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Old 19-06-2018, 05:09   #1
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Cable sizing and breakers for solar

I have two 260W panels I'm planning to connect in parallel to a Victron SmartSolar 100/50, max 10m away. The panel specs are:

Vmpp: 30.9V
Impp: 8.41A
Voc: 37.5V
Isc: 8.93A

Victron's excel calculator gives max current @ mpp of 45.9A

So is that simply a 20A MCB (or fuse) from the panels to controller, then a 50A MCB from controller to batteries? Then 6mm2 cables from panels to controller and 16mm2 from controller to batteries?

Thanks
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Old 19-06-2018, 14:45   #2
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Re: Cable sizing and breakers for solar

You only have to protect the wires from the panel to controller if the panel amperes would be too much for that class wire. I have 10 awg Renogy panel wires to 8 awg extensions to my junction box. My panels will never produce enough amperes to effect the wire. Where the controller connects to the battery there must be a fuse at the battery. Yours, 60 amp fuse, if your wire is heavy enough. And it should be.
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Old 19-06-2018, 15:14   #3
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Re: Cable sizing and breakers for solar

Great, thanks Zil!
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Old 19-06-2018, 15:47   #4
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Re: Cable sizing and breakers for solar

Yep, solar panels are by definition a current limited source, no fuse/breaker protection is required. You might want one for convenience if you have reason to disconnect the panels but otherwise no need.

Any cable connected to the batteries should have protection at the battery end. What you're protecting against is a problem/short in the solar controller that allows a large current from the battery - the current to the battery will be limited by the panel's capacity.
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Old 19-06-2018, 19:15   #5
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Re: Cable sizing and breakers for solar

Unless the SC vendor specifies it, in which case I'd follow those instructions.
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Old 19-06-2018, 19:29   #6
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Re: Cable sizing and breakers for solar

terah-
Fuses and breakers all have break times as well as loads. So a 50A device might trip after 50 amps for ten minutes--or thirty seconds. Check the blow time on the devices you select, look for something that will blow fairly quickly since in theory you should never see 50 amps on the controller output at all, for any length of time.
There are also thermal breakers (when overheated they trip) and more expensive magnetic breakers, which can be reset more times without wearing out. I'd go for the magnetic breakers (more expensive) if everything else was equal.
On breaker sizing...there are different schools of thought of that too. Some people would say to size the breaker 10% higher than the maximum expected load, and in your case that's 50A and probably just fine. Others would say to size it higher, so something like a constant high load on a very hot day would not be any problem.
The pros here can give you opinions on that.
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Old 20-06-2018, 10:00   #7
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Re: Cable sizing and breakers for solar

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...
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Old 20-06-2018, 12:33   #8
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Re: Cable sizing and breakers for solar

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Originally Posted by mvmojo View Post
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...


True, but be careful. In a series configuration, if any panel becomes shadowed, it will diminish the overall array output if no bypass diodes are employed. That shadowed panel becomes the equivalent of a series resistor. Some panels incorporate a reverse-polarity diode shunting the panel output terminals to bypass current around a shadowed or damaged panel, which will resolve this issue. If you decide use a series array, you can add these diodes yourself if they are absent.

With a parallel-wired array: wiring from each panel should be of a sufficient gauge to withstand the maximum total current from all of the panels combined (the combined Isc of all of the panels). Fusing each panel individually is an alternative. A fault in one panel that causes a short circuit in that panel will sink all of the current from the entire PV array if no series steering diodes are used.

