Electrical Wire Size Calculator
Enter your load, wire length and allowed voltage drop to find the correct copper wire size — with the recommended standard size, actual voltage drop and breaker suggestion.
Result
| Load current | - |
|---|---|
| Minimum copper area (voltage drop) | - |
| Ampacity of recommended size | - |
| Estimated power loss in cable | - |
Standard copper wire sizes (reference)
Typical ampacity in conduit for copper conductors. Ampacity changes with installation method, insulation rating and temperature, so treat these as reference values.
| Size (mm2) | Approx SWG | Ampacity (A) |
|---|
How to use
- Choose whether you know the load in amps or in watts. If you enter watts, also enter the system voltage so the current can be calculated.
- Enter the one-way wire length from the source to the load in meters.
- Set the allowed voltage drop percentage (3 percent is a common home-wiring limit).
- Click Calculate Wire Size. The tool shows the recommended standard size, the actual voltage drop with that size, and a suggested breaker.
Tips
- The recommended size is the smallest standard size that satisfies BOTH the voltage-drop calculation and the current ampacity — safety first.
- For long runs to motors, pumps or outdoor lighting, voltage drop usually decides the size, not the load current.
- Aluminium wire needs a larger size for the same load — this tool is for copper only.
- If your load is larger than 120 mm2 can handle, split it across parallel cables with professional design.
- Prefer AWG gauges? Convert between AWG and mm2 with the AWG Wire Size Converter.
Frequently Asked Questions (FAQ)
How does the wire size calculator work?
You enter the load as amps or as watts plus voltage, the one-way wire length in meters, and the allowed voltage drop percentage. The tool uses the copper voltage-drop formula, area = 2 x 0.0172 x length x current divided by (voltage x drop fraction), then recommends the next standard size up.
Which standard wire sizes does the tool check?
It checks standard copper sizes from 1.5 to 120 mm2 with their typical ampacity values, along with approximate SWG equivalents. The recommended size must satisfy both the voltage-drop calculation and the current-carrying ampacity for your load.
What should the allowable voltage drop percentage be?
For home wiring a common limit is 3 percent from the source to the load point. The tool defaults to 3 percent, and you can change it between 1 and 10 percent. A lower limit gives a thicker, safer recommendation.
Why does wire length matter so much in the result?
Voltage drop grows with the round-trip length of the cable, so the formula uses twice your one-way distance. Long runs to motors, pumps or outdoor lights often need a thicker wire than the load current alone would suggest.
Is this a substitute for professional wiring design?
No. The result is an estimate for planning only. Always follow your local wiring regulations and have the final selection checked by a licensed electrician before installation.