Battery-to-Inverter Cable Sizing: Gauge, Length and Lugs

Size the cable from the inverter's maximum current at battery voltage, keep the run as short as you can, and accept very little voltage drop. Victron's wiring guide recommends a drop of no more than 2.5 percent, which is 0.3 volts on a 12-volt system. Then terminate it with lugs that match the cable, a proper crimp tool and the torque the manual gives. The inverter manual's minimum cable size overrides any rule of thumb here.

Why this run is different

The cable between a 12-volt battery and an inverter carries far more current than any other wire in a small build, because low voltage means high amps for the same watts. Victron's guide notes that raising the system voltage lowers the current and allows thinner cable, and it suggests 12 volts only up to a certain inverter size, about 3000 VA, with 24 or 48 volts for larger ones. Undersized cable heats up and drops voltage, and an inverter that sees low voltage under load can shut down or alarm even with a full battery. This guide sits on top of the basics in our wiring basics guide, and the architecture trade-offs are in the 12V DC versus AC inverter guide.

Gauge: start from current, not from the cable

1. Find the maximum current. Use the inverter's rated continuous and surge ratings, and its input current at full load from the manual. Surge matters for motors and compressors; the surge rating guide explains why.
2. Find the cable length. Count both conductors, positive and negative, from battery to inverter.
3. Choose the drop you accept. Victron recommends no more than 2.5 percent.
4. Read the chart. Use the inverter maker's required cable size, or a chart for your cable type, and round up, not down.

Victron also gives a rough rule for runs up to about 5 meters total: divide the current in amps by 3 to get the cable cross-section in square millimeters. Treat it as a sanity check rather than a design method, because it depends on the cable and the assumptions behind it. US cable is sold by AWG number, so use the cable maker's conversion chart or an installation chart, and never guess a conversion. When a single cable is hard to find or handle, Victron says two cables in parallel are acceptable if their combined cross-section equals the recommended size.

Length: shorter is cheaper and safer

The same guide stresses keeping runs short, since a shorter cable can be thinner for the same drop and wastes less energy as heat. In practice that means putting the battery and inverter close together, ideally in the same ventilated enclosure, with the inverter near the battery and the long run on the AC side, where the current is lower for the same power. Do not coil excess cable into a tight bundle under load, since enclosed cable runs hotter. If you must route through a wall or conduit, check the cable maker's ratings. Where multiple batteries are paralleled, Victron says the interconnecting cables should be equal in length and thickness so the batteries share current evenly.

Cable type

Use fine-stranded, flexible copper cable. Victron advises against coarse-stranded, non-flexible and AC building wire, and against solid core, and recommends tinned copper marine cable in damp locations. A shed, trailer or boat is often damp or at least condensing, so tinned cable is a sensible default. Leave a little slack and add strain relief so the weight and vibration of the cable do not load the terminals.

Lugs and crimps

A good cable with a poor lug is a poor connection. The checklist from Victron's guide:

• Match the lug to the cable size and to the stud, and use a crimp tool made for that size, not pliers or a hammer.
• Insulate uninsulated lugs with heat shrink or covers that keep a dropped wrench away from the terminal.
• Stack hardware in order: washer, spring ring, then nut, with the lug flush against the terminal and nothing between the lug and the surface.
• Use the torque in the product manual, not a guess. Our torque tool guide covers the tool side.
• Tug-test each crimp after making it, and avoid bullet connectors and butt splices on high-current runs.

Common mistakes to avoid

• Sizing from the average load. The cable must carry the inverter's maximum current, not what you usually run. A kettle or a compressor start can pull far more than the lights.
• Measuring only the positive cable. Victron says to check the drop on both the positive and the negative conductor.
• Unequal parallel cables. Two cables of different length or thickness share current unevenly, so cut them to match.
• Loose or dirty terminals. A connection that is not flush and torqued adds resistance right where the current is highest.
• Leaving the lug bare. An exposed lug beside the battery terminal invites a short from a dropped tool.
• Ignoring ventilation. Batteries and inverters need airflow and room, so follow both manuals when you plan the enclosure.

Fuse and verify

Every battery-to-inverter run needs a fuse at the battery, matched to the cable, the inverter manual and the battery's possible short-circuit current. Victron lists the selection criteria as current, voltage, interrupt rating, speed and type. Fuse types differ in voltage rating and interrupt rating, and your inverter manual states the fuse size it expects. When the system is built, measure the voltage drop at full load on both the positive and the negative cable, as Victron advises, with the inverter running a real load. Warm cable, warm lugs or a low-voltage alarm at moderate load are signs to stop, power down and recheck the run.