Using steering diodes (use Schottky diodes) from each panel output will prevent this scenario and prevent backcurrent losses in shadowed panels as an added benefit, but the design should accommodate the possibility that a steering diode in series with a damaged panel could also short. This can happen from a lightning hit that will certainly exceed the diode peak inverse voltage rating. Often the diode PN junction shorts in diodes under this circumstance.
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Old 20-06-2018, 12:47   #9
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Re: Cable sizing and breakers for solar

I'm under the impression (maybe incorrectly, haven't shopped them in a long time?) that most panels incorporate diodes to minimize output reduction due to shading. Mine ten 100 watt panels are 16 years old and they incorporate diodes. I do not have them wired in series, but if I were to ever have to replace/rewire them, I'd set it up so every two panels were in series to increase mppt efficiency.
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Old 20-06-2018, 13:00   #10
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Re: Cable sizing and breakers for solar

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. Should some component fail or inadvertently get shorted you are not protected. The fuses/breakers should be big enough to be stable under normal use, but trip under extreme circumstances. My panels are rated at 320 watts or about 10.7 amps each. The controller can put out 60a. I have a 30 amp fuse to the panels and 90 amps to the batteries.
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Old 20-06-2018, 13:07   #11
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Re: Cable sizing and breakers for solar

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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. Should some component fail or inadvertently get shorted you are not protected. The fuses/breakers should be big enough to be stable under normal use, but trip under extreme circumstances. My panels are rated at 320 watts or about 10.7 amps each. The controller can put out 60a. I have a 30 amp fuse to the panels and 90 amps to the batteries.
The point is your panels can never produce 30A, hence the fuse will never blow.
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Old 20-06-2018, 13:36   #12
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Re: Cable sizing and breakers for solar

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The point is your panels can never produce 30A, hence the fuse will never blow.
True, unless the wires between the fuses and the panels get shorted and batter power feeds backwards through the system. I am assuming that will take very unusual circumstances, but then again it's very unusual circumstances that fuses are there to protect against. Now the controller says it has short circuit protection, but a fuse is pretty cheap protection against the scenario that the engineers didn't think of. Besides there are circumstances under which you might want to disconnect the panels from the controller or the controller from the batteries. Breakers or fuses are a pretty easy way to do that. My controller for instance requires that power from the batteries be removed before any programming changes take effect. Cycling the breaker makes that very easy.
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Old 20-06-2018, 13:41   #13
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Re: Cable sizing and breakers for solar

Everyone agrees that fuse/circuit breaker protection is needed between the battery and the controller. Most manufacturers also recommend additional fuse/circuit breaker protection between the controller and the panels. I have never heard an explanation why this is required on boat sized systems, but as it is helpful to be able to disconnect the panels, a suitable circuit breaker solves both requirements.

Be careful with the fuse/circuit breaker ratings if using high voltage panels. Switching or interrupting high voltage DC is not easily done and some 12v marine equipment cannot do this safely.
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Old 20-06-2018, 14:10   #14
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Re: Cable sizing and breakers for solar

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True, unless the wires between the fuses and the panels get shorted and batter power feeds backwards through the system. I am assuming that will take very unusual circumstances, but then again it's very unusual circumstances that fuses are there to protect against. Now the controller says it has short circuit protection, but a fuse is pretty cheap protection against the scenario that the engineers didn't think of. Besides there are circumstances under which you might want to disconnect the panels from the controller or the controller from the batteries. Breakers or fuses are a pretty easy way to do that. My controller for instance requires that power from the batteries be removed before any programming changes take effect. Cycling the breaker makes that very easy.
The chances are higher of human error mis-wiring than the controller fault allowing battery voltage to get to the panels.

A solar disconnect switch is a better choice to open the circuit between the panels and controller.
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Old 20-06-2018, 14:11   #15
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Re: Cable sizing and breakers for solar

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Originally Posted by noelex 77 View Post
Everyone agrees that fuse/circuit breaker protection is needed between the battery and the controller. Most manufacturers also recommend additional fuse/circuit breaker protection between the controller and the panels. I have never heard an explanation why this is required on boat sized systems, but as it is helpful to be able to disconnect the panels, a suitable circuit breaker solves both requirements.

Be careful with the fuse/circuit breaker ratings if using high voltage panels. Switching or interrupting high voltage DC is not easily done and some 12v marine equipment cannot do this safely.
Most controller manufacturers recommend a disconnect between the panels and controller, I've never seen one recommend a fuse/circuit breaker.
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