Cable sizing calculator — useful spreadsheet

Just found this thread whilst troubleshooting a dodgy connection in my van setup, and I've got to say — cable sizing is one of those things that looks boring until it absolutely isn't. Lost a morning last month tracking down voltage drop across undersized cables. Turned out I'd grabbed some old automotive cable that wasn't fit for purpose at all.

The spreadsheet approach is genuinely solid. I've been using a basic calc myself for the emergency backup system, mainly because I got paranoid after reading about someone's shed fire caused by improper sizing. Now I cross-reference everything twice — once by hand, once through the sheet.

What I'd add though: don't just plug numbers in and trust the output blindly. Understanding why you're getting the result matters. Ambient temperature, cable routing (bundled vs. spaced out), length of run — all of it changes the picture. The Victron documentation on this is actually quite good if you're willing to wade through it.

For the van, I ended up with a mix of proper marine-grade cable and some Fogstar components, which came with their own sizing guidelines. Made life easier not reinventing the wheel.

Has anyone else got experience with spreadsheets that specifically account for different cable types and insulation ratings? The standards shift depending on whether you're working with automotive, marine, or general industrial cable. Would be curious if there's a more comprehensive tool out there that handles all three.

Absolutely mate, cable sizing bites you hard when you get it wrong. I learned that the painful way with my garden office setup — undersized the DC run from battery to inverter and nearly melted the insulation.

The thing that caught me out was not accounting for voltage drop properly. Thought I could get away with smaller cable to save a few quid, but then the Victron was throttling everything and I couldn't work out why.

Are you finding the dodgy connection's actually a sizing issue, or is it genuinely just a loose terminal? I've had both, and the terminal failures are often harder to spot because they'll work intermittently. Might be worth measuring actual voltage drop under load if you haven't already — sometimes that's the quickest way to confirm whether the cable's the culprit or something else is going on.

The thing that got me was realising how much voltage drop I was losing across undersized cables from my array to the charge controller. I'd spec'd everything properly on paper, but hadn't accounted for the actual run length in my shepherds hut setup — ended up with nearly 3% drop that I could've eliminated for the cost of a couple of metres of 10mm² cable.

What made it worse was that it wasn't dramatic enough to trigger obvious failures, just a slow bleed of efficiency. @GafferTapeKing's point about it "absolutely not being boring" is spot on — that morning you lose tracking down why your system isn't performing as expected costs way more than getting it right first time.

I'd definitely recommend working backwards from your inverter draw rather than just going off solar panel specs. The calcs look tedious but they genuinely matter.

Had this exact nightmare on my narrowboat. Ran 6mm² cable from my Victron mppt to the battery bank—seemed plenty—but was seeing 0.8V drop under full charge current. Didn't sound like much until I realised my Lifepo4s were throttling the charger because they thought the voltage was higher than it actually was.

Swapped it for 10mm² and everything sorted itself. The spreadsheet approach is spot on because you need to factor in both distance and your actual current draw, not just what you think you'll theoretically use.

@QuietMaker, that voltage drop is insidious—easy to miss if you're only glancing at the numbers. Worth checking your array-to-charge controller run specifically, as that's where it tends to bite hardest.

Yep, voltage drop is a killer. I used a Victron calculator tool ages back on the boat and it was eye-opening—turned out I needed nearly double what I'd planned for my solar array run to the charge controller.

The other thing nobody tells you: it's not just about the distance, it's about the current. I was running a 3kW inverter off undersized cables and the whole system was throttling itself. Swapped to proper gauge and suddenly everything worked properly.

@RetiredNurse 6mm² sounds tight for a full battery bank setup depending on your cable length. How far is your run? If you're losing more than 3% voltage you'll notice it in charge times.

The spreadsheet approach is solid if you've got a decent one—saves the mental maths. Though honestly, once you've done it once or twice you don't forget the lesson.

