rated protective device (fuse or breaker) at the supply end.

The calculator assumes a continuous load and a uniform cable temperature. Real installations may require derating for ambient temperature above 30°C, multiple cables bunched together, or insulation with a lower temperature rating. These factors can reduce the cable’s current-carrying capacity by 10–50%.

Frequently Asked Questions

How do I convert kW to cable size? You can’t directly – you need to know the voltage, phase, power factor, length, and acceptable voltage drop. This calculator does the conversion for you. The result is the minimum cross-sectional area in mm².

What is the maximum voltage drop allowed? It depends on your local code. A common rule is 3% for lighting circuits and 5% for power circuits, measured from the supply point to the load. Some utilities allow up to 10% for service drops.

Does cable length really matter? Yes, significantly. Doubling the length doubles the voltage drop for the same cable size. That’s why long runs (e.g., from a house to a detached garage) often need much thicker cable than short runs.

What does mm² mean in cable sizes? It is the cross-sectional area of the copper or aluminium conductor. A 2.5 mm² cable has a conductor area of 2.5 square millimetres. Larger area means lower resistance and higher current capacity.

Can I use this calculator for DC circuits? Yes, for DC the formula simplifies: the phase factor is 1 and the power factor is 1. Just enter the DC voltage and the load in watts (divide by 1000 to get kW). The voltage drop calculation is the same.

Why does the calculator ask for power factor? Because AC motors and many electronic loads draw reactive power, making the current higher than the real power suggests. A power factor of 0.8 means the current is 25% higher than it would be at unity pf, so the cable must be larger.

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in p | Power (kW) | kW | 7.5 | 0
in v | System voltage (V) | V | 230 | 0
in ph | Phase | Single=1, Three=1.732 | 1 | -
in pf | Power factor | - | 0.9 | 0
in l | Cable length (m) | m | 20 | 0
in vd | Max voltage drop (%) | % | 3 | 0
in r | Resistivity (ohm·mm²/m) | - | 0.0175 | 0
out area | Required cable size | mm² | (2 * l * p * 1000) / (ph * v * pf * (vd/100) * v * r) | 2
out drop | Actual voltage drop | V | (2 * l * p * 1000) / (ph * v * pf * area) | 2
out pct | Voltage drop percentage | % | (drop / v) * 100 | 2