Cable Size Calculator

Inputs

• Load in kW
• System — three-phase 415 V or single-phase 230 V
• Conductor — copper or aluminium
• Route length
• Permitted voltage drop

Method

1. Current = load ÷ (√3 × V × 0.85) for three-phase, or load ÷ (V × 0.85) for single-phase, at an assumed 0.85 power factor.
2. Steps through the standard sizes from 1.5 to 400 mm² against an approximate derated ampacity table for copper (aluminium at 80% of it), then checks the voltage drop over the route length from the conductor's resistivity, and selects the smallest size that satisfies both.

Assumptions

• Power factor 0.85 for every load.
• The ampacity table already carries a typical derating for grouping and ambient temperature; it is one typical installation condition, not yours.
• Reactance is ignored in the voltage-drop check, which is acceptable at these sizes and lengths but not for very large cables.

Limitations

• No short-circuit withstand check — the size must also survive the fault current for the protection's clearing time.
• No derating for the actual installation method, grouping, soil thermal resistivity or ambient; those can change the answer by a size or two.
• HT cables, bus ducts and parallel runs are outside the tool.