Marine Cable Sizing and Voltage Drop for Lithium Systems
Understand marine DC cable sizing for lithium batteries, inverters, alternators, solar and high-current loads.
Cable size is determined by more than current.
Consider:
- continuous current
- peak current
- run length
- acceptable voltage drop
- ambient temperature
- bundling
- termination
- conductor insulation rating
- circuit protection
- marine environmental exposure
Why 12V systems are demanding
A 3,000W inverter can require around 260A from a 12.8V bank at high output.
A relatively small resistance at 260A creates substantial voltage drop and heat.
Voltage-drop concept
Voltage drop = Current x resistance
For a two-conductor DC circuit, consider the complete current path, not only the positive cable length.
Connections count
A large cable with a poor crimp can still fail.
Inspect lug quality, correct crimp tooling, lug-to-stud fit, torque, strain relief, cable support, heat-shrink, corrosion protection and bend radius.
Make the installation verifiable
A credible marine installation should be inspectable, documented and recoverable after a protection event. The battery label alone cannot show whether fault current, isolation, charging control, mounting and essential services have been addressed.
- Document cable sizes, protection ratings, isolation points, charging settings and BMS warning or shutdown logic.
- Confirm that terminals are protected, equipment is restrained and the location is suitable for heat, water and service access.
- Have the completed system commissioned and retain the drawings, settings, test results and equipment manuals aboard.