Cable Size Calculator
Estimate a suitable cable cross-sectional area based on load current and allowable voltage drop.
For preliminary sizing only. Always verify with local electrical code, insulation rating, installation method, grouping factors, and ambient temperature corrections.
Why cable sizing matters
Choosing the right cable size is about safety, efficiency, and long-term reliability. If a cable is undersized, it can overheat, cause nuisance tripping, increase fire risk, and produce excessive voltage drop at the equipment. If it is oversized, you may spend significantly more on material and installation than necessary.
A good cable size calculator helps you quickly narrow down the correct conductor cross-sectional area (mm²) before final engineering review. The key is balancing two constraints: thermal current-carrying capability (ampacity) and acceptable voltage drop.
How this cable size calculator works
This calculator uses practical engineering approximations to estimate a recommended cable size:
- Current estimation: You can enter current directly, or enter power (kW) and let the tool calculate current using voltage, power factor, and phase type.
- Voltage drop criterion: It calculates minimum conductor area needed to keep voltage drop within your selected limit.
- Current density criterion: It checks a minimum size from current density (A/mm²), useful for a quick thermal sanity check.
- Standard cable selection: It rounds up to a common standard cable size.
- Ampacity check: It verifies estimated ampacity and moves to the next standard size if required.
Input fields explained
1) Load input type
If you already know load current, use the Current (A) option. If you know rated power, use Power (kW) with a realistic power factor.
2) System type and voltage
Select single-phase or three-phase and enter line voltage. This determines current calculations and voltage-drop behavior.
3) Cable length
Enter one-way length. The formula internally accounts for return path behavior depending on phase type.
4) Material
Copper has lower resistivity than aluminum, so for the same current and voltage drop limit, aluminum usually needs a larger cross-section.
5) Voltage drop limit
Typical design targets are 2% to 5%, depending on local standards and equipment sensitivity. Critical loads usually need lower drop.
6) Design margin and current density
A design margin accounts for continuous loading and future expansion. Current density is a simplified thermal parameter and should be adjusted by installation context.
Example sizing workflow
Suppose you have a 3-phase, 400 V motor load at 22 kW, power factor 0.9, with a 55 m run and 3% voltage-drop target. Set material to copper, margin to 25%, and calculate. The tool will estimate design current, determine required area from voltage drop and current density, and select the next standard cable size that also satisfies ampacity.
Common mistakes to avoid
- Using total route length when the calculator expects one-way length (or vice versa).
- Ignoring correction factors for ambient temperature and cable grouping.
- Not matching breaker/fuse rating and cable ampacity coordination.
- Assuming power factor equals 1.0 for all loads.
- Skipping startup/inrush considerations for motors and compressors.
Final engineering checks before installation
After using any online cable size calculator, validate against your governing standard (NEC, IEC, BS, AS/NZS, or local utility requirements). Confirm insulation temperature class, installation method (tray, conduit, buried), derating factors, short-circuit withstand, and protective device settings. For mission-critical work, review by a licensed electrical engineer is essential.