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Check the cable you have

DC voltage drop calculator: 12V, 24V and 48V

Enter a copper cable size, current and run length. See how many volts reach the load, compare the loss with your target, and open the same cable circuit in Crimp.

For 12 V, 24 V and other 1–60 V DC circuits. Choose mm² or AWG cable and metres or feet. This checks voltage loss; cable current capacity and fuse protection need their own checks.

Length units

Voltage at the start of this cable run, under the load you are checking.

Current through this cable, not the battery’s Ah capacity. Use a known operating current.

Source to load along the route. We add an equal-length return of the same cable size.

Conductor area, not the outside diameter. Length-unit changes keep your selected cable.

Use the equipment’s voltage requirements too. Meeting a percentage alone does not verify the circuit.

Check your cable

Choose a cable and complete the five inputs to see voltage drop and voltage at the load.

Voltage drop is unknown until your inputs are complete.

Need to choose a new cable? The wire and cable size calculator considers both ampacity and voltage drop.

Worked example

What happens to 12 V over a 5 m run?

A 20 A load, 6 mm² copper, 5 m outgoing and 5 m return. These are example inputs, not a cable recommendation.

Outgoing + return cable

4.86% drop

Above the 3% comparison target

Voltage lost in the cable
0.583 V
Voltage at the load
11.42 V
Power dissipated in the cable
11.67 W

Voltage-drop comparison only. Ampacity, fuse protection and the equipment’s minimum voltage are not verified.

The full circuit contains 10 m of conductor. At 20 A, its calculated loss is 0.583 V, leaving about 11.42 V at the load from a 12 V supply. That is 4.86%, above a 3% target.

Shortening the route or increasing the conductor area reduces this loss. If you change system voltage, calculate the new load current too: a fixed-power appliance draws a different current at 24 V.

The calculation keeps current constant. Actual equipment may draw more current, reduce output or stop as its supply voltage falls. Check the manufacturer’s operating range.

Formula and method

Include the outgoing and return conductors.

Voltage drop = 2 × current × one-way length × 0.0175 ÷ conductor area

Use amperes, metres and mm² to obtain volts. Percentage drop = drop ÷ supply voltage × 100. Load voltage = supply voltage − drop. The 0.0175 Ω·mm²/m copper-resistance assumption is the same one used by Crimp’s engine, at approximately 25°C.

AWG choices use nominal conductor areas; 6 AWG and 16 mm² are different sizes. Feet are converted to metres without changing the physical run. Cable insulation thickness is not conductor area.

What needs a separate check?

This model covers two equal-length, same-size copper conductors with a fixed load current. Hotter wire, terminals, fuses and switches add resistance. Chassis returns, unequal conductors, shared feeds, parallel cables, aluminium, AC and engine cranking need a different assessment.

The 3% and 10% comparisons follow the common critical/non-critical distinctions in the Blue Sea guidance below. They do not establish installation compliance or guarantee that a particular appliance will work.

A low voltage drop does not verify ampacity, insulation, bundling, fuse ratings or battery-end protection. The editor starts with its existing planning assumptions; select the real equipment and installation conditions.

Common questions

Checking an existing DC cable

Do I enter one-way or round-trip length?

Enter one way. A 5 m route is calculated as 5 m out plus 5 m back. If you already measured 10 m of equal outgoing and return conductors, enter 5 m.

Can I check a 24 V or 48 V cable?

Yes, with a known DC load current. The supported supply range is 1–60 V. For identical current, length and area the loss in volts is unchanged, while the percentage depends on supply voltage.

Sources

Check the calculation basis.

Blue Sea SystemsWire sizing: circuit length and voltage-drop targetsExplains full outgoing-and-return circuit length and 3% critical / 10% non-critical voltage-drop distinctions. These targets do not replace equipment limits or an installation review.National Bureau of Standards (NIST)Copper Wire Tables, Handbook 100Nominal American Wire Gauge conductor dimensions. AWG selections retain their actual nominal area; nearby metric sizes are not treated as identical.CrimpCrimp sizing methodologyThe existing engine's copper-resistance assumption and limits. This tool uses the same voltage-drop calculation without selecting a cable or fuse.
Keep goingChoose a wire or cable sizeCheck ampacity and voltage drop when choosing a new conductor.Keep going12 V vs 24 V systemsCompare current at equal power before changing system voltage.Keep goingSingle battery boat diagramExplore a simple accessory circuit with lights, electronics and a fuse block.Keep goingWhat Crimp checksUnderstand the editor's planning checks and the equipment details still to verify.

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