Ground Cable diameter Lynx MultiPlus

Been staring at that Victron wiring diagram for the Lynx distributor setup and the one thing that always gets me is whether the ground cable spec is consistent with what the MultiPlus actually demands under load — because on paper it looks fine until your inverter decides to pull serious amps at 2am and you've undersized your negative return.

On my cabin setup I ran 70mm² for the ground throughout and honestly I'd not go smaller for anything above a 2kVA MultiPlus. The Lynx Power In makes it dead easy to keep things tidy but it does lull you into a false sense of security — neat terminals don't compensate for undersized cable.

Couple of questions worth throwing out to the group:

• Is anyone running the Class T fuse on both positive and matching the ground cable diameter, or are people still treating the negative as an afterthought?
• For a 12V MultiPlus 3000VA setup, the fault current potential is enormous — are folks actually calculating ground cable cross-section based on fault current duration, or just mirroring the positive?

The Victron diagram linked in similar threads tends to show the ground bonding point at the Lynx, which is fine, but in a motorhome chassis-earth situation things get murkier fast.

Would be good to hear from anyone who's had a Fogstar lithium pack in the mix too — BMS fault current behaviour changes the equation slightly compared to lead-acid.

Hey @WayneTaylor, good question. The Lynx distributor itself doesn't carry the negative through its busbar in the same way it handles the positive, so the ground path is slightly separate from what the Lynx diagram implies. Your MultiPlus ground cable should really match your positive battery cable spec - so if you're running 70mm² positive cables, run 70mm² negative too. The critical one people miss is the chassis bond from the battery negative, which wants to be at least 16mm² but ideally 25mm² for a MultiPlus 3000 and above. What size MultiPlus are you running? That'll determine the exact current rating you need to size against.

@MoorRoger makes a fair point about the Lynx negative path. What I'd add from setting up my static caravan system — the MultiPlus ground cable needs to match the positive cable spec exactly, so if you're running 70mm² positive you want 70mm² negative back to the busbar or battery negative terminal. Victron's own documentation is a bit vague on this but their tech team confirmed it when I asked directly.

One thing worth checking — are you running the Lynx BMS alongside it? That changes where your main negative terminates. What size inverter are you using, the 1200VA or something bigger? That'll determine whether 70mm² is even adequate or whether you need to step up to 95mm².

The ground cable wants to match your positive cable spec — if you've run 70mm² positive from the MultiPlus, don't go skimping to 35mm² on the negative because "it's only earth" or you'll be chasing voltage drop gremlins at 2am in a layby.

@GrumpySpanner is spot on about matching the spec, and I'd add a practical note from running a 3kVA MultiPlus on my narrowboat — the length of that negative run catches people out more than the diameter itself.

I had a situation where my earth return was travelling a long route back to the battery bank because I'd routed it neatly rather than directly. Voltage drop was causing the MultiPlus to behave oddly under load. Shortened the path, same 70mm² cable, problem gone.

Victron's own documentation is a bit vague on this but the principle is simple: treat that ground cable as equally important as the positive. Same cross-section, shortest practical run, solid terminations with proper crimped lugs — not the pinch-and-hope variety you find in bargain marine chandleries.

@GrumpySpanner and @HazelDawn have the matching spec covered well. One thing I'd add — don't forget the ground path includes the chassis bond in a motorhome setup. That joint is often the weak link, not the cable itself.

On my static setup I use a proper crimped lug with heat shrink, torqued to spec. Seen too many installs where someone's done a lovely 70mm² run then bodged a M8 bolt through a painted panel for the chassis bond. Kills the whole thing.

Victron's own docs are worth reading carefully — they're pretty explicit about earth/negative conductor sizing in the MultiPlus installation manual. Nothing revolutionary, just matches your positive spec as already said, but the termination quality matters just as much as the cable diameter.