KW to Cable Size Calculator
Estimate a practical cable cross-sectional area from load power, system voltage, and voltage drop limit.
How this kW to cable size calculator works
Cable sizing starts by converting electrical power (kW) into current (amps). Once current is known, we estimate the cable cross-sectional area (mm²) using two checks:
- Current carrying capacity check (via selected current density and derating factor)
- Voltage drop check (using run length and allowable percentage drop)
The calculator takes whichever requirement is larger, adds your chosen design margin, and rounds up to the next common standard cable size.
Core formulas used
1) Convert kW to load current
Single-phase: I = (kW × 1000) / (V × PF × Efficiency)
Three-phase: I = (kW × 1000) / (√3 × V × PF × Efficiency)
2) Area based on current density
Required ampacity current = Load current / Derating factor
Area by current = Required ampacity current / Current density
3) Area based on voltage drop
Single-phase: A = (2 × I × ρ × L) / ΔV
Three-phase: A = (√3 × I × ρ × L) / ΔV
Where ρ is conductor resistivity in Ω·mm²/m (approx. 0.0175 copper, 0.0282 aluminum).
Typical standard cable sizes
| Common sizes (mm²) | Typical use trend |
|---|---|
| 1.5, 2.5, 4, 6 | Lighting and small branch circuits |
| 10, 16, 25, 35 | Small motors and sub-main feeders |
| 50, 70, 95, 120 | Larger feeders and industrial loads |
| 150, 185, 240+ | High-current mains and long-run heavy-duty installations |
Why cable length matters so much
Two installations with the same kW can need very different cable sizes if run lengths differ. Longer cables increase resistance, which increases voltage drop. If voltage drop rises beyond your target (for example 3%), equipment may run inefficiently, start poorly, or overheat.
Copper vs aluminum cable sizing
- Copper has lower resistance and usually allows smaller size for the same duty.
- Aluminum is lighter and often cheaper, but typically requires a larger cross-section.
- Terminations, lugs, and code rules can differ between materials.
Practical sizing checklist
- Determine true load power in kW.
- Select single-phase or three-phase supply correctly.
- Use realistic power factor and efficiency values.
- Measure one-way cable length accurately.
- Set target voltage drop from your project standard.
- Apply derating for grouping, ambient temperature, and installation method.
- Round up to the next standard cable size.
Important note on standards and safety
This tool provides a design estimate only. Final cable selection must follow your local electrical code and manufacturer ampacity tables, including insulation type, ambient temperature, grouping, installation method (tray, conduit, buried), short-circuit withstand, protective device coordination, and earthing requirements.