want to expand my solar system but unsure if sub panels can handle it...

after buying an electric vehicle and a hot tub I'm looking to expand my existing 3.5kw system by adding another 4kw system.  Both do/will use enphase microinverters.   While not an electrician I have done virtually everything from build my house, installing EV chargers and helping my contractor friend on my last solar install.  Same gig this time but now I'll be taking on more of the roll of planner while my contractor friend is the helper ;)

In any case I've read about the 120% rule and am still a bit unsure.  My house has a 200amp main feed.  This feeds two 100 amp sub-panels with 100 amp breakers, both in different locations, but both would make good locations for the new solar to feed into.  However, these are not dedicated PV sub panels.  One runs the upstairs of my house, the other runs a small rental house.  I'd like some clarity on if the 120% rule applies only to the backfeed of the solar (i.e. 100amp panel X 1.2 = 20 amp max breaker for the PV feed), or do you also have to take into account the load of all the other breakers in the sub panel?  


  • TecnodaveTecnodave Registered Users Posts: 418 ✭✭✭✭

    The 120 percent rule mainly applies to the main panel, but can also be applied to the sub panels. To be legal the second grid tie in needs to be on the second sub panel. If you are metering this second subpanel this will credit your renter, not you.  Does your main panel have enough breaker spaces for the second solar system to be connected there?  That may be the better location for the second inverter.

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • barndoorbarndoor Registered Users Posts: 2
    Thanks for the help.  For clarity, there is only one meter, with the rental house having it's own sub panel.  It's roof would be a convenient location with all day southern exposure.  But I could also expand up at my main house, which is almost as good.

    I can see how the 120% rule ultimately applies to the main, since everything makes it back there.  Where I'm struggling is how to figure in all the other circuits.  I could feasibly feed into my main panel, though getting the wires to there will be challenging, but in that main panel there are car chargers, sub-panel circuits, etc.  In other words lot's of stuff drawing lots of current.  Does that matter, or does the 120% rule only apply to the rating of the panel?  It would seem to me that the other circuits must be factored in, but in doing research I haven't found any mention of a load calc being needed.  
  • billybob9billybob9 Registered Users Posts: 141 ✭✭
    I don't believe you can pass an NEC building code inspection on your main breaker panel unless your back feed breakers are on one end of the panel and your Main breakers are on the other end. Mid main panel breakers break the 120% rule by being to close to the back feed. Just in case yours are in the middle.
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