calculator for cable size

How this cable size calculator works

Correct cable sizing balances safety, efficiency, and cost. A cable that is too small can overheat, cause excessive voltage drop, and reduce equipment life. A cable that is too large can increase project cost with little additional benefit.

This calculator uses a practical engineering approach:

• Compute load current from power, voltage, phase, and power factor.
• Apply a design safety factor for continuous duty.
• Apply derating for ambient temperature and grouped circuits.
• Check voltage drop against your selected limit.
• Choose the smallest standard cable size that passes both checks.

Inputs explained

1) Load power, voltage, and phase

Current is calculated from your electrical load:

• Single phase: I = P / (V × PF)
• Three phase: I = P / (√3 × V × PF)

Where P is power in watts, V is voltage, and PF is power factor.

2) Temperature and grouping derating

Cables carry less current in hotter environments or when multiple circuits are grouped together. The calculator reduces allowed ampacity using correction factors.

3) Voltage drop limit

Even if ampacity is sufficient, long cable runs can still fail performance requirements due to voltage drop. This tool checks that the estimated drop stays below your selected threshold.

Practical sizing guidance

• Use 3% voltage drop for sensitive final circuits when possible.
• For feeders, 5% may be acceptable depending on local code.
• Always verify insulation type, installation method, and local standards.
• Motor starts and harmonics may require a larger cable than steady-state load suggests.
• Check short-circuit withstand and protective device coordination separately.

Example scenario

Assume a 7.5 kW load at 230 V single phase, PF 0.9, one-way run of 35 m, max drop 3%, copper conductor, ambient 30°C, one circuit, and 125% safety factor. The calculator will estimate the design current, apply derating, then evaluate standard cable sizes until both ampacity and drop limits are met.

Important note

This is a planning calculator, not a replacement for licensed electrical design. Final cable selection must follow your local electrical code (NEC, IEC, BS 7671, or other applicable standards), equipment datasheets, and site conditions.