Cable sizing for solar pump and MPPT controller display

Got my Victron SmartSolar humming away nicely in the van, but spent a genuinely embarrassing amount of time staring at cable runs before I finally committed — turns out "just wing it" isn't a valid sizing methodology, who knew.

Few things worth flagging for anyone else going through this:

The pump side will bite you if you undersize — inrush current on a 12V pump is no joke, and a voltage drop at that point means your pump either struggles or just gives up entirely like me on a Monday morning.

My rough rules of thumb for DC runs:

• Keep voltage drop under 3% for solar input runs
• Pump circuits? I'd argue even tighter — 2% if you can manage it
• Shorter runs win every time, so mount your MPPT as close to the batteries as is sensible

Been running 6mm² on my pump circuit and 4mm² for the MPPT to battery, which has been solid enough in the conversion — Fogstar 200Ah LiFePO4 handling the buffering nicely.

Anyone else gone bigger than "technically necessary" just for peace of mind? I've seen folks throw 10mm² at everything and honestly I respect the commitment.

Also curious whether anyone's noticed the MPPT display readings drifting when the pump kicks in — mine shows a brief voltage sag that I think is just the display catching the inrush, but would love to know if that's a known quirk with the Victron units or something worth investigating properly.

Drop your cable sizing setups below — always good to see what others are running in the real world rather than just the theoretical calculator outputs.

@MoorKev honestly same situation when I did my van build last year — ended up paralysed by indecision for days.

Can I ask what cable lengths you're working with between your panels and the MPPT? That's what really tripped me up. I kept finding conflicting advice about acceptable voltage drop percentages — some sources saying 3%, others saying 5% max on the DC side.

Also did you use a dedicated cable sizing calculator or just work it off tables? I tried a few online tools but wasn't sure which ones accounted for the derating you need inside a cable conduit or trunking properly.

Currently planning my second van build and want to get it right from scratch rather than retrospectively wishing I'd gone a size up on the wire gauge like last time.

@MoorKev the golden rule that saved my motorhome sanity: voltage drop ≤ 3% on DC runs, so grab the 12v cable sizing calculator on 12voltplanet.co.uk and stop eyeballing it like a nervous electrician.

Quick cheat sheet for common van solar runs:

Tinned marine-grade cable from Batt Man or BEP Marine is worth the extra quid — regular automotive cable corrodes faster than my patience in a Tesco car park.

@MarineGeoff makes a solid point on the 3% drop rule — I'd add that for the panel-to-MPPT run specifically, I tend to target ≤1% if the cable run is longer than 3–4 metres. The MPPT is already doing voltage conversion work; give it clean input to work with.

For my shepherds hut install I used 6mm² for the PV side (roughly 8m run, 400W array) and 10mm² on the battery side — the higher current post-conversion justifies upsizing there.

One thing often overlooked: crimped lugs rather than twisted-and-clamped conductors make a measurable difference in connection resistance over time, especially in a vehicle with vibration. Fogstar sell decent tinned wire that takes lugs well.

Victron's MPPT Connect app will actually show you input voltage under load — useful sanity-check once everything's wired.

@BordersExplorer that's a useful distinction on the panel-to-MPPT run — does that tighter tolerance apply even when the MPPT is doing its voltage conversion anyway? I'd assumed the controller compensates somewhat.

Asking because I'm mid-way through cabling a garden office setup (about 8m panel run from the roof) and I've been sizing based purely on the 3% rule. Currently spec'd 6mm² for that run but wondering if I should be going to 10mm² given the length. Running a Victron SmartSolar 100/30 if that changes anything.

Also, separate question — does anyone factor in future-proofing when sizing? Tempted to slightly overspec now rather than re-run cable later, especially buried conduit sections.