Cable sizing and breakers for solar

[noelex 77]

Originally Posted by DotDun

Most controller manufacturers recommend a disconnect between the panels and controller, I've never seen one recommend a fuse/circuit breaker.

Here is a typical example. This one is from Renogy:

“There are three different locations that we recommend installing fuses or breakers: first, between the charge controller and battery bank, second, between the charge controller and solar panels, and third would be between the battery bank and inverter.”

https://www.renogy.com/blog/how-to-f...-solar-system/

[Dsanduril]

Originally Posted by mvmojo

Your mppt controller will be more efficient if you wire the panels in series. This will increase the input voltage, but reduce the amps allowing you to use smaller wires from the panels. Discuss with the manufacturer...

Actually just the opposite, see graph below (for a specific model but typical). MPPT controllers use a buck type voltage converter (or a boost if they raise the voltage) and those are universally more efficient when input voltage is closer to output voltage and lose efficiency as voltage differential increases.



You do get a gain in wire sizing efficiency but as noted above shading becomes a greater issue, and you lose at the MPPT controller.

[DotDun]

Originally Posted by noelex 77

Here is a typical example. This one is from Renogy:

“There are three different locations that we recommend installing fuses or breakers: first, between the charge controller and battery bank, second, between the charge controller and solar panels, and third would be between the battery bank and inverter.”

https://www.renogy.com/blog/how-to-f...-solar-system/

I called Renogy tech support, they couldn't give a good answer except, "the fuse is there to protect the controller".

Again, there is no engineering answer why they suggest a fuse.

[Dsanduril]

If you have a short in the panel wiring the most that can happen is that Isc will flow continuously. No damage to the panels occurs and no overcurrent device that allows full power to flow in the proper direction will trip. Thus no point. A short in this situation can get hot enough to cause a fire, but no overcurrent device will help. You could try an arc-fault device if you are concerned.

The batteries, on the other hand can provide a tremendous amount of current and thus an overcurrent device should be installed at the battery end of the connection. If you believe that a fault inside the controller could lead to a flow from the batteries toward a short on the solar side (thus two faults at the same time) then you might install an overcurrent device between the controller and the solar panels but, again, it should be at the controller end (closer to the batteries). The only source of overcurrent is the batteries, not the solar panels.

[john61ct]

Originally Posted by Captain Bill

I completely disagree with the statement that no fusing is required because solar is a current limited source. While the panels are indeed current limited, the other end of the wire is connected to a large battery bank that is not.

That was only said about between SC and panels.

No one says the battery side shouldn't have CP.

[john61ct]

Panels are actually fine with getting shorted, no problem even at full output.

[john61ct]

Originally Posted by Dsanduril

Actually just the opposite, see graph below (for a specific model but typical). MPPT controllers use a buck type voltage converter (or a boost if they raise the voltage) and those are universally more efficient when input voltage is closer to output voltage and lose efficiency as voltage differential increases.

Certainly not true for Victrons. Their 75/15 is designed so that 40+V is much more efficient than "12V" lower voltage panels.

Right up to 65V, then their 150 versions fine with 140V.

Something about giving the MPPT algorithm more headroom to find optimal power point, that's how they get 15+% better output than PWM.

And this is all well before, nothing to do with the buck conversion.

[a64pilot]

Due to length mostly, I ran #2 cable from panels to controller and from controller to battery.
I used a combiner box for the panels as a handy way to connect to a bus, plus it came with individual cartridge fuses so I can disconnect or isolate a panel if needed, I don’t know maybe one day one will be damaged and I can isolate it by merely pulling a fuse, plus the combiner box is supposedly lightning protected, I say supposedly cause I’m not sure anything can really protect from lightning, there is just so much power.

[Dsanduril]

Originally Posted by john61ct

Certainly not true for Victrons. Their 75/15 is designed so that 40+V is much more efficient than "12V" lower voltage panels.

Right up to 65V, then their 150 versions fine with 140V.

Something about giving the MPPT algorithm more headroom to find optimal power point, that's how they get 15+% better output than PWM.

And this is all well before, nothing to do with the buck conversion.

