DC Cable Size Chart — AWG Wire Sizing Reference

AWG (American Wire Gauge) runs backward — the higher the number, the thinner the wire. 16 AWG is thinner than 4 AWG. Metric sizing uses cross-sectional area in square millimeters, which works the opposite way: a larger number means thicker wire. A 4 AWG wire is approximately 25 mm², and a 12 AWG is about 4 mm². The Cable Reference Table at the top of this page shows the metric equivalent for each AWG size.

Electrical resistance is proportional to wire length. A longer wire has more total resistance, which causes more voltage to drop across the wire before it reaches the load. A circuit designed for 12V may only deliver 10.8V (10% drop) or 11.64V (3% drop) at the far end of a long run. Using a thicker wire reduces resistance per foot, so less voltage is lost over the same distance. That is why the chart calls for heavier gauge wire as cable length increases for the same current load.

Non-critical circuits (10% voltage drop) include general lighting, fans, and accessories where small voltage fluctuations do not affect how they work. Critical circuits (3% drop) cover equipment that needs stable voltage to operate correctly. Inverters, solar charge controllers, navigation electronics, bilge blowers, and anything where the manufacturer specifies a tight input voltage range. When in doubt, use the 3% (critical) column. The extra wire cost is minimal compared to equipment damage or intermittent failures caused by voltage drop.

Yes, and it is usually a good idea. Larger wire has lower resistance, produces less heat, and drops less voltage across the run. The chart shows minimums. If your cable run falls between two rows or you plan to add loads to the circuit later, go up one gauge size. The only downsides are slightly higher material cost and a small increase in weight, neither is a concern for most installations.

Undersized wire heats up under load. At its mildest, this causes excessive voltage drop at the load, which makes devices malfunction or draw more current trying to compensate. At its worst, the wire overheats, melts its insulation, and starts a fire. Wire fires in marine and RV applications are a common cause of boat and vehicle losses, and undersized cabling is a frequent factor. The fuse or breaker protects against short circuits but does not protect against a wire that is simply too thin for the sustained load.

Ampacity varies by installation type, temperature rating, and whether the wire is bundled with others. As a general reference for copper wire in open air: 16 AWG handles up to 13A, 12 AWG up to 20A, 10 AWG up to 30A, 8 AWG up to 50A, 6 AWG up to 65A, 4 AWG up to 85A, 2 AWG up to 95A, 1/0 up to 150A, 2/0 up to 175A, 4/0 up to 230A. Reduce these by 20% for wiring routed through enclosed spaces or bundled tightly with other cables.

Stranded wire is made up of many thin copper strands twisted together. It bends more easily and resists fatigue from vibration, which makes it the right choice for marine, RV, and automotive applications where the wire flexes or the vehicle moves. Solid wire is a single copper conductor, cheaper and easier to terminate in screw terminals, but it cracks under repeated bending. For DC battery systems in any mobile application, use stranded copper wire.

Standard automotive wire is bare copper and corrodes quickly in saltwater and high-humidity marine environments. Tinned marine-grade wire has each copper strand coated in tin, which resists corrosion and extends service life significantly. The American Boat and Yacht Council (ABYC) standard E-11 requires tinned copper wire for marine electrical systems. Electrically, tinned and bare copper wire perform identically. The tinning protects against corrosion, not heat or current.

Yes. Wire insulation has a temperature rating (typically 60°C, 75°C, or 90°C), and sustained heat from the environment or from bundled wires reduces how much current the wire can safely carry. For wiring routed through engine compartments, enclosed bilge spaces, or anywhere ambient temperature regularly exceeds 30°C, derate the ampacity by 20–30%. When wires are bundled tightly together, apply a similar derating factor. The voltage drop values in this chart assume normal operating temperatures.