Inverter Wiring and Cable Sizing

An inverter is the one device on your boat that will happily pull hundreds of amps through a cable thinner than a pencil if you let it. A 2000-watt inverter running a microwave can draw close to 200 amps from a 12-volt battery bank, and that current has to travel through copper you chose. Get the copper right and the thing runs cool and quiet for a decade. Get it wrong and you have a fire hazard bolted to the bulkhead.

This is the part of an inverter install that trips up practical, capable owners, because the wiring feels like an afterthought once the shiny box is mounted. It is not. The cable is the install.

Here is how to size and wire it properly, step by step, using real numbers you can copy for your own boat.

Step 1: Find the inverter’s real current draw

Start with the number that actually matters, which is amps on the DC side, not watts on the AC side. Watts are the same on both sides of the inverter, but volts are not, and that is what makes the DC cable so demanding.

The quick rule for a 12-volt system: divide the inverter’s continuous watts by 10 to get a safe working amp figure. That already bakes in inverter inefficiency (most run around 85 to 90 percent) and a little headroom. A 2000-watt inverter lands at roughly 200 amps. A 1000-watt unit lands near 100 amps.

If you are on a 24-volt bank, divide by 20 instead, which is one of the quiet advantages of going to higher voltage. The same 2000 watts pulls only about 100 amps, so your cable and fuse shrink and get cheaper.

Step 2: Measure the round-trip cable length

This is the step people get wrong most often, and it is the difference between a warm cable and a dangerous one. Voltage drop cares about the total distance the current travels, down the positive cable and all the way back through the negative.

So if your battery bank sits 4 feet from the inverter, you do not have a 4-foot run. You have an 8-foot round trip. Measure the real routing, not the straight-line distance, because cable follows the hull, goes around stringers, and dips under the sole.

On a boat I rewired last spring, the owner had measured 5 feet and bought cable for 5 feet. The actual routing around a fuel tank made it 9 feet each way, an 18-foot round trip, and his original cable was two full sizes too small. It got hot enough to soften the insulation. We caught it before it caught anything else.

Step 3: Pick the gauge from a voltage-drop chart

Now put the two numbers together, amps and round-trip feet, and read off the cable size. The goal is to keep voltage drop (the volts lost as heat in the cable itself) at or below 3 percent for a critical run like this. More drop means less power at the inverter, more heat in the copper, and a battery that works harder than it should.

Marine cable is measured in AWG (American Wire Gauge), where the numbers run backward: smaller number, fatter wire. Once you pass 1 AWG the sizes are written 1/0, 2/0, 3/0, spoken as “one-aught,” “two-aught,” and so on, each one fatter than the last.

These are conservative real-world sizes for a 3 percent target with tinned marine cable. When you land between two sizes, or your run is long, go up a gauge. Copper is cheap compared to a haul-out.

Step 4: Use the right cable, not just the right size

Gauge is only half of it. The cable itself matters as much as the number stamped on it.

Use fine-stranded, tinned marine cable. Fine strands survive the constant vibration and flexing that would crack solid or coarse wire over a season. The tin coating fights the corrosion that turns bare copper green and resistive in salt air. Welding cable is beautifully flexible and often used, but it is not always tinned, so on a saltwater boat I lean toward tinned marine battery cable every time.

Skip the thin, stiff, red-and-black stuff sold at auto parts counters for “up to 400 watts.” It has no business carrying inverter current.

Step 5: Fuse the cable within inches of the battery

Here is the rule that saves boats: the fuse protects the wire, not the inverter. Its whole job is to blow before the cable overheats if something shorts. So you size the fuse to the cable and inverter draw, and you place it as close to the battery positive terminal as you physically can, within 7 inches per ABYC.

Size the fuse just above the inverter’s continuous DC draw. For that 2000-watt, 200-amp inverter, a 250-amp fuse is typical. For a 1000-watt unit near 100 amps, a 125-amp fuse fits.

Use a fuse rated for the job. A Class-T fuse is the gold standard near a big battery bank because it can safely interrupt the enormous short-circuit current a healthy bank can dump. An ANL fuse is acceptable for smaller inverters. A cheap blade fuse is not, at these currents.

Step 6: Make clean, tight terminations

A perfect cable ruined by a bad crimp is the most common failure I see. Loose or corroded lugs create resistance, resistance creates heat, and heat at 200 amps finds the weakest joint fast.

Crimp your lugs with a proper hydraulic or hammer crimper, not pliers. Then seal each joint with adhesive-lined heat shrink so salt air cannot creep in. Torque the terminal nuts to spec and add a star washer so vibration does not back them off.

Keep the positive and negative runs the same length and route them together where you can. It keeps the install tidy and makes voltage drop predictable on both legs.

A worked example you can copy

Let me put the whole thing together with a boat I actually spec’d. A 34-foot sailboat, 12-volt house bank, owner wants a 2000-watt pure sine inverter to run a coffee maker and charge tools at anchor.

Continuous draw: 2000 divided by 10 equals 200 amps. The battery bank is under the aft berth, and the inverter mounts in a locker 6 feet of routed cable away. Round trip: 6 plus 6 equals 12 feet.

From the chart, 200 amps at a 12-foot round trip lands us at 2/0 AWG tinned cable to hold 3 percent drop. We fused it with a 250-amp Class-T fuse mounted 5 inches from the battery positive, crimped 2/0 lugs with a hydraulic crimper, and sealed every joint. Total cable, lugs, and fuse came to about $150. That inverter has run three seasons without a warm cable or a nuisance trip.

Where inverter wiring fits the bigger picture

Cable sizing is one piece of a system that has to agree with itself. The inverter, the battery bank, the charging source, and the AC side all lean on each other, and a weak link anywhere shows up as heat or lost power somewhere else.

If you are building the whole 12-volt setup from scratch, start with the overview in our boat inverter and charger guide, then use the wider marine electronics setup guide to see how the inverter, batteries, and instruments share ground and power. For the safety standards behind the fusing and cable rules, the BoatUS technical library is a solid, plain-language reference.

Do the measuring once, buy copper a size up when in doubt, and put the fuse where it belongs. That is the whole difference between an inverter you forget about and one that keeps you up at night.