12V Cable Sizing Calculator

Safety first: Disconnect shore power and turn off other charging sources before opening panels or changing RV wiring. The fuse or breaker protects the wire. This calculator is guidance only—not a substitute for the cable, fuse, breaker, and installation requirements in your equipment manuals or local code.

Charging voltage vs 12.0 V — why it matters

While a converter or charger is in absorption, the bus typically sits around 14.4–14.6 V, not 12.0 V. Treating “3% of 12.0 V” as the only target (~0.36 V) often oversizes cable for charging runs. Absolute voltage drop (volts) is the primary engineering metric; % drop should use the reference voltage for your use case.

Example (copper, ~0.27 V drop): charger at 14.4 V → about 14.13 V at the battery ≈ 1.9% at 14.4 V—often fine for a converter feed. The same drop is ~2.3% of 12.0 V, which looks worse only because the wrong reference was used. Blind “3% of 12 V” is why many calculators push 2 AWG when 4 AWG is already adequate for that application.

How to read the results: Practical recommendation is the smallest common AWG that keeps absolute drop in a sensible band for the use case. Strict % target is the engineering size that meets your selected max % at the reference voltage. Next size up shows a lower-drop option and notes when upsizing is probably unnecessary. Prefer copper for high-current inverter and battery interconnects when possible—CCA has higher resistance for the same AWG.

How to use this cable sizing calculator

Use continuous current (not only a brief surge) and measure one-way distance from source to load. The calculator doubles that distance for the round trip. Choose Charging / converter output when sizing converter-to-battery or charger feeds (reference 14.4 V on 12V systems). Choose Under load / resting when the bank is near 12.0 V / 24.0 V. Compare the practical card to the strict % card—do not treat the thickest mathematical result as the only answer.

When to upsize

Upsize for an inverter, battery interconnect, long run, high ambient temperature, bundled cables, starting surge, or any installation where the manual specifies a larger conductor. Confirm ampacity, fuse or breaker rating, routing, lugs, and terminations with the manufacturer and local requirements. Volt Resources provides educational guidance only; a qualified RV electrician should review unfamiliar or high-current work.

Method note

We use standard DC resistance values for copper AWG 0000–18 and calculate voltage drop = current × resistance × round-trip length. CCA is estimated at 1.65× copper resistance for the same AWG. Recommendations prefer common stock sizes (0000, 000, 00, 0, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18). Practical (charging): absolute drop ≤ 0.50 V or ≤ 3.5% of the 14.4 V (28.8 V on 24V) reference. Practical (under load): ≤ 0.40 V (12V) / ≤ 0.80 V (24V) or ≤ 3% of system voltage. Strict: your selected max % of the reference voltage. Absolute volts are always shown first.

For this 30A circuit, I recommend 10 AWG tinned copper wire. See the wire on Amazon.ca