2 strings or 3

Installing a grid tied 5k solar system and went MAX on the panels. 16 panels 300W that are 37V. The SMA 5.0 Inverter that I'm using has DC inputs for 3 strings. So I can do 2 string in series with a total of 4800W ( 2400W per string ) or I can do 3 strings a 1500W, 1500W and 1800W. If I had a shading Issue, 3 strings would be best. I called SMA and they said it didn't matter as each string is independent. So OK I have no shade but was wondering about wire heating of DC down to the Inverter ( about 30 feet) and size of wire as I'm going to J box and EMT down to the Inverter. Either way running in series from one panel to the other positive to negative is pretty easy but if all 8 panels are on the same line it makes trouble shooting much easier. Originally I was going to use ENPHASE inverters but the cost was about $1000 more and I also had no shading issues. BG

Comments

  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    With no shading, I'd generally go for higher string voltage and lower current, as long as there's no chance of exceeding voltage limits.  Keeps wire size and voltage drop down.

    For max string voltage, you need to figure max string Voc at record low temp in your location, with a bit of a safety nargin.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • littleharbor2littleharbor2 Solar Expert Posts: 1,639 ✭✭✭✭
    Just thought I'd throw this leapfrog wiring diagram into the mix in the hopes that it might apply to your situation.  A wiring option that makes your home run wire is a bit more balanced and save some wire.




    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 540 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • billybob9billybob9 Registered Users Posts: 141 ✭✭
    Estragon...Thanks for the Wire size and DC voltage drop reminder.
    Littleharbor2... OMG if the wire from the positive side of the panel will be long enough to skip a panel and connect to the wire of the negative side of the 3rd panel," you're right " no long line to even it out. In my case it would have been 30 feet. This is what I will do with either 2 or 3 string connection to the Inverter. Thank you... 
  • BB.BB. Super Moderators, Administrators Posts: 32,229 admin
    Solar panels are (more or less) current sources and they share current pretty well and will forgive some wiring length differences.

    Not like batteries which are voltage sources... Their output (and input/charging current) are very dependent on voltage and "matched" wire runs (matching resistance).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • billybob9billybob9 Registered Users Posts: 141 ✭✭
     Bill

    The difference in the wire length from the Daisy Chain to the Leap Frog is 30 feet. So the resistance in the wire has decreased by about a third of what it was. If what Estragon said about going with the Highest voltage is the best way to go then the 2 strings option seems like the way to go. I don't know what most solar installers do but would like to find out.
  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    One thing to note/check is the orientation of panels in the diagram (portrait).  In this orientation, you'd have a fair bit of extra  coiled mc4 panel lead wire with adjacent panel series wiring. If your mounting area etc lends itself to this, and you have long strings, leapfrogging makes sense to me.. 

    In my case, I mounted in landscape for various reasons, and in that orientation I'm not sure the mc4 cables would reach without short extenders using the leapfrog wiring.  My combiners are also roughly equidistant from the ends of each string (of three panels each into 150v CC), so leapfrogging wouldn't make much difference anyway.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators, Administrators Posts: 32,229 admin
    BillyBob,

    I agree that making wire runs shorter reduces losses (and/or saves the cost of long runs of heavy gauge cables to keep overall losses to a minimum).

    And Estragon's issue of solar panel pigtails being Pretty Long for portrait installations vs Too Short for landscape (using leapfrog) installations. I also don't like "extra" connections/connectors... In my humble opinion, connectors tend to be the "weak points" / most prone to failure in most electrical installations--And I would suggest trying to avoid adding any "extra" connectors if possible (even if longer wire runs/heavier cables were required).

    I was trying to address the issues of current balance/sharing between parallel strings and solar panels vs batteries. Solar panels are current sources and much less affected by "unbalanced" wire run resistance... Batteries are voltage sources and are much more sensitive to even relatively minor resistance variations in parallel current paths.

    More or less a 5% to 10% voltage (10% of Vmp~18 volts = 1.8 volt difference) "mismatch" on parallel solar panel connections don't make much difference. A 1.8 volt difference on a 12 volt FLA battery is the difference between fully charged and pretty much dead.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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