Victron Multiplus 12/3000 keeps throwing overload fault on startup — is my cable run too long?

Fitted a Multiplus 12/3000/120 in the motorhome back in March. 200Ah of Fogstar Drift LiFePO4, Victron SmartShunt, and a 4m cable run from the battery to the inverter using 70mm² welding cable. Terminations are all done properly with copper lugs and a 300A ANL fuse inline. On paper, that should be plenty.

The fault keeps appearing on heavy startup loads — specifically a 2kW induction hob and occasionally even the microwave on first start. The Multiplus throws an overload LED flash sequence (5 flashes, so overload/high temp according to the manual) and shuts down for 30–60 seconds before recovering. Ambient temp in the hab is around 18°C so I don't think thermal is the culprit.

My suspicion is voltage sag on that 4m run. Even at 70mm², a 250A inrush over 4m each way is going to drop voltage noticeably, and if the Fogstar BMS is also twitching under the spike, the Multiplus could be seeing a low-voltage condition and interpreting it as overload rather than undervoltage — not sure if that's how the fault logic works though.

Has anyone else had this with the 12/3000 specifically? I'm wondering whether shortening the cable run is worth the hassle, or whether tweaking the PowerAssist settings or the low-battery cutoff threshold in VictronConnect would actually address it. Open to suggestions before I start ripping out cable trunking.

AndyReid | 847 posts

@Curly, 4m is getting on a bit for 70mm² at 12V — that's where I'd start looking. At 3000W you're pulling 250A+ and voltage drop over that run could be tripping the low voltage protection on startup, which often presents as an overload fault.

Worth checking a few things: are both positive and negative runs 4m each (so 8m total)? That doubles your resistance. Also, what are your low voltage disconnect settings in the Victron Connect app?

Honestly for that cable length at 12V, I'd be looking at 95mm² minimum, ideally 120mm². The Multiplus has a brutal inrush on startup.

What size fuse/breaker are you running between battery and inverter? That could be nuisance-tripping too.

AlexYoung76 | 312 posts

@Curly — just to add to what @AndyReid is getting at, the Multiplus 12/3000 can pull serious surge current on startup, we're talking potentially 500-600A briefly depending on the load. Even with decent 70mm² cable, 4m total run (both positive and negative combined, I assume?) creates enough voltage drop that the unit sees what looks like a low-voltage/overload condition before it's even properly loaded.

What's your battery BMS doing during that spike? Fogstar Drift has a decent BMS but if it's momentarily disconnecting under that inrush, the Multiplus will fault every time. Worth checking the SmartShunt history for any voltage dips below 10V during startup attempts.

Quick test — does it fault immediately at switch-on, or only once a load is connected?

EdKelly | 1,203 posts

@Curly — just to add to what @AndyReid is getting at, the startup surge is the killer here. The Multiplus 12/3000 can pull well over 300A in those first few milliseconds, and any extra resistance in that cable run will cause the voltage to sag badly right at the inverter terminals. That trips the low voltage protection before it's even properly started.

Worth checking your terminal connections too — loose lugs on welding cable are surprisingly common and will compound the problem significantly. Have you got a multimeter? Measure the voltage at the inverter terminals under load rather than at the battery — that'll tell you immediately if the cable is your culprit. If you're seeing more than a 0.5V drop you've found your issue.

DailyDream | 2,156 posts

@Curly — one thing nobody's mentioned yet: check your terminal connections at both ends before you do anything else. Welding cable is brilliant stuff, but if those lugs aren't properly crimped or the bolts aren't torqued down properly, you'll get resistance that makes even a good cable run behave like a longer, thinner one. A dodgy connection can cause exactly the voltage sag that triggers overload faults on startup.

Also worth checking — has the Fogstar BMS been configured correctly? Some LiFePO4 packs will throttle current output if the BMS hasn't been set up for high-draw scenarios, which again looks to the Multiplus like a voltage collapse under load.

Sort the connections first, it costs nothing and you might be surprised. 👍

DevonCruiser | 847 posts

@Curly — worth checking what transfer switch setting you're running in VE.Config. There's a parameter called "Dynamic current limiter" that can help with motor loads on startup, but more relevantly, have a look at your "AC output current limit" — if it's been left at a conservative value it'll fault before the surge even has chance to settle. Also, have you ruled out the BMS on the Fogstar doing a momentary disconnect under that inrush current? The Drift's BMS has a peak discharge rating and if your cables have any resistance adding voltage drop, the BMS might be seeing a bigger sag than you'd expect and cutting out briefly — which the Multiplus then reads as an overload condition. Worth watching the SmartShunt data during a fault to see if you get a corresponding current spike.