Cable Size Calculator
What size cable do you need for a solar panel, battery, or inverter circuit? Enter the current, length, and voltage to compare voltage drop and estimated heat limits, and find a starting cable size in mm². Your local wiring rules always decide the final size.
For planning only. This tool gives a preliminary estimate. It is not a design, and a qualified professional must check it against the datasheets and your local electrical rules.
Your cable size
Start from 16 mm² copper cable (about AWG 4). Voltage drop sets this size.
Preliminary estimate. Must be verified by a qualified professional.
Current capacity here is not from a wiring standard. A qualified electrician must confirm the size and protection under your local rules.
Assumptions used
- Estimated capacity
- 62 A
- Heat and grouping factor
- 1.00
- Running temperature
- 70°C
How each size compares
A cable must pass both checks. Bigger cable lowers the drop, but costs more.
What if?
Save this result, change any setting, and compare.
How this is calculated
Voltage drop
Cable resistance turns some energy into heat and lowers the voltage at the far end. We calculate the drop from cable length, area, and material at a running temperature of 70°C.
DC or single-phase: 2 × I × L × R. Three-phase: √3 × I × L × R × power factor
Current capacity
Cables heat up as current flows. We use a conservative rule of thumb, reduced for hot surroundings and for circuits grouped together. It is not taken from a wiring standard.
Capacity = rule of thumb × temperature factor × grouping factor
Choosing the size
We pick the smallest common size that passes both checks. Long, low-voltage runs are usually limited by voltage drop, and short, high-current runs by heat.
Worked example: 30 A over 10 m at 48 V
A DC run with a 2% limit needs at least 16 mm² copper. It drops 0.77 V, or 1.6%.
Doubling the length doubles the voltage drop, which is why low-voltage systems need much thicker cable over long runs.
How accurate is this?
Voltage drop is standard physics, but current capacity here is a conservative rule of thumb. Real ratings depend on insulation, installation method, and your country's wiring rules. This tool cannot replace those tables, and a qualified electrician must select the final cable.
Common mistakes
- Using one-way length as the full loop on DC circuits
- Checking voltage drop but not current capacity
- Ignoring hot roofs and bundled cables
- Using household cable for solar DC circuits
Limits of this tool
- It does not replace the tables in your national wiring rules
- It does not size fuses, breakers, or earthing
- Solar DC circuits often need specific rated cable and extra safety factors
What to do next
- Have a qualified electrician confirm the cable and protection
- Check your string design with the string voltage calculator
Frequently asked questions
What size cable do I need for solar panels?
Many home strings use 4 mm² or 6 mm² solar cable, but the right size depends on current, length, and local rules.
What is an acceptable voltage drop?
Many designers aim for 1% to 3% on solar circuits. Your local rules may set a limit.
How do I calculate voltage drop?
On a DC circuit, multiply two by the current, the one-way length, and the resistance per metre.
Is aluminium cable okay for solar?
It can be used in some larger circuits, but it needs a bigger size and suitable connectors. Ask a qualified electrician.