Calculating cable cross-section: How to find the right conductor cross-section
Choosing the right cable cross-section (conductor cross-section) for DC installations is crucial for the safety and performance of your system. Cables that are too thin cause voltage drops and overheating, while oversized cables are unnecessarily expensive and unwieldy. So what do you need to consider when determining the right cable cross-section? Here are the most important factors and tips:
Power and cable length
The required cross-section depends primarily on the maximum current flow (amps) and the length of the cable. The higher the current and the longer the cable, the thicker the cable must be to keep voltage losses to a minimum. As a guideline, a voltage drop of less than 2.5% should be aimed for. This means that the voltage at the consumer should be barely noticeable lower than at the battery, even under load. It is best to use a voltage drop calculator (e.g., in the Victron Toolkit app or online) – enter the system voltage, current, and cable length, and the tool will recommend a minimum cross-section.
Use manufacturer specifications
Victron provides clear recommendations for cable cross-sections and fuse sizes for DC connections in its product manuals. Therefore, always check the manual for your inverter, charger, or solar charge controller. There you will usually find tables or information such as “recommended battery cable cross-section: 35mm² (with appropriate fuse)”. These recommendations are based on internal tests and should be adhered to as a minimum. If in doubt, choose a thicker cable, especially for inverters, as these sometimes require larger cables and higher fused circuits due to pulsating AC loads (ripple).
Observe voltage level
Keep in mind that higher system voltages (24V or 48V instead of 12V) require less current for the same power output, thus allowing for thinner cables. For example, at 1200 W power output, approx. 100 A flows in a 12V system, but only 50 A in a 24V system. If you anticipate that your power requirements (e.g., large inverter) will be in the kW range, consider switching from 12V to 24V – this will drastically reduce current levels and cable cross-sections. (More on this in the section on system voltage.) For smaller consumers and lighting in motorhomes, however, 12V is common and practical.
Rule of thumb
A simple estimate for cables up to approx. 5 m in length is the “rule of thirds”: current (A) ÷ 3 = required cross-section in mm². Example: For 90 A current, a cross-section of approx. 30 mm² should be selected. This rule of thumb takes into account a voltage drop within a reasonable range. However, it does not replace the exact calculation for longer distances – here, the resistance increases with distance, which is why longer cables must be disproportionately thicker.
Protection and safety
The right cable always needs the right protection. Every cable connected to the battery must be protected close to the source with a suitable fuse or circuit breaker. The fuse protects the cable from overload and short circuits. Its value depends on the cable cross-section and the connected consumer. Victron often provides recommendations for fuse sizes in its manuals or data sheets (e.g., 400 A fuse for a 3000VA/12V inverter) – you should follow these recommendations. Use high-quality fuse holders or busbar systems (e.g., Victron Lynx Distributor) for a clean installation.
Conclusion
Carefully calculate cable cross-sections based on current, length, and permissible voltage drop. Use tools such as the Victron Toolkit app or online calculators. Follow the manufacturer's recommendations and secure cables correctly. This will help you avoid voltage losses and dangerous cable fires and ensure that your battery power reaches consumers efficiently.
FAQs: Selecting the correct DC cable cross-section
The required cable cross-section depends primarily on three factors:
- Maximum current (amps)
- Total cable length (round trip)
- Permissible voltage drop
In practice, a voltage drop of less than 2–3% is considered ideal. The easiest way to calculate this is to use a voltage drop calculator: enter the voltage, current, and length, and you will get the minimum required cable cross-section.
Pro tip:
If you are unsure, choose the next higher available cross-section to create reserves for peak loads and future expansions.
Manufacturers take additional factors into account, such as thermal load, continuous load behavior, contact resistance, and possible current peaks of certain devices (e.g., inverters). Many simple calculators do not fully reflect these influences.
Pro tip:
Always treat the information in the device manual as a minimum recommendation and, if in doubt, select a larger cross-section.
The fuse protects the cable—not the connected device.
The correct fuse size therefore depends on:
- the cable cross-section
- the maximum possible current carrying capacity
- the expected operating current
Important: Every cable that is connected directly to the battery must be fused as close to the battery as possible.
Pro tip:
Use high-quality fuse holders and make sure that the fuse always matches the cable used to reliably prevent overheating in the event of a fault.
The higher the system voltage, the lower the current required for the same output. This allows thinner cables to be used and losses to be reduced.
Example for 1200 W:
- 12V system: ~100A
- 24V system: ~50A
- 48V system: ~25A
Higher voltages are particularly suitable for powerful systems with inverters or long cable lengths.
Pro tip:
For systems with a power output of approx. 3000 W or more, plan to use 24V or 48V wherever possible in order to significantly reduce cable costs and voltage losses.
The rule of thumb is suitable for rough estimates for short cable lengths (up to approx. 5 m). However, it is not sufficient for longer cables, higher currents, or precise installations, as it does not take into account the permissible voltage drop, cable resistance over longer distances, or DC ripple.
Pro tip:
Use the rule of thumb only as a guide—for correct results, always use a voltage drop calculator or the manufacturer's recommendation.
The most common mistakes are:
- Calculating only the one-way distance instead of the round trip
- Neglecting peak loads (e.g., for inverters)
- Undersized fuses or missing fuses
- Using CCA cables (copper-clad aluminum) instead of pure copper
- Failure to observe the manufacturer's specifications
- Use of motor vehicle tables that are unsuitable for continuous loads
Pro tip:
Only use high-quality, pure copper cables and check all connections carefully to minimize contact resistance. In the marine sector, it is also recommended to use heat-shrink tubing that is protected against salt water to prevent corrosion.
