Sizing the combiner box circuit breaker?

kaipo_boy
kaipo_boy Solar Expert Posts: 143 ✭✭
in Off Grid Solar & Battery Systems #1
OK, you guys have convinced me that I'm swimming upstream by trying to put together everything I'll need when the e-panel already comes ready to go, so I guess I'll be picking one of them up. However, I'm trying to size my circuit breakers for my PV combiner box and need some advice.  From what I gather, the strings (5 of them) will be 2 panels in series, for Vmp of 8.06A each string at around 70v in good sun. So saying, should I have the circuit breakers for each string in the combiner box at 12A? or 10A?  120% of 8.06A is around 9.67A so I think I'm supposed to go with the 10A breakers.... is that right?
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,102 ✭✭✭✭✭
    #2
    Series fuse rating will be on the data label on the back of your solar panels.

    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,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    If you cannot find the series fuse rating--Then a starting (not official) recommendation would be:
    • 8.06 Amp Imp * 1.25 Isc derating * 1.25 NEC fuse/wiring derating = 12.6 Amps
    Or, round up to 15 amp breaker/fuse.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • kaipo_boy
    kaipo_boy Solar Expert Posts: 143 ✭✭
    #4
    My thanks, all!
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #5
    BB. said:
    If you cannot find the series fuse rating--Then a starting (not official) recommendation would be:
    • 8.06 Amp Imp * 1.25 Isc derating * 1.25 NEC fuse/wiring derating = 12.6 Amps
    Or, round up to 15 amp breaker/fuse.
    Some fuses/breakers are designed to operate indefinitely at their rated current without blowing/tripping.   If you use such a fuse/breaker you only need one of the 1.25 deratings.  Therefore you can use a 12 amp circuit breaker such as this: Midnite Breakers for Solar Arrays.  Of course, as littleharbor points out, if your panels have a series fuse rating, use it.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    edited April 2016 #6
    Remember that most are rated at 40C, that has been an issue with high load circuits here in the past.  On a hot day many times the breakers would trip.  40C isn't far from the normal afternoon high here, panel in hot sun I am sure can be much higher..
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Also, remember that breaker+fuses have AC and DC ratings--And DC ratings are usually much lower (operating voltage--Plus as said above, sometimes breaker/fuses are rated "differently" and not the same test conditions in different countries). As an example:
    Uploaded on Mar 17, 2011

    Why not to use an AC breaker on a DC PV circuit:

    Testing a standard household Square D Homeline 15A circuit breaker on a 290vdc 12A PV array (28 Evergreen 120's in two parallel strings of 14, in about 90% full sun).Thorough destruction of the breaker, with fire shooting out of the sides for a while and molten copper spraying out several times. The arc was finally extinguished only by the terminal melting off the breaker... if it hadn't, it is unknown if the breaker would have eventually opened or not.

    https://www.youtube.com/watch?v=HLX9cdB4TFQ

    And remember that DC Arcs are much more difficult to extinguish than AC Arcs:

    https://www.youtube.com/watch?v=Zez2r1RPpWY

    Lots of details here... Answering one question "exactly" does not mean that the research+design is done (or done safely).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #8
    solar_dave said:
    Remember that most are rated at 40C, that has been an issue with high load circuits here in the past.  On a hot day many times the breakers would trip.  40C isn't far from the normal afternoon high here, panel in hot sun I am sure can be much higher..


    A proper breaker for kaipo_boy's Combiner situation is the MidNite MNEPV 150 VDC-rated breaker.  These appear to have an operating temperature range of -40 to + 65 C,  and are very inexpensive,  quality breakers.  The Trip curves apply at 30 degrees C:
    http://www.midnitesolar.com/pdfs/Q-Series(1).pdf

    BB,  that dramatic video is very compelling on why NOT to use a breaker that has NO DC rating whatsoever (SQ D Homeline breaker).   The SQ D QO, QOB,  and QOU breakers DO have a limited Rating for DC (handle Ratings from 15 to 70 A,  one,  two,  or three poles),  and only to 48 VDC,  at that.

    Not to pick the nit too much.  But,  YES,  use products that are RATED for the application,  install components like breakers,  in boxes/enclosures that are RATED for those breakers in that application,  and so on ...

    FWIW,   Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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