Do I need a disconnect/switch on each parallel string (3 strings in parallel)?

tchss777tchss777 Registered Users Posts: 6 ✭✭
Hey all, apologies as I'm sure this has been covered already (I'm not very good at searching for this particular question).

I'd like to know if I need to have a disconnect/switch between my parallel solar strings. I'm mainly concerned about the safety/reliability of "hot plugging" parallel strings together during install (if there was enough potential between the strings). Each string will be fused, and the panels are identical (on paper at least). All of the panels will be close together and facing the same direction during install.

My panels:
- ~100Voc each
- Isc = ~6A
- 3 bypass diodes per panel

My setup:
- MPPT Victron charger
- 3 parallel strings
- 2 panels in series per string
- ~200Voc per string
- ~6A per string
- 8A fuse per string
- 3 strings will be combined inside of my combiner box

Because of my design and space constraints, I'd really like to avoid have 3 switches for each string. Instead, I'd like to use a single switch (contactor) on the PV+ rail after all 3 strings are combined together. Is this acceptable by NEC standards?

Threw together a rough wiring diagram: https://imgur.com/a/uvhmsts

Thanks in advance for the help! :smile:

Comments

  • mcgivormcgivor Solar Expert Posts: 3,299 ✭✭✭✭✭
    Welcome to the forum 

    Since you're using fuses, why not use circuit breakers which will serve both purposes https://www.amazon.com/MidNite-Solar-Photovoltaic-Circuit-Breaker/dp/B004EQK7YA not withstanding space consideration. The contactor would need have DC rated contacts  and consumes energy assuming normally open and non latching. 

    The dissconntion of the 4 position connector in the example would be fine if the contactor were open, but the question is why the connector ?  No comment on NEC standards.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • tchss777tchss777 Registered Users Posts: 6 ✭✭
    edited October 2018 #3
    Thanks mcgivor!

    I think circuit breakers will be less reliable than fuses over the longterm for this setup (specing for automotive). Also my power distribution will largely be on a custom PCB, so fuses and PCB contactor will take up less space overall.

    The 4-pos solar panel connector allows the system to be easily installed and serviced. The overall system needs to be flexible and modular 
  • mike95490mike95490 Solar Expert Posts: 8,666 ✭✭✭✭✭
    You need to insure the contactor is open before plug /  unplug any fuses or MC4 connector cables.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.BB. Super Moderators, Administrators Posts: 30,219 admin
    Plugging/unplugging MC4 connectors under load can cause arcing and ruin the connector (using circuit breakers to turn off the array is a good way to let you service/debug the system) (as the guys above said).

    Fuse holders (particularly the "touchless fuse holders" are not safe to unplug under load (they can arc and catch fire if you are not careful).

    Another question, you have Voc~100 VDC rating for the panels (what is the Vmp and Imp rating of the panels). That seems unusually high for typical solar panels (usually, Vmp is around 30-36 volts, sometimes a bit higher--for typical panels--Which is Voc~35-45 volt range).

    Are you looking at the right rating (there is a maximum voltage ratings for the panel which can run from 100-600-1,000 volts)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tchss777tchss777 Registered Users Posts: 6 ✭✭
    @BB, the plugging/unplugging would only occur when the circuit is off (contactor open, hard requirement for any installation), so no arcing if I'm not mistaken. That said, I was concerned about any potential difference between the panels in parallel (when that connection is first made). Sounds like mcgivor was saying that wouldn't be a problem if contactor was open. For debug, I was planning to use small switches to force the contactor open (normally SW controlled, but easier to manually control during debug).

    And absolutely agree, any fuse replacements are only allowed to be performed when the contactor is open.

    I want to support a couple different panel options, and I've seen a few out there in the 70-90 Voc range (though less common).


  • BB.BB. Super Moderators, Administrators Posts: 30,219 admin
    Sounds like you are being careful... I don't see any connector/powering issue.

    Sounds like you are looking at 400-500 Watt panels?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tchss777tchss777 Registered Users Posts: 6 ✭✭
    Glad to hear! Thanks for your help! Yes, I was hoping to source panels in that range. But we'll see :)
  • littleharbor2littleharbor2 Solar Expert Posts: 1,494 ✭✭✭✭
    Which Victron controller has a higher than 200 volt rating. On cold clear mornings you could see a higher than 200 volts, open circuit at the controller. What nominal battery voltage are you going to be charging? You would probably be fine with all panels in parallel. 

    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, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • tchss777tchss777 Registered Users Posts: 6 ✭✭
    These guys: https://www.victronenergy.com/solar-charge-controllers/smartsolar-250-85-250-100

    Battery bank is still up in the air, but most likely 12V or 24V range.
  • mcgivormcgivor Solar Expert Posts: 3,299 ✭✭✭✭✭
    Choose a higher voltage for best results in all but very small systems, as all ballance of system equipment, wiring fuses etcetera, will be be smaller making it more cost effective and voltage drop will be less of an issue. A note of caution, don't be tempted to purchase an inverter of far greater capacity than needed, calculate the loads carefully to understand the inverter size needed and battery capacity required. Generally when building a system this would be the starting point, the final part of the equation would be the PV required the charge the batteries, so your approach is a little bass akwards, a common beginner mistake so don't be offended, we all started at the beginning and all made mistakes.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • tchss777tchss777 Registered Users Posts: 6 ✭✭
    edited October 2018 #12
    @mcgivor Thanks for the advice! Much appreciated!

    EDIT: do you have a recommendation on inverter brands? I tried a Cotek inverter one time, but never could get the serial control working.
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