Multiplus Parallel - additional cable to slave

Been doing some research on parallel Multiplus setups and could do with some collective wisdom here.

I'm looking at running two Multiplus units in parallel on my motorhome build — likely 24V/3000VA units, similar to what I've seen discussed elsewhere. I understand the master/slave relationship and the RJ45 comms cable between them, but I keep seeing conflicting information about whether you need additional DC cabling from the batteries to the slave unit, or whether the slave can just pull current through the master.

My understanding is that both units need their own properly rated DC cable runs directly to the busbar — you can't just daisy-chain from master to slave and call it done. Is that correct? Seems obvious from a current-handling perspective but I want to confirm before I start cutting cable.

A few specific questions:

• Does the slave unit need its own dedicated fusing/MCCB, or can they share protection?
• Is there anything specific in VE.Configure or VictronConnect I need to set differently for the slave, beyond the standard parallel configuration?
• Any gotchas with the comms cable — length limits, quality of cable matters?

Running a Fogstar Drift LiFePO4 bank so the charge parameters need to be spot-on across both units equally.

Has anyone actually built a parallel Multiplus setup in a motorhome or similar space-constrained install? Would love to know how you managed the physical cable routing — it gets tight quickly.

@WattLiz running twin 3000VA on 24V in a motorhome is a decent shout — I did something similar on my static caravan build last year.

One thing worth confirming early on: both units need identical firmware before you even attempt pairing via the RJ45 cable. Victron's documentation mentions this but doesn't shout about it loudly enough.

Also worth asking — have you sized your DC cabling symmetrically between both units? Equal cable lengths from the busbar to each Multiplus matters more than most people expect. Unequal resistance causes one unit to shoulder disproportionate load.

What battery chemistry are you running? LiFePO4 with a Victron-compatible BMS changes the configuration somewhat compared to AGM.

@WattLiz one thing that caught me out when I was researching parallel Multiplus setups for my static caravan — the interconnect cable between master and slave needs to be kept as short as physically possible, and identical length on both DC positive and negative runs to each unit. Victron are quite specific about this in their parallel wiring guide.

Are you planning to use a shared DC busbar arrangement? That's generally the cleaner approach rather than running separate cables back to the battery from each unit. What cable cross-section are you working with? At 24V/3000VA you're looking at serious current on those DC cables and most people underestimate it badly first time round.

Something worth flagging that I don't think's been mentioned yet — make sure your DC cabling between master and slave is absolutely symmetrical in length and cross-section. Even a small difference in resistance can cause one unit to carry a disproportionate load, which throws off current sharing and can cause weird behaviour or faults.

On my cabin build I used a proper busbar arrangement rather than daisy-chaining the battery connections, which kept everything balanced. Victron's own documentation is quite specific about this but it's easy to overlook when you're deep in the build.

Also worth checking that both units are the same firmware version before commissioning — paralleling mismatched firmware versions has caused headaches for a few people I've seen post about it here.

@WattLiz one thing I'll add from bitter experience — when I paralleled two Multiplus units on my boat build, I massively underestimated how critical the battery cable lengths are between units. Both cables from master and slave to the busbar need to be identical length and cross-section, not just roughly similar. I used 16mm² when I should've gone 25mm² and ended up with the slave unit running noticeably warmer than the master under heavy load.

Victron's own documentation mentions this but buries it somewhat. Worth reading the parallel installation guide properly before you start cutting cable — not something I did first time round, to my cost.

Also make sure your RJ45 interconnect cable is the proper Victron one, not a generic ethernet lead. Learned that the hard way too.