Note: This tool sizes cable for voltage drop only. Always verify ampacity, insulation rating, ambient temperature, bundling, and local electrical code.
What this DC cable size calculator does
This calculator helps you estimate a suitable cable cross-sectional area for low-voltage DC systems such as solar installations, battery banks, RV wiring, marine electronics, and off-grid power runs. It uses your system voltage, current, cable length, and allowed voltage drop to calculate a minimum conductor size in mm² and a nearest practical recommendation.
In DC systems, voltage drop can become significant very quickly, especially at 12V and 24V. Choosing the right cable size keeps equipment efficient, reduces heating losses, and helps motors, inverters, and charge controllers perform as expected.
How the calculation works
The tool uses the standard resistance-based voltage-drop model for a two-conductor DC circuit (outbound + return path):
- A = required conductor cross-section (mm²)
- ρ = resistivity constant of conductor (copper or aluminum)
- L = one-way cable length (m)
- I = design current (A)
- Vdrop = allowable voltage drop in volts
The calculator then rounds up to a common metric cable size and gives the nearest AWG equivalent for quick reference.
Input guide
1) System voltage
Enter your nominal DC voltage (for example 12V, 24V, 48V). Lower-voltage systems are more sensitive to voltage drop.
2) Load current
Enter the expected continuous current in amps. If your load has startup surges, account for that in your design process.
3) One-way cable length
Enter the physical one-way distance from source to load. The calculator automatically doubles this for round-trip path resistance.
4) Maximum voltage drop percentage
Typical design targets:
- 1–2% for sensitive electronics and long battery/inverter feeds
- 3% for general-purpose DC branch circuits
- Up to 5% only where allowed and performance impact is acceptable
5) Design current factor
A factor such as 125% adds design margin so your final cable choice is more conservative.
Practical example
Suppose you have a 12V system, 40A load, 8m one-way run, and want a maximum 3% drop. With a conservative 125% design factor, the required area is roughly 39 mm², so a practical recommendation is 50 mm² copper.
This is why low-voltage, high-current systems often require surprisingly large cable sizes.
Best practices before final installation
- Confirm ampacity for insulation type and installation method.
- Check local code for minimum conductor size and overcurrent protection requirements.
- Use correct lug/crimp tools and torque settings to reduce connection resistance.
- Protect cables from heat, vibration, UV exposure, and abrasion.
- For critical runs, validate with a loaded voltage measurement after installation.
FAQ
Is AWG or mm² better?
Both are valid. Many regions specify mm²; AWG is common in North America. This calculator provides both for convenience.
Why does cable size increase so much at 12V?
Because allowable voltage drop in volts is small. For example, 3% of 12V is only 0.36V, so resistance must be very low.
Does this replace electrical design by a professional?
No. It is a planning tool. Final design should be verified against electrical code, manufacturer data, and safety standards.