12V Battery Cable Size Calculator

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The voltage drop along a DC cable loop is estimated from the loop resistance: Vdrop = I × Rloop.

• Rloop = 2 × (Length) × (Resistance per length) (the factor 2 covers the return path).
• Resistance increases with temperature using: R(T) = R20 × (1 + α × (T − 20°C)).
• If “continuous load” is enabled, current is scaled by 125% for design margin.

Final size is chosen as the smallest cable that satisfies the selected voltage drop limit and, when enabled, the derated ampacity check.

1. Enter the expected load current and one-way cable length.
2. Choose a voltage drop target (percent or volts) and cable material.
3. Optionally enable continuous load and growth allowance for extra margin.
4. Keep ampacity screening on, then set ambient, bundling, and installation method.
5. Press Calculate to see the recommended size, losses, and comparisons.

1) Why does cable size matter so much at 12V?

At low voltage, even small resistance causes a large percentage drop. That reduces device performance and increases heat loss. Upsizing cable quickly improves efficiency on long runs.

2) Should I use percent or volts for voltage drop?

Percent is easy to compare across systems, while volts is straightforward for sensitive equipment. For most 12V loads, 3% is tight and 5% is acceptable.

3) What is “one-way length” and why is round-trip used?

One-way is the physical run from battery to load. Current must return, so the loop includes both conductors. The calculator automatically doubles the length for loop drop.

4) Why does aluminum usually require a larger size?

Aluminum has higher resistivity than copper, so the same gauge has more resistance. A larger cross-sectional area is needed to keep voltage drop and heating within limits.

5) What does “continuous load uses 125%” mean?

Continuous loads run long enough to reach higher temperatures. Designing at 125% current adds margin, reduces heating, and aligns with common electrical sizing practice for sustained loads.

6) How accurate is the ampacity screening?

It provides a conservative estimate using typical cable ampacity values with simple derating for ambient, bundling, and constrained routing. Real ampacity depends on insulation, airflow, and standards. Verify against your cable datasheet and local code.

7) What if my result shows a large power loss?

High loss means wasted energy and extra heat. Reduce length, reduce current, increase system voltage, or select a thicker cable. For 12V inverters, upsizing cable often pays off.

8) Do I size differently for starter motors or short bursts?

Short bursts can tolerate more drop, but starter cables still need very low resistance for reliable cranking. Use thicker cable, short runs, solid terminations, and verify temperature and routing conditions.