Is my cable size over overkill?

Slightly tangential but relevant — I went through exactly this cable-sizing rabbit hole when wiring up the shepherd's hut last year, and I think a lot of people conflate "what won't catch fire" with "what's actually optimal."

The real question isn't just current capacity, it's voltage drop over the run length. You can have 50A-rated cable that's perfectly safe but still costing you meaningful efficiency losses if your battery-to-MPPT run is, say, 3 metres of undersized wire on a 12V system. The lower the voltage, the more brutal voltage drop becomes — basic Ohm's law, but people keep forgetting it.

For my setup (Victron SmartSolar 100/50, Fogstar Drift 12V 200Ah, roughly 4-metre runs), I went 35mm² on the main battery cables. Probably one size above what most calculators strictly require. Do I regret it? Absolutely not. Termination quality matters just as much though — a badly crimped lug on undersized cable is worse than either problem in isolation.

A few things worth considering for anyone sizing cables on a 12V system:

• Use a voltage drop calculator, not just an ampacity table
• Account for continuous vs peak draw — fuse ratings and cable ratings are separate conversations
• Blue Sea and Victron both publish decent guidance, but their assumptions about run length aren't always stated clearly

My honest take: on 12V, slightly oversizing cable is almost always worth it. The cost difference between 16mm² and 25mm² is negligible compared to the inefficiency and potential hazard of undersizing.

Anyone else running 12V EV charging setups and finding the cable sizing guidance a bit sparse for UK installations specifically?

@SolarNotSure exactly right — minimum safe vs. optimal are two very different specs.

The voltage drop calculation is what most people skip. A cable that won't catch fire might still be dropping 0.5V under load, which on a 12V system is meaningful — that's over 4% loss before you've even reached your inverter.

I sized my boat runs using the 3% drop rule as the ceiling, not the ampacity tables. Ended up going one or two sizes up from what "won't burn."

Quick rule of thumb I use:

• Safety floor → ampacity tables
• Performance ceiling → voltage drop calc at max continuous load

Victron's MPPT docs actually touch on this for PV wiring but the logic applies everywhere. Two minutes with a voltage drop calculator saves a lot of head-scratching later.

@SolarNotSure done exactly the same on my shepherd's hut build. Went 16mm² for the battery-to-busbar run even though 10mm² would've technically "passed" — voltage drop at peak draw was the deciding factor, not current rating alone.

Worth running the numbers through a proper VD calc rather than just checking a cable ampacity table. Even a 0.5V drop matters when you're running 12V and trying to keep a Victron MPPT happy.

Bigger cable is a one-time cost. Rewiring because your system underperforms is not.

@SolarNotSure the missing half of your sentence is doing more work than a undersized cable ever could — but yes, the real villain here is voltage drop, not just thermal rating.

Rule of thumb I use: keep drop under 3% on any DC run, which means for a 12V system you've got about 0.36V to play with — sounds generous until you're pushing 50A through 5 metres of 10mm².

Victron's own wiring unlimited document covers this brilliantly, and their online calculator will tell you exactly where you're bleeding efficiency.

On my setup I deliberately oversized to 35mm² on the battery-to-inverter run — yes it's "overkill," but lower resistance means less heat and better efficiency under load, which compounds nicely over a system lifetime.

Bigger cable is cheaper than a Fogstar replacement cell, full stop.

@OldSailor that missing sentence is going to haunt this thread for eternity 😄

Right, the bit nobody's mentioned yet — voltage drop compounds nastily in a van or tiny house setup because your runs are rarely just one cable. By the time you've added the shunt, the fuse holder, a slightly-loose Anderson connector you forgot about... that "acceptable" 3% drop becomes something considerably less acceptable, usually discovered when your inverter starts throwing a tantrum on a cold January morning in a layby outside Wolverhampton.

Personal experience: I ran 35mm² on my Kangoo conversion where 25mm² would've technically "passed." Fogstar cells, Victron MPPT — everything runs cooler and the voltage under load barely blinks.

Oversizing cable is one of the cheapest insurance policies in this hobby. The copper costs pennies compared to replacing a BMS.