The Boat Mill

Battery Cable Voltage-Drop Calculator

Check how much voltage a DC cable run loses at a given load. Enter the current, the one-way length from the source to the load, the wire gauge and the system voltage. The calculator doubles the length for the return conductor and reports the drop in volts and as a percentage.

When to use it

Whenever you add or move a DC load: a new windlass, a bow thruster, a fridge, a bilge pump moved aft. Too much drop makes motors slow and hot, starves electronics into resetting, and in the worst case leaves a pump unable to keep up. Too thick a cable costs money and weight. This finds the line between them.

Length is measured along the route the cable will actually take, from the battery or panel to the load. The return is assumed to run the same length back, so the calculator uses twice the number you enter.

Calculator

Load current (A)

One-way cable length (ft) Round-trip = 2 × one-way (pos + neg).

AWG (copper lookup)

Ω per 1000 ft Filled from AWG; override anytime.

System voltage (V)

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The formula

R = Ω per 1000 ft × (2 × one-way length) ÷ 1000

drop (V) = current × R

drop (%) = drop ÷ system voltage × 100

The resistance values are for copper at room temperature. Hot engine rooms raise resistance a few percent.

3% or 10%?

Widely used marine practice sets two limits. Circuits where low voltage could affect safety (panel feeds, navigation lights, bilge pumps, electronics) are sized for no more than 3% drop. Other loads, such as cabin lights, may be sized for 10%. Windlasses and thrusters are usually sized to their maker’s cable table, which is often tighter than 10% because they draw so much current.

Worked examples

Windlass, 100 A, 10 ft run, 4 AWG, 12 V. R = 0.2485 × 20 ÷ 1000 = 0.00497 Ω; drop = 100 × 0.00497 = 0.50 V (4.1%). Over 3%, which a windlass can usually live with, but 2 AWG brings it to 2.6%.

Bilge pump, 5 A, 25 ft run, 14 AWG. R = 2.525 × 50 ÷ 1000 = 0.126 Ω; drop = 0.63 V (5.3%). Too much for a safety load; 12 AWG gives 3.3% and 10 AWG 2.1%.

Same windlass on 24 V. Half the current for the same power: 50 A on 4 AWG drops 0.25 V, which is 1.0% of 24 V.

Percent drop at 12 V, 20 A load

Gauge	10 ft one-way	15 ft	20 ft	30 ft
14 AWG	8.4%	12.6%	16.8%	25.3%
12 AWG	5.3%	7.9%	10.6%	15.9%
10 AWG	3.3%	5.0%	6.7%	10.0%
8 AWG	2.1%	3.1%	4.2%	6.3%
6 AWG	1.3%	2.0%	2.6%	4.0%
4 AWG	0.8%	1.2%	1.7%	2.5%

Wiring safety

Voltage drop is not the only limit. Every conductor also has a maximum current (ampacity) that depends on its insulation rating and whether it runs in an engine space. A cable can pass the voltage-drop check and still be too small to carry the current safely.
Fuse or breaker every positive conductor close to the source, sized to protect the wire, not the load.
Use tinned, stranded marine cable and crimped, sealed terminals. Solid household wire cracks with vibration.
Measure the drop on a finished circuit with a meter across the load while it runs. If it differs a lot from the calculation, look for a bad crimp or a corroded connection.

FAQ

Why double the length?

Current goes out on the positive and back on the negative; both conductors add resistance.

My wire is metric.

Choose “Custom” and enter its resistance in ohms per 1000 ft, or convert: 1 Ω/km = 0.3048 Ω/1000 ft.

Does the warning mean the circuit is unsafe?

It means the drop is above the 3% or 10% guideline. Decide based on what the load is.