So 2 or 3 panels in parrallel...

Photowhit

Okay, I'm old...

When I was first doing solar electric systems, I rarely worried about breakers of fuses for panels in parallel.... Many/most weren't 12 volt system (panels) weren't done with breakers in the 90's
...but I do recall that it was 2 or 3 panels as rules were developed. I never came upon  system or reason to worry about 3 identical panels in parallel until recently. I acquired some 310-330 watt panels a while back and they are 72 cell panels and have a VMP of around 38 volt, IMP of around 8.7 amps. And a series fuse rating of 20amps.

Wonder if these would meet the specs of panels which could be put 3 in parallel, as 2 wouldn't be able to produce more than the 20 amp series fuse/breaker limit.

Guess I could hunt up the information, So feel free to chide me on my laziness! 

BB.

Placing voltage sources (like batteries) in parallel--More than 3 parallel strings, you have more issues with trying to balance string resistance so that each battery string correctly shares bus voltage and its 1/N share of current (basically the resistance of the wiring and batteries themselves "steer" the current between parallel strings). You can do more, but I see it as diminishing returns (expense of wiring, checking lots of battery cells for water levels, keeping wiring tight and clean, etc.).

With solar panels, they are "current sources"... Basically, when you put them in parallel, they simply output the current based on the amount of sun hitting the solar cells (more sun, more current)--Assuming you have reasonably close Vmp-string voltages.

The limitation of parallel solar strings... It is not the proper sharing of string current among the panels... It is that if you have a shorted panel or wiring (short to ground, internal + to - short, etc.), you need to look at the current on the Solar Bus and see how much is available.

With typical crystalline solar panels (say the larger ones)--They may have 8 amps Imp/Isc per panel. And a "series fuse" protection value of 15 Amps... If you have three panels (or strings of panels) in series, if one string gets shorted, then the two other panels are current sources with 2x8amps=16 amps of current that can be dumped into the shorted panel. The 15 amp fuse/breaker is to prevent the feeding of the rest of the panels/strings into a shorted panel/string.

The 1 and 2 parallel string array--You do not need fuse/breaker, because a shorted panel cannot be "set on fire" as the other panel/string is only ~8 amps and the panels are rated to carry 15 amps without overheating.

The 3x rule is pretty iffy... By math, 2 good strings feeding 1 shorted string--The available current is usually just over 15 amps... When you have 4 or more parallel strings, then the series fuse/breaker is really needed for fire safety (Imp/Isc of 3 other strings will quickly blow the 15 amp fuse.

And that is why we have the combiner boxes... Typically filled with 3 or more breakers/fuses.

With one or two parallel strings, a combiner box is not usually needed. However, some folks will still install a combiner... Makes debugging/working on system easier (to shut down strings for testing or maintenance). Or if they plan on adding more parallel strings in the future.

-Bill

Photowhit

So after reading all that...

The panels should be fine to hook up as 3 in parallel. Since the Series rating is for a 20 amp fuse or breaker and they have an IMP of 8.5 amps. 

I like that you answered everyone else's questions...lol. I did give all the pertinent information. IMP and Series fuse/breaker rating.

-sorry for giving you a hard time, it's late('r in Missouri) and I do appreciate your reply.

mcgivor

The 20A fuse would allow current to flow into a shorted/damaged panel, a 15A fuse would be better as it would be more likely blow, just a thought.