Circuit breaker/fuse rating?

kaipo_boykaipo_boy Solar Expert Posts: 143 ✭✭

I'm trying to rate the capacity I should get for my circuit breakers or fuses, both at the PV combiner boxes and as a safety switch just before the DC power enters the charge controller... do you just figure the max amps your system will have and use the next higher rated breaker or fuse? or do you go a couple steps past that or even more? (to prevent a stray triggering in the event of a weird surge or something that might turn off your system while you are gone)

Also, I didn't know anyone was still using fuses anymore, thought they'd all gone to breakers; but looks like there is a small price premium for resettable breakers over fuses; but I don't like keeping spares around, so will probably just pay slightly more and get breakers. Is this how most people do it now? its related to the above question, because if the rating you choose is a lot higher, there probably won't be instances of unintentional tripping.

aloha,

walt

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    In general, the purpose of circuit breakers is to protect the wiring.  A breaker must be rated less than the ampacity of the wire it is protecting.

    The cable from the combiner to the controller must be able to handle more current than the array can produce.  The breaker in that cable is more for convenience (as a switch) than for safety because the array can't produce enough current to trip the breaker or exceed the ampacity of the cable.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • littleharbor2littleharbor2 Solar Expert Posts: 1,405 ✭✭✭✭
    Midnite Solar dc breakers are pretty inexpensive. True dc rated PV fuses are actually not cheap combined with the cost of the holders.

    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.

  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    For typical North American NEC rated fusing and wiring... They are designed to be operated at 80% of rated current.

    For example, a 15 Amp 14 AWG circuit (i.e., typical house wiring):
    • 15 amps * 0.80 = 12 amps continuous
    Working the other way, if you have, for example a 20 amp load, then 1/0.80 or 1.25x continuous load for breaker/wiring rating:
    • 20 amps * 1/0.80 = 20 amps * 1.25 NEC derating = 25 amps minimum branch circuit
    NEC is a lot more complex (insulation temperature, ambient temperature, size of conduit, numbers of wires in conduit, etc.)... But the above is a basic starting point.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • kaipo_boykaipo_boy Solar Expert Posts: 143 ✭✭
    Thanks Bill! As usual, very helpful.   In a sort of related question, I am looking at safety disconnect boxes/switches and noticed that none of them rate their boxes in DC current; only AC (usually 240v 3 phase); do you guys use these boxes to cut DC current and just use one of the throws in the box? or split the positive into 3 smaller wires and use all 3 throws? (seems a shame to waste them) how do the AC amps compare to the DC rating for these types of switches?
  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    The "right way"--You need to find DC rated switch gear (and/or you DC rated breakers as your disconnects).

    There is no way (that I know of) that you can assume a ratio of AC to DC ratings (current and voltage switching ratings).

    Can you let us know what size disconnect(s) you are looking for? Perhaps some other folks here can supply some links to ones you can use.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Raj174Raj174 Solar Expert Posts: 696 ✭✭✭✭
    Along with the combiner box I'd recommend getting a Midnite epanel. It comes with the 175 amp breaker installed with 
    busbars, mounting locations for PV and CC breakers and more. It makes installation, wiring and connections much easier.

    I believe that this the model for your inverter.

    http://www.solar-electric.com/installation-parts-and-equipment/midnite/misoe/epaforouin/mne175sts-l.html
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 195AH LiFePO4 no BMS, 4500W genset.
  • kaipo_boykaipo_boy Solar Expert Posts: 143 ✭✭
    Thanks, guys. This is a lot more involved that I thought...
    Bill, it will be max 115vdc at around 24A.
    Raj174, am I reading that right? the inverter mounts to the e-panel's door? the Outback 3524 probably weighs close to 60 lbs...
  • Raj174Raj174 Solar Expert Posts: 696 ✭✭✭✭
    edited April 2016 #9
    yep, that's how it mounts. The epanel allows easy installation of things like SPDs, bypass switches and a limited number of AC breakers.

    This a photo of mine during installation....
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 195AH LiFePO4 no BMS, 4500W genset.
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