Attaching 100/30 MPPT to battery

Been wrestling with this one on my boat build recently, so good timing on this thread.

The Victron SmartSolar 100/30 manual specifies cable cross-section in mm² rather than AWG gauge (as it should for us lot), recommending 16mm² for the battery connection — which is roughly equivalent to the #6 AWG that gets thrown around in American discussions online. Worth being aware of that conversion when you're trawling forums for advice, because it causes a fair bit of confusion.

For my setup I went with 16mm² flexible marine-grade tinned copper from Batt Man, kept the run as short as physically possible — maybe 300mm from the controller terminals down to the busbar. Tinned copper is worth the extra few quid if you're in a damp environment like a boat or shepherd's hut where condensation gets everywhere.

A few things I'd flag:

• Fuse as close to the battery as possible — I used a 40A ANL inline
• Victron's own terminals on the 100/30 aren't enormous, so double-check your crimps fit before buying bulk cable
• Flexible stranded cable is definitely preferable over rigid — easier to route and less stress on the terminals with any vibration

One thing I'm still unsure about is whether there's any real benefit going over 16mm² on a short run, or whether that's just overkill. The controller only outputs 30A max so it seems unnecessary, but curious what others are doing.

Anyone else running this controller? What cable spec did you end up with?

@VoltChloe the manual's mm² spec is the right starting point but don't neglect the actual run length to your battery bank — that's where people come unstuck.

On my van build I ran 35mm² for a relatively short 800mm run to the Fogstar Drift 200Ah, but I've seen folks online use "technically adequate" 16mm² over longer distances and wonder why they're dropping voltage under load.

Worth also mentioning: the 100/30's battery terminals are rated for specific torque — it's in the manual and people routinely ignore it. Over-tightened or under-tightened lugs cause more grief than undersized cable in my experience.

Crimped lugs rather than soldered on that gauge cable, and keep the run as short as physically possible. Every extra 100mm matters more than most realise.

@KentVanLifer is right about run length being critical, but I'd add — voltage drop calculation is only half the picture. With a 30A MPPT you're looking at 16mm² minimum for anything over about 1.5 metres one-way run, and that's assuming decent tinned marine-grade cable rather than the cheap stuff that degrades in damp environments (particularly relevant on @VoltChloe's boat build).

Also worth considering: the 100/30's battery terminals will only physically accept up to a certain lug size, so if you're running 16mm² or 25mm² you'll need appropriate bootlace ferrules or copper tube lugs, crimped properly with a ratchet tool — not a hammer and a prayer.

In my tiny house setup I used Victron's own cable recommendations as a floor, not a ceiling, and upsized accordingly.

Great points from @KentVanLifer and @WheresMeWires67. One thing worth adding for a boat install specifically — marine environments demand tinned copper cable rather than standard automotive or domestic wire. The humidity and salt air will corrode untinned copper surprisingly quickly, even in enclosed runs. You'll often find the degradation hidden under the insulation where you can't see it until you've got a proper voltage drop problem or a fire risk on your hands. Ancor and Belden both do decent tinned marine cable in the sizes you'll need for a 30A MPPT. Also make sure your terminal connections are properly crimped with heat-shrink adhesive-lined terminals rather than soldered alone — solder joints can crack with vibration over time on a moving vessel.

@ShaunKelly87 is probably going to mention tinned copper — and rightly so — but nobody's said yet that the 100/30's battery terminals are only rated for 16mm² max, so going massive on cable won't help if you're fighting a bootleg ferrule into an undersized terminal block; use the correct crimped lugs and a proper torque driver (4Nm per the manual), otherwise your "perfectly sized" cable becomes a resistance heater at the connection point itself — seen that exact failure mode more times than I'd like to admit in my motorhome before I wised up.

@FormerCop good shout on the battery terminals — I actually made a right mess of mine first time round on the cabin setup. Ended up with a loose connection that was arcing slightly, only caught it because the wire was warm to touch.

For the boat I went 16mm² on the battery side, kept the run under a metre. Picked up decent cable from Batt Man but honestly Fogstar's own recommendations are worth a read if you're running their cells — they're pretty specific about it.

One thing nobody's mentioned — fuse placement. As close to the battery positive as possible, not halfway down the run.