Cable sizing and breakers for solar

[john61ct]

No I just mean add more controllers, 1:1 is my ideal.

[Dsanduril]

Originally Posted by noelex 77

...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, ...

Originally Posted by Cpt Pat

... 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....

Both true, but the OP described an array of two identical panels with
Isc at ~8.5A. If you put a 10A or 15A fuse in the lead of each panel it will still never trip. If panel A has a dead short, panel B can still only supply the 8.5A Isc into that short through the 10A or 15A fuse. And if the short is in panel A then the current from that panel will never see the fuse, so it can't add to the current and trip the fuse.

In a larger array with many panels in parallel the situation is different. But in a one or two panel array....

[noelex 77]

Originally Posted by Dsanduril

Isc at ~8.5A. If you put a 10A or 15A fuse in the lead of each panel it will still never trip. If panel A has a dead short, panel B can still only supply the 8.5A Isc into that short through the 10A or 15A fuse. And if the short is in panel A then the current from that panel will never see the fuse, so it can't add to the current and trip the fuse.

Yes, I agree. The only faults I can think of would require a minimum of three panels (and probably in most situations four panels) connected in parallel before there was a danger and additional fuses were required between the solar panels and the controller (assuming idendical panels). If mixed sizes of panels are used even two panels in parallel could create a potential problem.

However, I would caution that there may be some faults I have not considered and many manufacturers recommend a fuse/CB between the panel and the controller without qualifying the number of panels in parallel before this requirement is needed.

The most important fuse/CB is between the controller and the batteries. This must be fitted in all cases. An additional fuse/CB between the panel and the controller is rarely fitted in marine installations, but perhaps in some circumstances it should be. Certainly in the case where three or more solar panels are connected in parallel there are potential electrical faults where a fuse between battery and solar controller would not provide adequate protection.

Every fuse/CB added to the system is another potential failure point, and source of voltage drop, so they should not be added unless required, but the commonly expressed opinion that additional fuses between the solar panels and controller is not required because the current will always be within safe parameters is not correct.

If a circuit breaker is used it will also have the useful secondary function of isolating the output from each panel.

[john61ct]

+1 ^^^^

[NorthCoastJoe]

Shorted wires can get hot enough to start a fire we all agree there. I have a fuse at each panel. One between controller and battery, and another at the battery before loads and breaker panel. Fuses get changed and contacts get cleaned every year. I also have a fuse on my starter cable just off the switch.

I may be over fused but I have no worries about a fire. ( ex-firefighter )

[darylat8750]

Originally Posted by Dsanduril

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.

I've read that the big gain from series is that the voltage gets high enough to start charging the batteries earlier in the morning and continues later in the afternoon. I can't remember where I read it but the poster noted that single change made a significant improvement in his system.

[Albro359]

Originally Posted by darylat8750

I've read that the big gain from series is that the voltage gets high enough to start charging the batteries earlier in the morning and continues later in the afternoon. I can't remember where I read it but the poster noted that single change made a significant improvement in his system.

That's my experience.
We have 3 x 200W panels wired in series...Voltage in the middle of the day typically 125V.
We are a 24V ship and we see quite early in the day the input voltage to the Victron MPPT over 29V and we start charging....and the same towards the end of the day. If the panels were wired in parallel, this wouldn't happen AND it would mean much much bigger cables from the panels to the controller.
BTW we have breakers on the feed into the MPPT and the feed out, the whole system can be totally isolated.

[Dsanduril]

Originally Posted by darylat8750

I've read that the big gain from series is that the voltage gets high enough to start charging the batteries earlier in the morning and continues later in the afternoon. I can't remember where I read it but the poster noted that single change made a significant improvement in his system.

Only marginally. The voltage/irradiance curve is logarithmic (while the irradiance/current curve is linear). As a result there is about a 10% difference in voltage between first usable light and maximum sun. If you have 30-36-cell panels going into a 12V battery or 60-72-cell panels going into a 24V battery then series connection can make a difference when irradiance < 100 W/m2. However, the OP has 30V panels into a 12V system and even at very low light these panels will be at ~26-27V, more than enough to start charging a 12V system (but not really enough to start charging a 24V system).

You have to balance this with:

1. Greater conversions losses with higher voltage deltas
2. Greater shading losses with series connected panels vs. parallel

Low light generation is by its nature low power output, each installation has to be evaluated individually to see if series connection for low light generation and smaller cable sizes balances against the conversion losses and shading losses during high light generation periods. It is not a one-size-fits-all proposition.

