Question about combining branch AC circuits

I will have 3 branch circuits, each producing 16 amps, total combined is 48 amps.  I am using Emphase M250 inverters.  According to the Enphase tech instructions I am supposed to put a 20 A breaker on each of these circuits before combining them at a none fused AC shutoff switch.  The cable from the AC shutoff switch to my Service Panel is 4 gauge (calculated to easily handle the total possible 48 amps).   That cable then connects through a 60 amp, 240 volt circuit on the Service Panel.


My question is, why to I need the 20 A breakers on each of the branch circuits.  It is my understanding that since each of the 3 branches are not only connected in-phase but also in parallel at the AC Shutoff switch.  Therefore these 20 A breakers appear to be redundant and not necessary.   Seems putting them in the circuits just adds another component to fail and add resistance.

I called Enphase their tech support was unsure, but said that is the way their engineers tested their circuit design, so that is what the recommend.


Need some feedback, thanks,

Bill Haas   

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Comments

  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    In the event that there were a short in the conductors between the enphases and the combiner, the combined current plus the current from the grid could flow through that short.  A breaker's function is to limit the current to what the wires can handle.  Also, the breaker acts as a switch to de-energize the branch circuit... very handy if you need to service the branch.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • JAZ140JAZ140 Registered Users Posts: 8 ✭✭
    The breakers will protect the12 gauge wire in the individual branch circuits in the event of a fault.  
    30 SolarWorld 310xl. SMA 7700tl-22
    20 SolarWorld 320xl. SMA 5000tl-22
  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    What the folks above said... Just to put some numbers to the explanation (help understanding).

    Say circuit "C" had a short circuit. That 12 AWG wire could be fed by the 60 amp main circuit + 2x 16 amps from the other two circuits, plus 1x 16 amps from the inverters in branch "C". That totals to 108 Amps (without tripping the 60 amp breaker back in the main panel) into a 12 AWG cable to a short (assuming a worst case short happened in the wiring at the "last inverter" in the "C" chain).

    Mixed load+generating system (like Grid Tied Solar) can create very complex fault conditions.

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