Cable size for solar connection

Been through this exact head-scratch myself when I wired up my first array, so worth laying it out properly.

For a 100/15 MPPT paired with a single 200W panel, you're looking at relatively modest current — roughly 5-6A depending on your panel's Isc. In theory 4mm² handles that comfortably, but I'd still argue it's worth thinking about voltage drop over cable run length rather than just raw current capacity.

Here's my rule of thumb:

• Short runs (under 5m): 4mm² is fine
• Runs between 5-10m: stick with 4mm², but keep an eye on your volt drop calculations
• Anything over 10m: seriously consider jumping to 6mm²

Voltage drop on the PV side hurts efficiency more than people realise, especially on cloudy UK days when you're already scraping watts.

I use twin core 6mm² solar cable (the proper UV-resistant stuff, not just flex) throughout my setup regardless, partly because it future-proofs things if I expand the array later. Picked mine up from BQ Cables — decent pricing compared to some of the inflated solar-specific suppliers.

Also worth double-checking your panel's datasheet for Voc and Isc before finalising anything. The 100/15 has a 100V max input, so a single 200W panel is well within limits, but good habit to verify.

What's your cable run length looking like? That's really the determining factor here. Others may have different preferences — be interesting to see what setups people have gone with.

@Spider12 nothing like an unfinished sentence to really build the suspense 😄 we're all sat here on the edge of our seats...

But yes, cable sizing — genuinely important and underrated. The golden rule I learnt the hard way: don't cheap out on cable runs. I used some dodgy unbranded stuff on my first

@Spider12 classic cliffhanger 😄 while we wait for the rest...

For a 100/15 with a single 200W panel you're typically fine with 4mm² on the PV side. I ran 6mm² on mine just because I had it spare and the run was about 8 metres — always worth upsizing if your cable run is longer than 5m to keep voltage drop sensible.

Key things to check:

• Fuse/breaker on the PV line close to the panel
• MC4 connectors rated to match your cable size
• Victron's wiring unlimited PDF is genuinely brilliant for sizing calcs

Don't forget the battery cable is often undersized by people — that 15A charge current needs proper treatment too.

@Spider12 and @BirchLover are basically writing the world's most suspenseful solar novel over here.

Anyway, while we wait for the thrilling conclusion — 4mm² cable will see you right for that setup in most real-world installs, assuming you're not running ridiculous lengths. Keep your runs under ~3-4 metres each way and your voltage drop stays sensible.

My static van runs a similar config with Fogstar cells and a Victron 100/15, used 6mm² purely because I had a roll of it doing nothing and I'm not made of restraint.

Rule of thumb: oversize your cable, never undersize it. Cable is cheap. Melted connectors and a wrecked MPPT are not.

MC4 connectors rated properly too — don't go buying the dodgy no-brand ones off the bay.

Wired my static caravan's garden office with a 100/15 and honestly just ran 6mm² everywhere from panel to controller and slept soundly — overkill maybe, but copper's cheaper than a meltdown and I already had a Fogstar bill that month giving me palpitations.

For short runs (under 3m) 4mm² is technically fine for a single 200W panel, but the moment you're routing it through any kind of conduit or the sun decides to actually show up in Britain for once, the extra headroom in 6mm² earns its keep.

Key thing @Spider12 and @BirchLover are presumably building up to: always size for the controller's max input current, not the panel's rated output — the 100/15 can handle 15A on the battery side, work backwards from there.

@BorderVanLifer 6mm² everywhere is fine but you're basically using a sledgehammer to crack a nut — for a single 200W panel the actual current rarely exceeds 12A, so 4mm² copper comfortably handles it with headroom, assuming you're not running absurd cable runs where volt drop becomes the villain of this particular story.

Rule of thumb: calculate your Isc (short-circuit current, on the panel datasheet), multiply by 1.25 as your safety factor, then size accordingly — and always factor in cable run length before committing, because a 10m run behaves very differently to a 2m run.

My Victron SmartSolar on the motorhome runs 4mm² no bother whatsoever.