[dstraton]

Are you sure that you can’t connect them in series? The Victron SmartSolar 100/50 seems to be MPPT, so you would start your harvest earlier in the day and carry on later with the higher voltage of a series setup, compared to parallel. The old solar controllers would waste the volts above a certain level, but MPPT converts the higher volts to extra amps. I’d run it past Victron.

[john61ct]

Yes MPPT, and that unit is fine up to 85-90Voc total panel input.

I doubt if there would be much greater power output past 60V though.

[Nicholson58]

Originally Posted by terah

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

Manufacturers nearly always recommend a too small gauge. You paid too much for the panels and MPPT to throw away potential as heat. My 220 watt panels came with #8 leads. I ran #6 from the other side of the connector. Panel cables from three panels join at an aluminum junction block in parallel. The line from the block to the charge controller is #4. The line out of the MPPT is the largest the terminal would accommodate. The MPPT is very close to the batteries.

If you woul like to learn more about wire sizing and solar installation check out Handy Bob Solar's web site. A long tirade against the average equipment purveyors but a few gems to glean. https://handybobsolar.wordpress.com

I installed two pole breakers in each panel wire pair so that both wires can be interrupted and the panel isolated. You need to be able to disconnect the panels because it is the only way to shut down the controller if necessary. I also have a two pole circuit breaker on the MPPT output. Collectively these breakers protect your wiring and equipment. An additional benefit is that you can isolate the controller and panels in the event of an electric storm. You will also have a battery sense wire pair. I added a two pole interrupt switch in order to complete the lightning isolation.

After three lightning strikes on Lake Michigan we noted that the only devices that survived were air gap isolated so we have plain old fashioned mechanical two pole toggle switches on every electronic device and the mast wiring passes through a 48 pin connector we pull if an electric storm approaches.

[mikeo16]

It seems with the Victron controllers (100/20), the wire receptacle slots from panel to controller and controller to batteries will only take a maximum of 10 gauge wire. How could you use a larger size wire?

[john61ct]

Butt connectors or ferrules.

The fat gauge is for the distance.

Put the SC very close to the target bank, in the same space.

[McG]

Howdy guys.
I am putting together a solar system and would like to get some input on which Circuit Breakers to use. I am connecting 9 x 100Watt panels. I planned on connecting 3 arrays in parallel, each containing 3 panels in series.
Spec:
Voc 21.2V
Ioc 6.25A
Vmp 17.6V
Imp 5.68A

......connected to an Outback Flexmax 80A controller to an 800AH battery bank.

I had a look at these breakers

1.I read somewhere online that a circuit breaker is oblivious to volts because it can in no way measure it. It makes sort of sense but why do most breakers have a voltage rating?

2. What breaker rating between SC and Batt is optimal for my application?

3. What breaker is optimal for use between Solar array and SC?

[noelex 77]

With 63v DC you need to careful about the voltage rating of any circuit breakers between the panels and the controller. High DC voltages such as this are much more difficult to interrupt that the equivalent AC voltages. Fire is a real risk.12V DC is very easy to switch and interrupt, but high DC voltages require much more care.

The breaker in your link is rated to 72v DC which would be fine, but it seems a copy of the Blue Seas 285 series. The Blue Seas circuit breaker is only rated to 48v. The circuit breaker in your link may have a better internal construction, but I suspect that is not the case. Personally, I would not trust the circuit breaker in your link on a 63v DC system.

The correct current rating will be listed on the panel specifications as the “maximum series fuse rating”. This rating is likely to be around only 15A so high current breakers like the one you listed (the minimum rating is 60A) will protect the wiring, but not the risk of an internal short in the solar panels.

Designing a system that protects against these internal shorts is difficult. It requires multiple breakers with high DC voltage ratings, but relatively low current ratings. Most people ignore these requirements, but I am not sure that is always wise especially if the solar panels are mounted on flammable materials such as a bimini or fiberglass cabin top.

Solar systems on have evolved very rapidly. The system I installed on my first yacht consisted of a single 10w panel with a maximum voltage of about 20v. The latest has over 1000w and a voltage of almost 70v.

With these high powered systems there is a lot of potential energy and voltages that can be lethal. When installing these high powered, high voltage systems many people are using practices and protection devices that were perfectly adequate and safe for small systems, but these large complex systems need higher standard of thought and planning.

Edit: one other consideration that I did not mention is that the circuit breakers in the link are thermal only. If you can fit thermal/magnetic circuit breakers these offer a higher level of protection by reacting faster to high short term overloads.