This tool provides an engineering estimate only. Always verify with local electrical code, derating tables, and manufacturer data.
How this 3 phase cable size calculator works
This calculator estimates the cable cross-sectional area (mm²) for a three-phase circuit using two core checks: ampacity (current-carrying capacity) and voltage drop. The larger requirement is selected, then a design margin is applied, and finally the nearest standard cable size is recommended.
You can calculate from either:
- Load power in kW (with power factor and efficiency), or
- Load current directly in amps.
Formulas used
1) Three-phase current from power
For power-based input, load current is estimated using:
I = P / (√3 × V × PF × η)
where P is watts, V is line-to-line voltage, PF is power factor, and η is efficiency (decimal).
2) Ampacity-based minimum area
A simplified current-density method is used:
Aampacity = I / (J × Ktemp)
J is a typical current density (A/mm²) chosen by material and installation method, and Ktemp is ambient correction.
3) Voltage-drop-based minimum area
A practical approximation for 3-phase voltage drop is applied:
Avd = (√3 × I × ρ × L) / ΔV
where ρ is conductor resistivity, L is one-way length, and ΔV is allowed voltage drop in volts.
Why both checks matter
A cable that survives current thermally may still fail voltage-drop requirements. Likewise, a cable sized for low voltage drop might be larger than thermal minimum. Good design requires both checks, then selecting the greater value.
Quick design guidance
- Long feeders: voltage drop usually drives cable size.
- High ambient temperature: thermal capacity drops, so cable size increases.
- Aluminium conductors: usually require larger cross-section than copper for the same duty.
- Motor loads: include power factor and efficiency to avoid underestimating current.
- Safety margin: include future expansion and real-world installation effects.
Worked example
Suppose you have a 45 kW motor on 415 V three-phase supply, PF = 0.9, efficiency = 95%, 50 m run, copper cable, 3% voltage drop limit. The calculator first finds load current, then checks ampacity and voltage drop. After adding margin, it rounds up to the next standard size.
This mirrors a typical design workflow used in preliminary sizing before detailed code checks.
Important limitations
This is a preliminary estimator. Final cable sizing should include:
- National/regional code tables (IEC, NEC, BS, AS/NZS, etc.)
- Grouping factors and installation-specific derating
- Harmonics, starting current, duty cycle, and fault level/short-circuit withstand
- Insulation rating and termination temperature limits
- Protective device coordination
FAQ
Is this calculator suitable for final sign-off drawings?
No. Use it for fast estimation only, then verify with standards and project-specific data.
Why does cable size jump quickly for long distances?
Because voltage drop scales with current and length. Longer runs often need much larger conductors.
Can I use this for aluminium armoured cable?
Yes for rough sizing. But always confirm exact cable construction and tabulated ratings from manufacturer/code tables.