I still remember the first time I saw a customer’s inverter cable glowing warm to the touch, not sparking, not smoking, just warm enough that you didn’t want to hold it for long. That cable was 4 AWG feeding a 2000 watt inverter across an 8 foot run. On paper it looked fine to the untrained eye. In practice it was undersized by two full gauges, and it had been quietly cooking itself every time he ran the microwave.
Cable sizing is not the exciting part of a lithium conversion. Nobody posts a photo of their wire gauge chart on Instagram. But get it wrong and you end up with voltage sag, nuisance shutdowns, or in the worst case, a fire risk hiding behind a nice looking battery box.
The quick version
- Cable size depends on three things: amperage, one-way cable length, and how much voltage drop you can tolerate (aim for 3% or less on 12V systems).
- A 100A continuous load over a 10 foot round trip typically needs 2 AWG, not 4 AWG, once you account for real-world voltage drop.
- Always size the cable to the fuse or breaker it protects, using the cable’s ampacity rating as the ceiling, not the load’s typical draw.
- Use fine-stranded, ideally tinned, copper cable rated for the temperature and location, not generic solid-core wire from a hardware store.
Why lithium changes the cable sizing math
Lead-acid batteries sag under load, which naturally throttles how much current you can pull before things get ugly. Lithium doesn’t sag nearly as much. A LiFePO4 bank from Battle Born, Li Time, or Ampere Time holds its voltage right up until it’s nearly empty, which means it will happily deliver its full rated current, and your cables need to be sized for that reality, not for the gentler behavior of the old AGM bank you replaced.
This is also why undersized cable is more dangerous with lithium than it was with lead-acid. The battery won’t limit the current for you. If your inverter asks for 250 amps during a surge, a healthy lithium bank will try to deliver it, and thin cable becomes the weak link that heats up.
The core battery cable size chart
This chart assumes 12V systems, copper cable, and a target voltage drop of 3% or less, which is the standard most RV and marine electricians work toward. Distances are round trip (positive plus negative combined), since that’s the total length current actually travels.
| Continuous Amps | Up to 10 ft round trip | 10-15 ft round trip | 15-20 ft round trip |
|---|---|---|---|
| 50A | 6 AWG | 4 AWG | 4 AWG |
| 100A | 2 AWG | 1/0 AWG | 1/0 AWG |
| 150A | 1/0 AWG | 2/0 AWG | 3/0 AWG |
| 200A | 2/0 AWG | 3/0 AWG | 4/0 AWG |
| 250A (3000W inverter) | 3/0 AWG | 4/0 AWG | 4/0 AWG (or dual runs) |
| 300A | 4/0 AWG | dual 2/0 AWG | dual 3/0 AWG |
Notice how fast the wire jumps in size once you get past 200 amps. That’s the point where a lot of DIY installers start second-guessing the chart and downsizing because 4/0 cable looks absurdly thick. Don’t talk yourself out of it. A 3000 watt inverter under load can pull 250 amps or more continuously, and surge loads (like a compressor starting) can spike well above that for a second or two.
Warning: Never size cable off the inverter’s average draw alone. Check the continuous amp rating on the inverter’s spec sheet, not the “peak” or “surge” number, then add margin. Xantrex and Victron both publish continuous DC input current for exactly this reason.
Doing the voltage drop math yourself
Charts are convenient, but your actual run is never exactly 10 feet or exactly 100 amps. Here’s the formula I use on every job:
Voltage Drop = (Current in Amps x Cable Length in Feet x 0.0002) / Cable Area Factor
Honestly, most of us just use an online voltage drop calculator (Blue Sea Systems has a good free one) rather than doing this by hand every time. Plug in your amps, one-way distance, and system voltage, and it spits out the minimum gauge you need.
A real example from a van build
A customer had a 280Ah lithium bank (two 280Ah cells configured as a single bank) fifteen feet from a 3000W Victron MultiPlus inverter, with the battery under the bed and the inverter mounted near the electrical panel up front. Continuous draw at full inverter load was roughly 260A. Round trip cable length was 30 feet. Chart says 4/0, but at 30 feet round trip we actually ran dual parallel 2/0 cables per leg to keep drop under 3% without needing cable so thick it couldn’t bend around the doorway. That’s a legitimate workaround when a single cable size gets impractical.
Matching cable size to your fuse, not just your load
A mistake I see constantly is people sizing cable to what they think they’ll use, then adding a fuse that’s rated way higher than the cable can safely carry. The fuse exists to protect the cable, not the other way around. If you run 2 AWG cable (rated around 115A insulated, depending on insulation type and ambient temperature), your fuse needs to trip before that cable overheats, generally in the 100-125A range, not 200A.
I’ve written a full breakdown of this relationship in our guide to fusing a lithium battery correctly, which pairs directly with this chart. Get the cable size right first, then size the fuse to protect it.
Cable type matters almost as much as gauge
Not all cable claiming to be “2 AWG” behaves the same. Cheap generic battery cable sometimes uses fewer, thicker strands with lower actual copper content than true welding cable or marine-grade cable. Fine-stranded cable (many small strands rather than a few thick ones) flexes better around corners and resists fatigue cracking from vibration, which matters a lot in a moving vehicle or a boat pounding through chop.
- Choose fine-stranded copper cable, not solid-core house wire
- Tinned copper for boats or humid climates, bare copper is fine for dry interior van runs
- Match insulation temperature rating to engine bay or hot enclosure locations (105C+ where relevant)
- Keep positive and negative cables the same length and gauge
- Use proper crimped or soldered lugs rated for the cable gauge, never a twist-and-tape connection
Getting the lugs right is its own skill, and it’s where a surprising number of otherwise well-planned systems fail. I cover the crimping process step by step in how to crimp battery lugs right, including why a $15 hardware store crimper isn’t the same tool as a hydraulic crimper.
Where busbars fit into the sizing picture
Once you’ve got more than one battery or more than a couple of loads, you’re not running single point-to-point cables anymore, you’re building a distribution system. Busbars simplify this by giving you one clean point to land multiple cables instead of daisy-chaining ring terminals onto a single stud. Each cable landing on a busbar still needs to be sized individually for what it carries, which is where people trip up: a busbar doesn’t average out your amperage, every leg gets sized on its own merits. Our guide on wiring a clean 12V system with busbars walks through laying this out so you’re not guessing at gauge for six different cables at once.
Tip: Label every cable at both ends with its gauge and destination before you close up the walls or the battery box. Future you, troubleshooting at 11pm in a Walmart parking lot, will be grateful.
When to just go one size up
If your calculated gauge lands right on a size boundary, or your run is close to hitting the next distance bracket, go with the thicker cable. The cost difference between 1/0 and 2/0 over a typical 10-15 foot run is usually $15 to $30 in copper, which is nothing compared to the labor of re-running cable later or the risk of chronic voltage drop robbing your inverter of performance. This is especially true if you think you might upgrade your inverter or add a second battery down the road.
If you’re still working through the broader install, our full walkthrough on installing a lithium battery bank safely covers how cable sizing fits into the rest of the job, from disconnects to grounding to mounting location. Cable gauge is one piece of a system that all has to work together, and getting this one piece right early saves you from tearing into finished walls later to fix a wire that was never quite big enough for the job you eventually gave it.
