Wire Your Rig

Wire gauge & voltage drop calculator

Pick the smallest copper wire that keeps voltage drop under your target and carries the current safely. Works for 12, 24 and 48 V.

Free printable: 12V Wire Gauge & Fuse Size Cheat Sheet (PDF, 2 pages), from the same formulas as this tool.

Enter values above.

The spreadsheet adds: up to 20 circuits side by side on one Wire & Fuse tab, each with its own fuse check, plus a check-a-wire-you-own box, all saved in your file. Free vs. spreadsheet

Already have a wire? Check it

Pick an AWG to check it against the inputs above.
Safety: this is a planning estimate, not an engineering sign-off. DC systems can start fires and lithium batteries can deliver very high fault current. Check every result against the manufacturer's instructions and current ABYC/NEC or local rules, put a correctly rated fuse or breaker close to the battery on every positive cable, and have a qualified installer check work you are not sure about. We are not responsible for how results are used.

How this is calculated

Voltage drop. ABYC's marine method gives the copper area needed for a target drop: CM = 10.75 × I × 2L ÷ (V × drop%), where CM is circular mils, I is amps, L is the one-way length in feet (×2 for the round trip, the negative wire counts too) and 10.75 is the resistivity constant used in the ABYC tables. ABYC calls for 3% on critical circuits (panel feeders, bilge pumps, electronics, navigation lights) and allows 10% on non-critical ones such as general lighting. We pick the smallest AWG whose area meets that.

Ampacity. The wire also has to carry the current without overheating. We use the ABYC table for 105 °C insulation, the lower inside-engine-space column if you choose it, and the bundling derates (×0.857 for 4–6, ×0.714 for 7–24, ×0.571 for 25+ conductors). The wire must be rated for at least the load and the fuse that protects it, so the result is the larger of the voltage-drop size and the ampacity size.

Cross-check. We also show the drop using the hot-copper resistance from NEC Chapter 9 Table 8, which comes out a little higher than the ABYC figure. If the two disagree about your target, go one size up.

Metric. 1 circular mil = 0.000506707 mm². We suggest the smallest common IEC 60228 size whose area is at least the recommended AWG's, which errs on the large side. Check the copper area and temperature rating printed on the cable you actually buy: "AWG-equivalent" automotive cable is often smaller.

Tested: these formulas are checked against a recalculated copy of our spreadsheet and published examples (ElectricalToolbox: 10 A over 15 ft one-way at 12 V and 3% needs 8,958 CM → 10 AWG, 0.31 V / 2.59% drop).

Planning a whole build?

The paid spreadsheets run these same tested formulas for a whole build in one saved file. Compare free vs. spreadsheet.

12V Load & Wire Gauge Calculator spreadsheet on Etsy, $9.00

Overland Rig Planning Bundle (12V + Payload + Trip spreadsheets) on Etsy, $18.99

Sources

  1. ABYC: What is voltage drop? (3% / 10% guidance). abycinc.org
  2. PKYS: ABYC voltage-drop tables (CM = 10.75 × I × L ÷ E). shop.pkys.com
  3. Blue Sea Systems: Allowable amperage in conductors / wire sizing chart (ABYC ampacity, bundling derates). www.bluesea.com
  4. NEC Chapter 9, Table 8: conductor properties (copper resistance, Ω/1000 ft). lorisweb.com
  5. ElectricalToolbox: marine voltage drop calculator (worked example used in our tests). electricaltoolbox.com
  6. Victron Energy: Wiring Unlimited (cable, fuse and inverter wiring guidance). www.victronenergy.com