Tools · 03
Wire Gauge Calculator
At 12V, wire sizing is a two-check problem: the wire must carry the current without overheating (ampacity), and it must deliver the voltage without starving the load (voltage drop). On long runs, drop — not heat — usually sets the size. This tool runs both checks and recommends the larger answer, the way a marine electrician would.
Inputs
Measure length along the actual routed path, source to load, one way. The math doubles it for the return leg.
Read this before you crimp anything
We are not electricians, and this page is not permission. It's arithmetic — the same voltage-drop formula and conservative ampacity values used in marine practice (ABYC E-11 territory). Verify against the standard, use marine-grade tinned stranded copper, fuse every positive conductor near the battery, and have your work checked before it lives three feet from where you sleep.
Ampacity values here assume 105°C insulation outside engine spaces and unbundled runs. Engine compartments and bundled looms require derating — go up a gauge.
The math, shown
- Voltage drop: Vdrop = 2 × length(ft) × current(A) × R(Ω/1000ft) ÷ 1000. The factor of 2 is the return leg.
- Resistance values: standard copper Ω/1000ft per AWG (e.g. 10 AWG ≈ 1.0, 4 AWG ≈ 0.25).
- Ampacity values: conservative ABYC-style ratings for 105°C-insulated conductors outside engine spaces (e.g. 10 AWG ≈ 60A, 4 AWG ≈ 160A). Cheap 60°C automotive primary wire is rated far lower — buy the good stuff.
- The recommendation is the smallest gauge that passes both checks. Going thicker is never wrong; it just costs money and bend radius.
Questions people actually search
What wire gauge do I need for a 12V fridge?
A typical 12V compressor fridge draws about 5A running with startup surges near 8–10A. On a 15ft one-way run at 3% allowable drop, that lands at 10 AWG — thicker than most people expect, because at 12V, voltage drop (not heat) is what sets the wire size on long runs. Undersized fridge wiring is the classic cause of mystery low-voltage compressor cutouts.
What does 3% vs 10% voltage drop mean and which should I use?
The marine-electrical convention (ABYC E-11) is 3% maximum drop for critical circuits — electronics, navigation, refrigeration, anything voltage-sensitive — and 10% for non-critical loads like general lighting. In a van, wire fridges, DC-DC chargers, inverters, and comms to 3%. When in doubt, use 3%: copper is cheaper than diagnosing brownouts.
How do I size the fuse for a wire?
The fuse protects the wire, not the appliance. Size the fuse at or above roughly 125% of the continuous load so it does not nuisance-blow, but never above the ampacity of the wire it protects. Every positive conductor should be fused as close to the battery as practical. If your calculated fuse exceeds the wire’s rating, you need thicker wire, not a bigger fuse.
Does wire length mean one-way distance or round trip?
This calculator asks for one-way distance from the power source to the load and doubles it internally, because current flows out on the positive and back on the negative. Measure the actual routed path — around door frames, under floors — not the straight line. Real runs are always longer than they look.