Boat Battery Cable Size Calculator

Calculate the minimum marine cable size needed to keep DC voltage drop within your chosen limit. Enter the current, one-way cable length, system voltage, and allowable voltage drop.

Required circular mils = (2 × one-way length × current × K) ÷ allowable voltage drop
K = 10.75 for the copper-conductor method used here.
Use the maximum expected load current.
Enter the distance from the source to the load. The calculation doubles it for the return path.
Tinned marine cable is copper with a corrosion-resistant tin coating. The coating does not justify using the 17.02 constant from the previous version.
Important: This calculator sizes the conductor for voltage drop. Final marine wire selection also requires a separate ampacity check based on installation conditions, conductor temperature rating, bundling, and applicable marine standards.

How Boat Battery Cable Sizing Works

Why voltage drop matters

A DC cable has resistance. As current flows through it, some voltage is lost along the cable. Longer runs and higher current increase the loss, while a larger conductor reduces it.

Why the calculator uses one-way length

Enter the physical distance from the battery, panel, or other source to the load. The current must travel back through the negative/return conductor, so the calculation uses twice the entered length.

3% vs. 10% voltage drop

For marine DC circuits, 3% is commonly used where voltage drop is important, while 10% is used for non-critical loads. A 5% option is included as a custom planning target rather than presented as an ABYC category.

Voltage drop is not the whole cable-sizing calculation

A cable can pass a voltage-drop calculation and still require a larger size because of ampacity. Installation temperature, number of conductors bundled together, conductor insulation rating, continuous load, overcurrent protection, and the applicable marine wiring standard must also be checked.

Copper vs. tinned copper

Marine cable is commonly made from stranded copper with tin plating to improve corrosion resistance. For this calculator, both are treated using the same K constant because the previous version's 17.02 value should not be used simply because the copper is tinned.

Worked example

Suppose a 12V load draws 80A and is 15 ft one way from the battery with a 3% maximum voltage drop. The allowable drop is 0.36V and the circuit length is 30 ft round trip.

Required CM = (2 × 15 × 80 × 10.75) ÷ 0.36 ≈ 71,667 circular mils. The next standard size in the table is 1 AWG at 83,690 circular mils. At 80A and 30 ft round trip, 1 AWG produces about 0.31V of drop, or about 2.96%.

Safety reminder

Use marine-grade stranded cable and properly rated overcurrent protection. This calculator is a voltage-drop planning tool and should not be treated as proof that a particular cable installation meets every applicable requirement.