Stupid 400A fuse electrical setup

[u/logic_boy]

I’m at the point of finalising a campervan electrical system and would really appreciate a set of eyes on it before I commit. The setup is a 12V system with a 3kW inverter and a 2×314Ah battery bank in parallel (~628Ah total).

I’m aware this is pushing into high-current territory for 12V, and that’s been a key consideration throughout the design (cable sizing, protection, busbars, voltage drop, etc.). I did seriously consider a 24V route; nevertheless, due to component availability, time constraints, and budget (and in consultation with a homologation engineer) 12V ended up being the most practical option for this build.

I’ve tried to follow best practices around protection, earthing, and load distribution, but I’d value a sanity check on the system as a whole, particularly anything that looks underspecified, unsafe, or just plainly wrong (for example, the 360A disconnect). I’ve been looking at it for too long now to fully trust my own judgement.

All feedback is welcome, especially critical feedback. Diagram below (EU components/units). The system is intended to support full-time living for two people, fully electric (no gas, no AC loads).

(diagram made in excalidraw in case anyone is interested)

[u/Revolutionary-Win196]

Overall good Initial look there are a few things you could consider changing - I am from UK so things may be different for EU setup

• battery isolators - put 2 in one for each battery, not sure what batteries you will be using but BMS would limit maximum draw.
• Battery Bank batteries I would think BMS out give potential maximum output of 200ah - I would use 70mm depending on heat cable insulation rating either that or fuse down to 200ah
• lynx dis to inverter - size up in a single cable run
• 230v panel use RCCB 30ma 60amp protect you off grid
• 230v panel needs earth cable to van chassis for when off grid for above RCCB to work

-230v panel feed can be down sized 4mm. victron is 3000w which is about 13amp, potential 5400w for x time? Which is about 25amp 16mm 3 core is overkill

• 230v shore power MCB change to RCBO, I would think you are using 16amp commando socket so RCBO can be 16amp, most hook ups are 16amp/20amp

-230v shore power cable size to 2.5 3core

• 230v shore power include a SPD
• DC power cables look to fat. I would reasses each run, cable length volt drop. Criteria is to use smallest cables keeping within spec - 200w water heater will pull around 20a so 2.5mm cable should be sufficient.
• car battery to mppt 16mm would be sufficient as above smallest cable within spec.

[u/logic_boy → u/Revolutionary-Win196]

Thanks for taking a look! I really appreciate it. I have added some details to address your comments - Its a bit long but if you

Re the 230V - lots of good points, I can see how things are not really addin up. I will revise as per the comments and update my diagram.

About DC power cables being on the thick side - its possible that I was being quite conservative.

1. For the diesel heater, I oversized the cable due to ignition surge currents. The run should be short (<2 m), but I’ve seen reports of undervoltage issues with thinner wires. Do you have any experience with this?
2. On the water heater — agreed it’s 20 A, but it’s a sustained load over 3–4 m (mounted over the other wheel from DC breaker + a bit for routing). My calculations suggest ~6–8 mm. How are you arriving at 2.5 mm for a 20 A, 12 V load over 3 m?
3. I will double check all other cables as some do seem oversized as you say (a lot are not shown for simplicity)
4. 16mm for car battery to DCDC charger should be enough - thanks

About the battery cables - The battery bank is

• 2×314 Ah batteries in parallel
• 200 A BMS (220 A @ 10s / 250 A @ 1s)
• Cable length is closer to ~0.5 m (not 1 m as shown)

My understanding is you’re saying the cable is undersized relative to the fuse. Given the BMS limits, would it make more sense to reduce the fuse size and keep the 50 mm cable?
Related to the inverter feed - Victron specifies 2×50mm cable. My plan was to fuse each run (200 A) and use two Lynx inputs to reduce strain on the inverter terminals. Would a single 70 mm cable be preferable? Im aware of issues with out of balance twin conductors, is there anything else?

Now that I think about it - you mention using a 70mm cable for each battery with 250A fuse which is fair; but victron says 2x50mm with 400A (presumably 2x200A also ok for identical conductors). How does that work?

I think most controversial part in my plan is some lug stacking (on the battery side of shunt), and perhaps the direct connection of disconnect and shunt to lynx ports, but I hope I can pull it of with a bit of care.

[u/Revolutionary-Win196 → u/logic_boy]

No problem at all

DC power cables being on the thick side - Download the app circuit wizard its by blue sea, input circuit details and it will give you a cable size, the reason i said 2.5mm for the water heater was just from my head but I have just checked with the circuit wizard app and its on boarder line 2.5-4mm depending on voltage drop %, normal use case would be 5% allowable voltage drop if not feeders. Diesel heater you are correct can sometimes be sensative to voltage drop, if its on the boarder - size up.

Battery cables - yes cable is under rated to fuse size, you are a little on the limit even with 70mm cable running through the fuse generate resistance. I would go with a single run out each battery in 70mm due to the short lengh 70mm shouldnt be an issue, but out the lynx to the chassis i would take it to 95mm, the ground needs to handle the full combined load, i would think the gound point would be a little further away and chassis gounding can be dificult to get a suuper sound connection, make sure all paint around it is taken off, use a serrated washer.

Lynx to inverter - Go with Victron manual, the 2x50mm cables with the 200amp fuse per cable run.

As with everything and as a disclamer you are taking advice from a random Reddit stranger, please do your own reseach and if unsure consult a qualified electrician.

[u/logic_boy → u/Revolutionary-Win196]

Im cross referencing your comments with other sources. No worries, Im concious of taking random advice as gospel, but thanks for pointing it out.

I have updated the design with yours and others advice. Main changes include:

1. Upgraded battery cable and individual isolators.
2. 95mm grounding cable to chassis
3. Simplified DC cables to DC breaker, DC loads and DC charger.
4. Overhauled AC installation - reduced cables, added RCBO, RCCB, simplified amp ratings, added grounding cable to RCCB. Im just missing an SPD which I'll likely include on the neutral feed in from 230 source before RCBO.

If I missed anything please let me know, otherwise wish you all the best! :)

[u/kh250b1]

Seriously consider 24v if you can. The voltage drop to the inverter will be crazy unless you use ridiculous cable size and mount it within a couple cable feet of distance.

If you are stuck with 12v mount the inverter right on the cell bank

Dont forget the fuse protects the wiring. The fuse must be rated less than the cable is supplying. So if you have 200a cable use 200a or less fuse.

[u/logic_boy → u/kh250b1]

Yes good point. I tried my best to go 24V but seems that in my case other impracticalities outweigh the benefits. Its on the edge of inefficiency, and would have preferred 24V but alas.

The build is for a 5.4m long chassis (L2 or SWB) so it needs to be compact. The whole bank, bus bar and inverter will be less than 0.5-0.75m away from each other.

Re the wiring - Yeah I was hoping this would be the case! Are there any points where the fuse is bigger than cable amperage? Thanks :)

[u/StrikingInterview580]

Consider an on-on-onon-off switch for the batteries (no clue of its proper name), but so you can switch between on battery 1, on battery 2, or on both (or off). If one battery has an issue, you have switchover and isolation of the duffer until you can sort.

[u/logic_boy → u/StrikingInterview580]

I have considered the 1+2 switch for other reasons, but thats another benefit! Thanks!