Free Tool
Cable Size Calculator
Undersized cable wastes power and can overheat. Enter your system voltage, load and cable run to get the right copper cross-section for a safe, efficient 12V, 24V or 48V installation.
The right cable size depends on the current it carries, the length of the run and the system voltage. Low-voltage DC systems need much thicker cable than higher-voltage ones for the same power, because voltage drop hurts more at 12V. Aim to keep voltage drop under about 3 percent on critical runs.
Interactive Tool
Cable Size Calculator
Recommended cable size
16mm²
41.7A over 3m one way, actual drop 2.28% (0.27V) at 12V
Shop cables and connectorsBased on copper cable at typical operating temperature. Always fuse the cable appropriately and follow the manufacturer's installation guidance for your equipment.
How this is calculated
The calculator first works out the current in your circuit. In watts mode that is simply load divided by system voltage, so a 500W load on a 12V system draws about 41.7A. It then sizes the copper using the voltage drop formula: required cross-section = (2 × 0.0175 × length × current) ÷ (voltage × allowed drop). The factor of 2 accounts for the round trip, current flows out along the positive cable and back along the negative, and 0.0175 ohm mm² per metre is the resistivity of copper.
The result is rounded up to the next standard metric cable size, from 1.5mm² to 120mm². We then run a second check against each size's approximate continuous current rating. If the voltage-drop size cannot safely carry your current, the calculator bumps it up to the smallest size that can, and tells you it has done so.
A 3% drop is a sensible default for most 12V, 24V and 48V circuits. Use 2% for sensitive loads such as fridges and charge controller runs, and 5% only for short, non-critical circuits like lighting. Low voltage systems are unforgiving over distance, which is why doubling the system voltage quarters the copper you need for the same power.
Frequently asked questions
What size cable do I need?
It is set by the current (amps), the round-trip cable length and your target voltage drop. Higher current and longer runs need thicker cable. Enter all three and the calculator returns the minimum safe cross-section in mm².
What is voltage drop and why does it matter?
Voltage drop is the voltage lost as current flows through resistance in the cable. Too much drop means dim lights, underperforming appliances and wasted energy as heat. Keep it under about 3 percent for power circuits.
Why do 12V systems need such thick cable?
Power is volts times amps, so at 12V you need four times the current of a 48V system for the same power. More current means more voltage drop and heat, so 12V runs need much thicker, more expensive cable, one reason larger systems use 48V.
What cable size for my inverter or battery?
High-current battery-to-inverter runs are usually the thickest cable in the system. Work out the inverter’s maximum DC current (watts divided by battery voltage, allowing for efficiency) and size for a short run with minimal drop.
