How to Size DC Fuse & Cable Gauge for a 2,000W Inverter on 12V (Formulas + Voltage Drop)

Been through this exact headache with my static caravan setup, so let me share what actually worked rather than what the YouTube crowd says.

The core formula is dead simple:

Watts ÷ Volts = Amps
2,000W ÷ 12V = 166A continuous

But here's where people go wrong — inverters aren't 100% efficient. Factor in ~85% efficiency and you're closer to 195A at the battery terminals. Then add a 25% safety headroom for surge loads and you're sizing fuse and cable for roughly 240A.

For cable gauge, I run 70mm² tinned copper between my Pylontech batteries and my Victron Multiplus. Anything less on a 12V 2kW system is asking for a fire risk. Some folks swear by 50mm² but I sleep better with 70mm².

Voltage drop is the sneaky one. Keep your battery-to-inverter cable run under 1 metre each way if humanly possible. Every extra metre on 12V costs you dearly — aim for under 0.5V drop under load or your inverter will start throwing low-voltage alarms before the battery is anywhere near flat.

ANL fuse sizing: I use a 300A ANL fuse mounted within 300mm of the positive terminal. Some go 250A but given surge headroom I'd rather not live on the edge.

Worth noting — Fogstar and Victron both publish their own cabling guides which align closely with this. Always worth cross-referencing your specific inverter's manual too, as surge ratings vary enormously.

Anyone running a 24V system will find this considerably more forgiving — half the current for the same power. Something

@Pylontech_Queen good start but worth flagging the voltage drop issue that catches everyone out on 12V systems.

At 2,000W your theoretical draw is ~167A, but factor in inverter efficiency (typically 85-90%) and you're realistically looking at 185-195A peak.

For cable sizing I'd add:

• Calculate at the battery terminal voltage (10.5V worst case, not 12V nominal)
• Voltage drop should be ≤3% across your cable run
• Formula: Voltage drop = Current × Resistance per metre × Cable length × 2

What cable run length are you working with? In my tiny house setup the short runs saved me from needing absolutely enormous cable, but static caravans sometimes have awkward routing.

Also — are you fusing at the battery? The fuse rating needs to protect the cable, not the inverter. Caught me out initially.

Good shout on both points. One thing that bites people — don't use nominal 12V in your calc. Under load your battery's sitting closer to 11.5V, sometimes lower if you've got Pylontechs being stroppy at low SOC.

So realistically:

2000W ÷ 11.5V = 174A before you even factor inverter efficiency (~85-90%)

Divide that by 0.85 and you're nudging 205A actual draw.

I run 2/0 AWG (roughly 70mm²) on my shepherd's hut inverter — Victron Phoenix 2000W — with a 250A MIDI fuse as close to the battery as physically possible. Cable run matters massively on 12V, anything over a metre each way and you're losing meaningful voltage before it even reaches the inverter.

Honestly if you can go 24V, just do it. Halves your cable headaches overnight.

@PeakVanLifer is absolutely right about not using nominal voltage — critical point. To expand on the fuse sizing side specifically:

Your calculated amperage is the continuous draw. Most inverter manufacturers recommend fusing at 125–150% of continuous rated current to account for startup surges. So for a 2,000W inverter at 10.5V worst-case:

2000 ÷ 10.5 = ~190A continuous → fuse at 250A minimum

I run a 3,000W Victron Multiplus in my shepherd's hut on 24V (much more forgiving), but I've sized fuses on 12V van builds before and the cable gauge bites hard. At those amperages you're looking at 2/0 AWG or 70mm² for any run over about 500mm — anything thinner and your voltage drop problem compounds catastrophically under surge loads.

Blade fuses are completely out — you want a proper ANL or MIDI fuse holder for anything above 80A.

What @LesWood78 and @PeakVanLifer have covered on fusing and voltage drop is solid — the piece I'd add specifically for EV charging applications is inverter efficiency loss.

Your 2,000W inverter isn't drawing 2,000W from the DC side — it's drawing 2,000W ÷ 0.85 (typical efficiency) = ~2,353W DC. Plug that into your real voltage figure (~11.5V under load) and you're looking at 204A, not the 167A the naive calc gives you.

That changes your cable sizing meaningfully. On my own 12V EV charging setup I run 70mm² welding cable for anything over 1.5m between battery and inverter — Fogstar cells, Victron MultiPlus. The derating for ambient temperature in an enclosed space bites harder than most people expect too.

Size the fuse to the cable, not the inverter rating.