Calculator inputs
Enter your load and installation details. Results appear above this form after submission.
Example data table
These examples are illustrative and help you understand how changing length, material, and load can shift the recommended conductor size.
| Use case | Phase | Load | Voltage | Length | Material | Drop limit | Illustrative size |
|---|---|---|---|---|---|---|---|
| Room air conditioner | Single | 2.2 kW | 230 V | 18 m | Copper | 3% | 4 mm² |
| Water pump | Single | 3.0 kW | 230 V | 30 m | Copper | 3% | 6 mm² |
| Workshop motor | Three | 7.5 kW | 400 V | 40 m | Copper | 5% | 10 mm² |
| Long outdoor feeder | Single | 5.0 kW | 230 V | 65 m | Aluminum | 3% | 25 mm² |
Formula used
1) Load current
Single phase: I = P / (V × PF × η)
Three phase: I = P / (√3 × V × PF × η)
2) Design current
Design current = Load current × (1 + Safety margin)
3) Required table ampacity
Required ampacity = Design current / (Ambient × Grouping × Installation × Insulation factors)
4) Voltage drop
Single phase: Vd = 2 × L × I × (R × cosφ + X × sinφ)
Three phase: Vd = √3 × L × I × (R × cosφ + X × sinφ)
5) Voltage drop percent
Drop % = (Vd / Supply voltage) × 100
This estimator uses approximate standard sizes, a fixed low-voltage reactance assumption, and temperature-adjusted conductor resistance. Final design should always follow local electrical standards and manufacturer data.
How to use this calculator
- Choose single-phase or three-phase supply.
- Enter the appliance or feeder load in watts or kilowatts.
- Fill in voltage, power factor, efficiency, and one-way cable length.
- Select conductor material, insulation, installation method, ambient temperature, and grouped circuits.
- Add your allowable voltage drop, demand factor, and safety margin.
- Press the calculate button to show the recommended size above the form, along with a comparison table and chart.
Frequently asked questions
1) What does cable size mean here?
Cable size means the conductor cross-sectional area, usually in square millimeters. Larger sizes carry more current and usually produce lower voltage drop over distance.
2) Why does cable length matter so much?
Longer cables create more resistance. That raises voltage drop and can cause poor equipment performance, heating, and reduced starting ability for motors or compressors.
3) Should I choose copper or aluminum?
Copper usually allows smaller sizes and lower resistance. Aluminum is lighter and often cheaper, but it usually needs a larger cross-sectional area for the same duty.
4) Why are there correction factors?
Temperature, grouping, and installation conditions reduce real cable carrying capacity. Correction factors adjust the base ampacity so the recommendation better reflects actual service conditions.
5) What safety margin should I enter?
Many users apply 10% to 25% for planning. The right margin depends on startup current, future expansion, supply stability, and installation policy.
6) Can I use this for air conditioners and motors?
Yes, as an estimate. Include realistic power factor, efficiency, and safety margin. Motor starting behavior and manufacturer instructions may still require a larger conductor.
7) Is voltage drop more important than ampacity?
Both matter. A cable can carry the current safely yet still drop too much voltage. Good sizing checks both limits before making a recommendation.
8) Is this enough for final installation approval?
No. Use it for planning and comparison. Final selection should be checked against local codes, breaker settings, insulation rating, cable construction, and site conditions.