When wiring your marine electrical system together, careful consideration should be given to proper cable sizing. Not all cables are created equal, and it is absolutely essential that the cables wiring your batteries to various loads are sized properly.
Improper cable sizing can create dangerous operating conditions and limit the effectiveness of your electrical system.
Although it can seem daunting, we’ll walk you through a couple of quick and easy ways to ensure your marine electrical system cables are the proper size.
A key component of any electrical system is the wiring used to connect each piece of the system. These wires are responsible for carrying current from one place to the next.
Properly sizing your wires is essential to ensure the proper amount of current transfers to power your loads. Improperly sized wires can drastically limit the functionality of your electrical system and create potentially dangerous operating conditions.
Why Cable Size Matters Even More With Lithium Batteries
Lithium batteries can deliver high current with less voltage sag than lead-acid batteries. That performance is one of their biggest advantages, but it also means the rest of the electrical system needs to be designed for the current it may carry.
Undersized cables, loose external battery connections, poor crimps, or excessive cable runs can increase resistance in the circuit. Resistance creates voltage drop and heat, and that heat can damage insulation, stress terminals, reduce system efficiency, and cause connected equipment to shut down unexpectedly.
A properly designed marine lithium system should use correctly sized cables, marine-rated wire, properly crimped lugs, secure terminal connections, and appropriately sized fuses or circuit protection. The battery’s internal safety features are important, but they work best when the surrounding electrical system is installed correctly.
How Do You Determine What Size Cable Your Marine Electrical System Needs?
While each of the wires in the photo above is rated for up to 600 volts (as noted on the side of the wire), they are clearly very different sizes. The gauge (or thickness) of the wire that your electrical system needs will be determined by the amperage, or current, flowing through your system and the total distance of the circuit.
The current of your electrical system can be determined by using the power equation. This equation tells us that amps multiplied by volts equals watts.
So, to solve for amps, you will want to divide the watts of the load you’re trying to power by the voltage of the system. For example: If you are trying to determine what size wire to use for a 3,000-watt inverter in a 12V system, dividing 3,000 watts by 12V will give you 250 amps.
⚡️ Note: A 3,000-watt inverter in a 12V system draws at least 250 amps before efficiency losses. In practice, the current may be higher, so always follow the inverter manufacturer’s installation instructions and use properly rated cable and overcurrent protection.
Then, you will need to determine the total round-trip circuit length. This will be how long your wire needs to be to connect your batteries to the loads you’re trying to power and back again (positive conductor plus negative return path).
Once you’ve determined the length, you can use a table, like the one below, to find where the length of your circuit and the amount of current flowing through it intersect to determine the size of the wire you need to use. 
Additional Things to Consider When Sizing the Wires for Your Sailboat Electrical System
Another important factor to consider is voltage drop. Voltage drop is “the reduction in voltage in an electrical circuit between the source and the load. Wires carrying electricity have inherent resistance, or impedance, to current flow.” This creates overall voltage loss as power moves through the system.
Depending on what you are powering, ABYC standards recommend an allowable amount of voltage drop. For some non-critical branch circuits, ABYC guidance allows up to 10% voltage drop. For critical circuits, such as bilge pumps, navigation lights, and safety-related electronics, 3% voltage drop is commonly used. A larger cable will help minimize the voltage drop due to less resistance.
A Simple Solution to Size Your Cables
With modern technology, various smart device applications are on the market that can help you calculate the size of wires you need. These apps allow you to input a variety of variables, and at the click of a button, you receive a recommended wire size. Warrior Sailing expert Connor Smith uses the Blue Sea Systems Circuit Wizard to quickly identify the size of wire his marine electrical system requires. The app simply prompts him to input:
- voltage of the system
- load current (in amps)
- length of the conductor (full length of the circuit)
- allowable voltage drop percentage
- insulation temp rating of the wire (printed on the size, generally 105°C for quality wires)
- if the system is installed in the engine room
- how many wires are bundled together
- the average duration of the load
- whether the circuit is protected by a fuse or circuit breaker
Then, he simply hits calculate and receives a wire sizing recommendation. This app, and others like it, can be extremely helpful in ensuring your cables are the proper size. By taking many variables into consideration, you can rest assured knowing that your system is operating safely and efficiently.
Want to Learn More?
Check out the full Marine 101 series on our YouTube channel or through the Academy page on our website. For further questions or help designing the perfect marine power system, our technical sales team would love to help! You can reach by calling (855) 292-2831 or emailing info@battlebornbatteries.com
