Cable Size Input Form
Example Data Table
| Example | Phase | Load | Voltage | Length | Material | Drop Limit | Typical Result |
|---|---|---|---|---|---|---|---|
| Small motor | Single phase AC | 5 kW | 230 V | 25 m | Copper | 3% | Check 6 mm² or larger |
| Workshop feeder | Three phase AC | 25 kW | 400 V | 60 m | Copper | 5% | Check 16 mm² or larger |
| Long lighting run | Single phase AC | 2 kW | 230 V | 90 m | Copper | 3% | Voltage drop controls size |
Formula Used
Load Current
Single phase current equals P ÷ V × PF × efficiency. Three phase current equals P ÷ √3 × V × PF × efficiency. Direct current equals P ÷ V × efficiency. Power is converted from kilowatts to watts before calculation.
Design Current
Design current equals load current × future margin. The required cable ampacity is checked after installation, ambient, and grouping derating.
Voltage Drop
Single phase drop uses 2 × L × I × (R cosφ + X sinφ) ÷ 1000. Three phase drop uses √3 × L × I × (R cosφ + X sinφ) ÷ 1000. Direct current drop uses 2 × L × I × R ÷ 1000.
Short-Circuit Check
Minimum conductor area uses S = I√t ÷ k. Fault current is entered in amperes after converting from kiloamperes. The k value depends on conductor material and insulation class.
How to Use This Calculator
- Select the supply type, conductor material, insulation, and installation method.
- Enter load power, or enter a known current if available.
- Add voltage, power factor, efficiency, and cable length.
- Set voltage drop, derating factors, future margin, and parallel runs.
- Enter fault current and breaker clearing time for withstand checking.
- Press the calculate button to view the recommended cable size.
- Use CSV or PDF buttons to export the same result.
Always verify final cable selection with your local wiring rules, protection device data, installation conditions, and approved engineering practice.
Electrical Cable Sizing Guide
Why Cable Size Matters
Cable sizing protects equipment, people, and buildings. A cable must carry load current without overheating. It must also keep voltage drop within a useful limit. Long runs often need larger conductors than short runs. This is because resistance rises with distance. A correct size improves performance and reduces wasted energy.
Current, Drop, and Heat
Current is the first design check. The calculator converts power into current using the selected supply type. It then adds a future margin. This helps when loads may increase later. The cable ampacity is reduced by installation factors. Conduit, burial, insulation, heat, and grouping can all reduce safe current capacity.
Voltage Drop Control
Voltage drop is another important limit. Motors may start poorly when voltage is low. Lights may dim, and electronics may become unstable. The calculator estimates drop using resistance, reactance, length, current, and power factor. For direct current, resistance is the main part of the drop.
Fault Withstand Review
A cable must survive fault current until the protection device opens. This calculator includes an adiabatic short-circuit check. It compares the selected size with the minimum area needed for fault energy. This is useful for feeders, submains, and distribution circuits.
Engineering Judgment
The result is a design aid, not a final approval. Real projects may need local code tables, soil temperature, installation depth, harmonic current, neutral loading, and manufacturer data. Use the exported report for review. Then confirm the final choice with a qualified electrical professional.
FAQs
1. What does this cable size calculator estimate?
It estimates load current, design current, voltage drop, cable loss, fault withstand size, and a recommended conductor area.
2. Can I use a known current instead of power?
Yes. Enter the known current value. The calculator will use that current instead of calculating current from kilowatts.
3. Why does cable length affect the result?
Longer cables have more resistance. More resistance increases voltage drop and heat loss. Larger conductors reduce both effects.
4. What is a derating factor?
A derating factor reduces cable capacity for real installation conditions, such as conduit, high temperature, or grouped circuits.
5. Why include a future load margin?
A margin allows spare capacity for later load growth. It can reduce future cable replacement costs and improve design flexibility.
6. What does the short-circuit check mean?
It checks whether the conductor can withstand fault energy until the protective device clears the fault.
7. Is copper always better than aluminium?
Copper has lower resistance and usually carries more current. Aluminium is lighter and often cheaper, but needs larger sizes.
8. Can this replace local electrical code tables?
No. Use it for planning and comparison. Final cable selection must follow local standards, protection rules, and site conditions.