Not trying to say that an MPPT isn't more efficient than a PWM. And Victron may be a particularly efficient MPPT and may have a better MPPT algorithm.

But, if you connect two 30V panels in parallel into a 75/15 with output at 12V (nominal) I am asserting that the actual power conversion (from 30V at x Amps to 12V at y Amps) will be more efficient than the same panels connected in series and sending 60V to the controller. Has nothing to do with the MPPT part of the unit, simply with the power conversion that must occur. That is generally the case with any buck conversion unit.

Would love to see something different but all I can find in the Victron specifications/manuals/datasheets is a 'peak efficiency' of 98%. If you can provide more detail I'd love to see it.

[noelex 77]

Originally Posted by DotDun

Again, there is no engineering answer why they suggest a fuse.

i agree. I have never seen a satisfactory explanation for this requirement.

I think the concern is current that is produced by the other panels wired in in the array could, in some potential faults flow through through a defective panel with for example an internal short. The internal wiring of solar panels is kept thin to minimise shading and e maximum permitted current is low usually around twice the Isc, so it would not require a large array to overheat this internal wiring,

A single panel is safe in dead short situation, but not if the fault causes the internal wiring of the panel to conduct the current from several panels.

[Cpt Pat]

Originally Posted by john61ct

Panels are actually fine with getting shorted, no problem even at full output.

Here's my situation: I have two 130 watt panels each with a short circuit current (Isc) of 8 amps. I also have twelve surface mounted thin 25 watt panels, each wired with short pre-manufactured 20 AWG leads passing through the deck that are then bonded to heavier 14 AWG wires for the remaining run to the controller. All the panels are wired in parallel.

If one of those 25 watt panels shorts, I will have as much as 28 amps flowing through 20 AWG lead wires (with a maximum current rating of 11 amps) to the shorted panel, resulting in smoke and possibly fire - not to mention whatever catastrophic destruction occurs inside the plastic encapsulated panel. So I have steering diodes AND fuses to each panel. I've already had one panel short in service because of a manufacturing defect that didn't actually allow it to be stepped on, as claimed.

I believe the safety of shorting individual panels is being inappropriately applied to the danger of shorting an entire parallel array at one bad panel.

If anyone wants to make a safety-related case for using a series configuration instead of parallel, here's an example of the hazards of parallel configurations. I don't have that option since I have dissimilar (130 and 25 watt) rated panels that won't work in series.

[john61ct]

Originally Posted by Dsanduril

And Victron may be a particularly efficient MPPT and may have a better MPPT algorithm.

But, if you connect two 30V panels in parallel into a 75/15 with output at 12V (nominal) I am asserting that the actual power conversion (from 30V at x Amps to 12V at y Amps) will be more efficient than the same panels connected in series and sending 60V to the controller.

I disagree, and I'd be willing to bet the cost of the gear required to test it.

[john61ct]

Originally Posted by Cpt Pat

I don't have that option since I have dissimilar (130 and 25 watt) rated panels that won't work in series.

Why would you do that?

Really all panels should match no matter how they're wired.

[Cpt Pat]

Originally Posted by john61ct

Why would you do that?

Really all panels should match no matter how they're wired.

(Ah, if we only lived in a perfect world and my 1963 vintage boat was designed for many identical solar panels. Better yet - I wish it came with a fusion power reactor, a plutonium 238 thermoelectric power generator, or a dilithium crystal moderated matter/antimatter reactor.)

Why? 12 times 25 watts of extra power that I otherwise wouldn't get from surfaces that cannot accommodate big aluminum and glass panels.

Matching isn't important in parallel arrays as long as the PV voltages of all the panels are the same. The array works fine - as expected. If the voltages are only slightly different, the MPPT controller calculates the best compromise.

[alctel]

Originally Posted by Dsanduril

Would love to see something different but all I can find in the Victron specifications/manuals/datasheets is a 'peak efficiency' of 98%. If you can provide more detail I'd love to see it.

Same - I have that exact model of controller and have one 100watt and one 50 watt panel wired up to each one in parallel - would be great if I could squeeze even more power out of it!