Cable Size Calculator
Estimate a practical conductor cross-sectional area using current, voltage drop, cable run length, and basic derating factors.
Tip: This calculator applies a 10% design margin and picks the nearest standard cable size.
What a Cable Size Calculator Does
A cable size calculator helps you estimate the minimum conductor cross-section needed for safety and performance. If a cable is too small, it can overheat, waste energy, and create excessive voltage drop. If it is oversized, cost goes up unnecessarily. A good design balances thermal capacity, voltage drop, installation conditions, and future load growth.
Key Inputs That Control Cable Sizing
1) Load current
Current is usually the strongest driver. Higher amperage generally needs a larger cross-sectional area.
2) Cable length
Longer runs increase resistance, which increases voltage drop. Long circuits often require upsizing even when ampacity seems acceptable.
3) Conductor material
Copper has lower resistivity than aluminum, so copper usually carries the same load with a smaller cross-section.
4) Allowed voltage drop
Typical targets are around 3% for branch circuits and up to 5% total for combined feeder + branch (depending on local code and project requirements).
5) Environment and grouping
Higher ambient temperature and grouped circuits reduce heat dissipation, so derating is required. That frequently pushes the final cable size up.
Formula Basis Used in This Tool
This calculator evaluates two limits and selects the larger requirement:
- Voltage-drop area from conductor resistance and run length.
- Ampacity area from current density adjusted by temperature and grouping factors.
The final recommendation is then increased with a small design margin and rounded up to the nearest common standard size (for example: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50 mm² and above).
Quick Practical Guidance
- Use accurate design current, not nominal equipment labels alone.
- Measure one-way cable distance realistically, including routing paths.
- Apply derating for hot rooms, roof spaces, ducts, and grouped runs.
- Check both normal operation and motor starting conditions where applicable.
- Always verify with your local electrical code and manufacturer data tables.
Example Use Case
Suppose you have a 32 A single-phase load at 230 V, copper conductor, 25 m one-way run, and 3% max voltage drop. The calculator checks whether voltage drop or thermal current capacity is the governing constraint. On short runs, ampacity often governs; on long runs, voltage drop usually governs.
Limitations and Safety Note
This tool is an engineering estimate, not a permit-ready design document. Real projects must also consider insulation type, installation method, fault level, protective device coordination, short-circuit withstand, harmonic content, and local code requirements. For final selection, consult a licensed electrician or electrical engineer.
FAQ
Is bigger cable always better?
Electrically, larger cable reduces resistance and drop, but it increases cost and can be harder to terminate. Correct sizing is usually best, not just maximum sizing.
Why does voltage drop matter?
Excessive voltage drop causes under-voltage at loads, poor motor performance, nuisance tripping, and energy loss in the cable.
Can I use aluminum instead of copper?
Often yes, but aluminum generally needs larger cross-sectional area for equal performance and requires compatible lugs and installation practices.