What size fuse for my solar charge controller?

Just upgraded my motorhome setup and gone for a 400W Renogy panel feeding into a Victron MPPT 100/20 charge controller. Need to sort the fusing between the panel and controller.

Looking at the Renogy documentation, they're suggesting something around 25A but I want to make sure I'm not undersizing it. The panel's rated at about 13A max under ideal conditions, so would a 20A fuse be safer, or should I stick closer to spec?

Also — do I need fusing on both sides (panel to controller and controller to battery bank)? Currently running a 200Ah LiFePO4 battery, so the battery side's obviously a bigger concern for fault protection.

Cheers for any guidance. This is my first proper solar setup and don't want to mess it up with dodgy fusing. I've got the DC wiring sorted with 10mm² cable if that matters for the sizing.

Cheers!

The Renogy 400W panel specs will tell you the short-circuit current (Isc) — usually around 13-14A for that wattage. You'll want a fuse rated for 20A between panel and controller, which gives you a proper safety margin without nuisance trips.

I've got a similar setup in my array (mixed 400W panels with a Victron 100/30), and 20A DC-rated fuses have been rock solid. Make sure you're using proper DC-rated fuses, not automotive ones — the arc characteristics are completely different and matter when you're dealing with higher voltages.

Placement-wise, fuse holder should sit as close to the positive terminal of the panel as physically possible. Running mine now for three years without a single issue, which isn't thrilling but exactly what you want from fusing.

What cable gauge are you running to the controller?

@DorsetExplorer the Victron manual's your best mate here — it'll specify the DC-side fuse rating. For a 400W panel you're looking at roughly 20A, maybe 25A to account for panel degradation over time (they're optimistic about their specs, bless 'em).

Mount it as close to the panel as physically possible — ideally within 30cm — otherwise you're just asking for a crispy cable if something goes wrong. Use a proper solar-rated fuse holder, not some bodged automotive nonsense.

Also check your cable gauge while you're at it; undersized cable + high sun = expensive regrets.

@DorsetExplorer the Victron manual specifies 32A DC-side fusing for the 100/20, but that's the controller's input limit — you actually want to size the panel-side fuse based on your specific panel's short-circuit current.

That Renogy 400W will have an Isc around 13A as @LiFePO4Nerd mentioned. Standard practice is to rate your fuse at 125% of Isc, which puts you at roughly 16-17A. I'd run a 20A fuse between the panel and controller — gives you headroom without being unnecessarily oversized.

The critical bit: ensure the fuse holder itself is rated for the DC voltage and current. I made the mistake early on using an automotive-style fuse holder which struggled with the voltage spikes. Get a proper solar-rated fuse holder with strain relief.

Also verify your cable gauge — for a 20A circuit at reasonable distance you'll want 6mm² minimum, ideally 10mm² if your run's over a few metres. Undersized cable and undersized fusing is a

@DorsetExplorer I've got a similar setup in my shepherds hut so can share what I learned the hard way. The thing is, you need to size the fuse based on the panel's short-circuit current, not just what the controller can handle.

For a 400W Renogy, you're looking at roughly 13A Isc. The rule of thumb is to use a fuse rated at 125% of that, so you'd want a 16A or 20A DC fuse between panel and controller. This protects the wiring from your array during a fault.

The Victron 100/20 itself has internal protection, but it won't protect the cables feeding into it if something goes pear-shaped on the panel side.

I'd grab a 20A ANL fuse holder with a 20A fuse — they're cheap as chips from any solar supplier and give you headroom. Mount it as close to the panel as practically possible. Saved me from a cable fire last winter when a connector corroded.

What gauge wiring are you running between panel and controller? That'll also

The manual spec is a good starting point, but here's what actually matters: work backwards from your panel's short-circuit current (Isc), not the wattage.

Your 400W Renogy will be around 20-22A Isc depending on conditions. The rule of thumb is fuse at 1.25x that figure, so you're looking at roughly 25-30A. The Victron 100/20 can handle up to 20A input anyway, so you're actually limited by the controller, not the panel.

I'd go 25A as a sensible middle ground — gives you some headroom without being daft about it.

Key thing: make sure your DC cabling between panel and controller is properly rated for that current too. I made the mistake of skimping on cable gauge early on and it caused voltage drops that were doing nobody any favours.

Use a quality DC breaker or fused disconnect (Victron make solid ones), and keep it as close to the panel as you can manage. Makes troubleshooting miles easier when something goes sideways.

Right, I've been down this exact road with my shepherds hut setup, and it caught me out initially. The 400W Renogy will give you roughly 10-12A under ideal conditions, but here's the thing—short-circuit current is what matters for fusing, not nominal output.

Check your panel's datasheet for Isc (short-circuit current). Renogy panels typically run around 12-13A Isc, so you'd size your fuse at 125% of that—call it 16A. That's your sweet spot between protection and not nuking the fuse on cloud cover.

@LakelandNomad's right about the controller's 32A limit, but that's the absolute ceiling. You want the fuse sized to protect the cable run, not the controller. What gauge wire are you running between panel and controller? If it's undersized, a smaller fuse makes sense.

Mount it as close to the panel as physically possible—ideally within 45cm. I use DC-rated ANL fuses with proper holders. Saves a lot of headaches when something goes sideways

Mate, for a 400W Renogy into that Victron MPPT you're looking at roughly 25A nominal, but the key is the short-circuit current — check your panel specs, it'll likely be around 13-14A Isc.

Rule of thumb: fuse at 125% of Isc minimum, so you'd be looking at a 20A fuse as your sweet spot. Victron's own guidance backs this up.

What matters more than the exact rating though is getting a DC-rated fuse (not automotive), proper inline holder, and keeping the cable run short. I learned this the hard way with my boat setup — too long a run and voltage drop becomes a nightmare.

What gauge cable are you running between panel and controller?

The 400W Renogy's short-circuit current is the real gotcha here — you're looking at roughly 30-32A Isc in ideal conditions, so a 40A DC breaker between panel and controller won't leave you scrambling when the sun hits just right.

I ran the same setup in my motorhome and went 50A initially, which is honestly overkill but gives you headroom if you ever stack panels. The Victron manual will say 20A matches the controller's input rating, but that's continuous — your fuse needs to handle transient spikes without nuking every time a cloud clears.

Use a DC-rated breaker or fused disconnect (not automotive blade fuses — they're too slow). Midnite Solar or similar quality gear actually survives the journey.

@DorsetExplorer, also double-check your cable gauge from panel to controller — undersized cable + high inrush = expensive magic smoke.

@DorsetExplorer — you'll want to size for Isc, not nominal. Renogy's spec sheet lists around 32A short-circuit current on that panel. I'd go with a 40A DC breaker between panel and controller as your safety margin. Been running similar on my narrowboat setup for two years without drama.