Cable sizing really is the unsexy topic that bites you when you least expect it. I spent about three months wondering why my battery bank wouldn't charge properly before realising I'd run 4mm² from the array like some kind of optimist. Turns out voltage drop doesn't care about your hopes and dreams.

@RetiredNurse and @GoldenGa

Right, this is exactly what I've been wrestling with for my motorhome setup. Got a 200Ah LiFePO4 battery bank and initially thought I could get away with 10mm² from the charger to the batteries—turned out that was a recipe for voltage sag when pulling more than 100A during rapid charging.

The thing that clicked for me was actually running the numbers properly rather than guessing. Swapped it out for 16mm² and the difference was immediate—temps on the cable dropped noticeably and my Victron BMV stopped registering those odd fluctuations.

@GafferTapeKing, what gauge are you running in your van? The annoying bit is that most online calculators assume ambient temps lower than what you get stuffed in a motorhome locker in summer. I've started adding a safety margin for that alone.

Has anyone else found the Victron sizing tool actually conservative compared to real-world results, or is that just me being cautious?

Spot on about the boring-until-it-breaks thing. I learned that the hard way with my garden office setup—undersized the DC run from the battery bank to the inverter and got voltage drop that made the whole system feel sluggish.

The spreadsheet approach is solid, but honestly the Victron sizing tool is brilliant if you haven't tried it yet @GoldenGaffer. Saved me guessing.

@BevJackson64 with 200Ah LiFePO4 you'll want to be proper generous on cable gauge—those systems can pull serious current and voltage drop becomes noticeable fast. What amperage are you planning for your main inverter feed? That's usually where people slip up.

The annoying bit is working backwards from what you think you need versus what actually works in practice. I'd rather oversize and have headroom than spend months troubleshooting phantom voltage issues.

@GafferTapeKing, glad you've got it sorted! Cable sizing really does catch people out, especially in vans where space is tight and the temptation to "just use what fits" is strong.

The thing that gets folks is that undersized cables don't always fail dramatically—they just run hot, lose efficiency, and gradually degrade. In a van with limited ventilation, that's a recipe for trouble.

@BevJackson64, if you're working with 200Ah LiFePO4, you'll want to be generous with your sizing if you're planning high discharge rates. The spreadsheet approach is genuinely helpful because it forces you to think through your actual requirements rather than guessing.

My tip: work backwards from your maximum expected current draw, not just your battery capacity. If you're ever going to pull 150A from that bank (even briefly), your cables need sizing for that, not for some theoretical continuous draw.

What gauge did you end up settling on for your setup?

I'm wrestling with this exact problem at the moment with my static caravan setup. Got a 5kW Victron inverter and I've been second-guessing whether my existing 16mm² cable run from the battery bank to the main breaker is adequate.

The thing that's caught me out is that most calculators assume ambient temperature of 30°C, but in winter my caravan sits much colder. Does anyone know if that actually makes undersizing safer in those conditions, or is it still conservative to just go one size up regardless?

Also, @BevJackson64 — with your 200Ah LiFePO4, what gauge did you end up running? I'm trying to avoid the false economy of undersizing now, but equally I don't want to rip everything out if 16mm² is genuinely sufficient.

The spreadsheet approach sounds useful though. Beats trying to calculate voltage drop in my head at midnight when something's gone wrong.

Right, the real killer is voltage drop over distance. In a van you're often running 48V battery to inverter, so undersizing cables looks fine until you're watching your Victron derate under load. @SueJohnson — for 5kW on 48V, you'll want proper 25mm² minimum if the run's more than a couple of metres. Use the actual spreadsheet to check your specific distances.

Learnt this the hard way with my first solar array — ran undersized cable from battery bank to Victron and the voltage drop was absolutely costing me efficiency. Rewired everything at 48V and the difference was night and day. @Boycie's spot on about distance being the killer. Worth spending the extra quid on proper gauge cable upfront rather than losing power month after month.