PV array breaker sizing and voltage rating

ctann
ctann Registered Users Posts: 12 ✭✭
edited January 2017 in Off Grid Solar & Battery Systems #1
Hi all,

Putting the last touches on my 4kW off-grid solar installation design. Right now I am running through the wiring and breaker sizing. This will be a 12x325W panel, Magnum PT-100 charger, Magnum PAE inverter, and 48V battery bank.

The panels have VOC = 46.1V, ISC = 9.48A and TC_VOC = -0.304%. I am sizing as 3 panels in series, 4 parallel strings.

My breaker sizing calc shows that I need 12A 100% duty cycle breakers (1.25*9.48), although wiring should be sized for 15A (I'm still a little confused about the two 25% derating factors). However, what worries me is the breaker voltage rating. 3*46.1 = 138.3V, but if I derate for my worst case ambient temperature (-10C), that comes to 153V - just pushing me past the 150V breaker rating.

So, two questions
 - Will 150V PV breakers meet NEC code for my system?
 - Are 150V breakers safe to use in this installation?

I really don't want to go to the 300V breakers - my current installation plan has exactly 4 open fuse slots in the panel, and going to the 300V breakers would require me to go to a larger 8-slot combiner box.

Thanks,
              Chris.
                     

ps - The breakers in question are the Midnite Solar MNEPV12 or MNEPV12-300. The combined PV source will be going through an MNEPV50-300 to the charge controller.


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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    153V - just pushing me past the 150V breaker rating
    What's the kill voltage for your charge controller ?   Usually it's 150V, that would worry me more than the breaker.  Most controllers have a high voltage logger for warranty exclusion .  Exceed that and you are SOL.

    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 ,

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  • ctann
    ctann Registered Users Posts: 12 ✭✭
    #3
    Well, I think I have answered myself for #1. http://www.solarabcs.org/about/publications/reports/expedited-permit/map/ shows my location (95120, San Jose CA) "Extreme Minimum" as 0 degrees C. I'm not sure where I got the -10 degrees C from that I was using before - probably from a gardening website when looking into planting fruit trees! :) That is a reasonably realistic number for this area though - maybe once every 20 years. At 0 degrees C, my VOC = 149V - so from a code perspective, it looks like I am OK with the 150V rated breakers.

    Still interested in #2, the safety aspect, but it seems that under any condition that generates significant current, the temperature of the panels will never be under 0 degrees C.
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  • ctann
    ctann Registered Users Posts: 12 ✭✭
    #4
    mike95490 said:
    153V - just pushing me past the 150V breaker rating
    What's the kill voltage for your charge controller ?   Usually it's 150V, that would worry me more than the breaker.  Most controllers have a high voltage logger for warranty exclusion .  Exceed that and you are SOL.

    Right, that is one of the reasons I like the Magnum PT-100. It has a maximum operating voltage of 187VDC, and an absolute max rating of 240VDC. I could actually just go up to 4 panels in series with this controller, but that is right on the limit. With the Outback charge controller I was looking at previously, I would have been limited to two panels in series, and 6 parallel strings.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #5
    I don't think VOC is relevant since it would only apply when there was in essence no current. Sillier things have been written into the code though... 
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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