size of fuse after a smart solar 100/50

Trudy · June 6

Hello all, what size fuse is best to install between the smart solar 100/50 and the lithium battery bank? I am 2nd owner of a 2025 Tab 400 that has 2 Smart Solar MPPT 100/50 charge controllers. They both had 30amp fuses after them, one of them blew and melted the plastic fuse holder. So I will replace that holder of course but the real question is: the maximum output current of the MPPT is 50 amps, so why wouldn't it be more appropriate to have 50 amp fuses between them and the battery bank? Otherwise it seems like they would just blow again even when the solar charge controller was just doing it's job appropriately…

Thank you in advance, Trudy

Grumpy_G · June 6

Fuses protect the wiring so simply putting in a larger fuse might cause wiring to get hot to the point of insulation melting. Check the wire gauge that is installed and make sure it can carry the current.

I'm not familiar with the specifics of the Black Canyon package, are the solar panels split between two controllers ?

Horigan · June 6

The solar controller needs to be more capable than what the solar panels can generate. The wires and fuses should be sized to the solar panel output, though typically 10AWG is used which would warrant a 30A fuse. You more likely have a short somewhere since 200W or 300W panel (I don't know what the 2025 400 has) isn't going to generate more than 25A (17A for 200W, 25A for 300W). Plus, having the fuse holder melt indicates the connection at the fuse holder was poor.

As @Grumpy_G states above, check what wire gauge is used for the solar wiring, then I would replace the fuse holder in the wires with a good crimping tool.

AnOldUR · June 6

The 310W solar panel that nuCamp had been using has a maximum power voltage of 39.6. That puts the amperage between the panel and controller at theoretical maximum of 7.8. I take this to mean that the wire size between the panel and controller can safely be 10AWG without a lot of voltage drop. The controller then drops the voltage and bump up the amperage for charging a 12.8 nominal voltage lithium battery. This is why wire size and fusing on the battery side of the controller must be larger. The output amperage could approach 30 in ideal conditions with a single 310W panel.

On my system using Victron's 100/30 controller I used 6AWG wire fused at 40 amps on about a three foot run between the controller and battery. Since your controller has the potential to output 50 amps you should be using a minimum of 6AWG wire fuses at a maximum of 50 amps.

Trudy · June 7

Thanks for your knowledgeable response. I have 2 310 watt solar panels on the roof in series to one MPPT 100/50 charge controller. The wire between that solar charge controller and the battery is 10 AWG. The other solar charge controller, also a 100/50, is the one I use to hook up a 220 solar panel, it's wire to the battery is also 10 AWG and both solar charge controllers had a 30 amp fuse in line but since one melted I ordered a new fuse holder and two 60 amp fuses. I don't have a crimper. I was planning to use wago connectors, is that a bad idea? Thanks

Grumpy_G · June 7

10 AWG wire is rated for minimum 30 amp so the fuse was correct. However some back of the napkin math shows that the two 310W solar panels can exceed 30 amps after the charge controller in real world conditions. You might get away with a 40A fuse if the wire run is short (say sub 5f ft) otherwise I personally would upgrade the wire to 8 AWG. Voltage drop from too thin wiring can fool the charge controller into going to float too early.

AnOldUR · June 7

@Grumpy_G said, "Voltage drop from too thin wiring can fool the charge controller into going to float too early."

When I upgraded our rooftop solar from 100W to 310W I replaced the 75/10 controller with a 100/30 located in the same spot under the drivers side bench. The existing 10AWG wire caused the exact situation that you describe. Rather than routing heavier from that location, I chose to move the controller closer to the battery. My choice of wire gauge and fusing after the controller was overkill, but I already had the wire.

I still think that since his solar controller has the potential to put out 50 amps, all wire and fusing after the controller should accommodate it. I'd guess that if RV's had the same regulations as residential wiring, that it would be required to meet code.

elbolillo · June 8

According to Victron's site, these are the recommended fuses for the 100/30 and 100/50 MPPT solar chargers:

Since you have to replace the fuses, I would recommend using a breaker instead. Makes isolating the solar charger from the batteries much easier.

size of fuse after a smart solar 100/50

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