RupertW
Well-Known Member
And you still won’t need a fuse but you may need to upgrade the cables so the normal max output of those upgraded solar panels can be handled by the cable.
Fuse between solar panels and MPPT controller?
- Thread starter scr0che
- Start date
lustyd
Well-Known Member
Why not 2x 6mm cables? If there's zero chance of overcurrent as you say, and 6mm is rated 50A then I'm confused by your posts
PaulRainbow
Well-Known Member
Nope, it won't fuse. I said earlier, even if you shorth them together, nothing bad happens.
Lots of good info online about how solar panels work. But again, as i said earlier, it's not like having a power source at the other end of the cable.
So ?
rogerthebodger
Well-Known Member
Vic It says a PV breaker isolator so I would read that as a way of breaking or isolating the circuit and not an overload. In other words its a 2 pole switch.
lustyd
Well-Known Member
Why would you need to uprate the cables if a fuse wouldn't be helpful? If there's no danger to the cable there's no danger to the cable.
rogerthebodger
Well-Known Member
lustyd
Well-Known Member
Please can you link to one? I don't understand how the panel would know to stop producing power specifically in that scenario when in all others it would provide the power
PaulRainbow
Well-Known Member
Because he has two solar panels, each needs a pair of wires. Cables are sized according to two criteria, current rating and voltage drop. On 12v systems voltage drop is a big deal. 1.5mmcable is rated at 21a, so enough current rating for one of the OPs panels, but a 5m run of 1.5mm cable will drop 2.29v at 18a, clearly no good. 6mm cable will drop 0.5v.
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rogerthebodger
Well-Known Member
Its all to do with internal impedance(resistance) of the panel.
The silicon cells have a much greater internal resistance than a battery so the more current you try to draw from the solar cell the lower the output voltage will come.
The solar cell at maximum efficiency has an internal resistance of Rs = 0.72V/0.34A = 2 ohms. Placing two solar cells in series adds the internal resistance so that it is 4 ohms. The 4 ohms is closer to 10 ohms and so increases efficiency of energy transfer.
A torch battery has an internal resistance of around 0.1 ohms and a car battery about 0.001 ohms. Our imaginary internal resistor obeys Ohm's law just like any other resistor.
PaulRainbow
Well-Known Member
There is no danger to the cable, but if you increase the current, you increase the voltage drop. So, the cables won't melt/catch fire, but you won't get all of the power you should.
VicS
Well-Known Member
You'll have more current but probably still less than the max current rating of cables sized to limit voltage drop. But if you add significantly more solar power you will have to upgrade the wiring anyway to avoids increased volts drop. That will automatically increase the max current rating of the cables so that they will again be well in excess of the increased max solar current .
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VicS
Well-Known Member
You are probably right... its logical anyway but usually "isolator" means a simple switch and " circuit breaker" means an overcurrent device. Here we have something referred to as a "breaker isolator" WTF?
rogerthebodger
Well-Known Member
I agree with you . This does question the Engineering knowledge/ correct terminology and basic electrical principles.
In realty a circuit breaker is just a way of breaking a circuit. We tend to assume it it operate by an over current device or a differential (residual) current device but it could just be a simple switch the "breaks the circuit"
RupertW
Well-Known Member
You uprate the cables in your scenario because you have said you uprated the solar panels. As I said, if the cables were already big enough for the uprated panels then of course still no purpose in having a fuse.
lustyd
Well-Known Member
Persistently ignoring the scenario isn’t helpful to the conversation. I was addressing the extremely common scenario of people changing panels without changing wiring etc.
If the above posts are true then worst case the power drops and the cables cannot get warm let alone cause a fire. All sources I’ve found do highlight cable heat for small cables though
noelex
Well-Known Member
This is an article that explains better than I can when (and why) circuit protection (fuses or circuit breakers) should be used between the solar panel and the solar controller:
How/When to Fuse a Solar Panel Array
How/When to Fuse a Solar Panel Array
RupertW
Well-Known Member
If you change panels and the cables are too small to take normal max output then change them. Putting a little fuse on that will blow whenever the sun shines seems the wrong solution.
lustyd
Well-Known Member
The fuse would be to protect against fire or damage in case someone doesn't change the wiring. Personally I prefer a fuse to blow rather than see a boat burn down, especially if it's anywhere near my own boat!
Given the extremely conflicting advice here and elsewhere on the web I still feel a fuse is a good option. It costs almost nothing and has no downsides whatsoever.
The link that Noelex posted would seem to suggest that undersize cable would in fact be dangerous.
VicS
Well-Known Member
Excellent.
That explains in detail what I said way back in #4
However Victron do not specify a max fuse rating for the OPs panels but we can deduce from the example in the link that it should be be about 25 amps.
If the short circuit current of the entire array exceeds 25 amps the individual fuses illustrated will be required
In the case of the OP's proposed system the total short circuit current will be about 20 amps. Therefore he does not require fuse between the panels and the controller.
If he adds another panel , reaching the 30amp maximum for his controller, he will have to add the fuses and probably upgrade the common 10mm x 5m feed to the controller
RupertW
Well-Known Member
Baffling but if a continuously blowing fuse is what you think best instead getting the correct cable I won’t argue any more.
