Backfeed Breaker Sizing Question

Hi all!  Thanks for this forum.  I am in the early stages of planning my system and one area I am encountering an issue/confusion is with the back feed into the main panel.  I will preface this by saying I have a very large house that uses quite a bit of power. It has 400amp service with 2 200amp main panels.

I have not entirely settled on panels yet, but the basic plan is an ~20kW system using the enphase microinverters. The thought is 4 branch strings of 13 panels each (max for the iq7+) for a total of 52 panels.  These would all meet in an enphase combiner box on their own 20amp breakers. Then that box would connect to one of my main panels.  Here is where I am a little confused.  The iq7+ generates 1.21 amps each so a total of 62.92amps for all 52.  Plus the 25% code requirement means 78.65amps so I need an 80amp breaker in the main panel to back feed correct?  

Assuming what I have said above is even correct, I still have the issue that I cannot get an 80amp breaker in either panel.  Both 200amp panels have 200amp main breakers thus leaving me only 40amps for the PV.  I understand I could try to get them to let me derate it, but in order to get the extra 40amps I would have to derate all the way down to a 150amp main which I am not certain I can even do either (I have not done a load evaluation).  I could maybe consider upgrading to a 225amp panel and derating to get there but that seems like a huge expensive pain.

So I guess my question is, first am I missing something or is what I am saying at least mostly accurate.  Second, what are my options?  What do people who are installing larger systems to the grid do?  It sounds like a line side tap is not generally allowed for residential systems.  I really appreciate any insight any of you can provide me. Thanks!!


  • BB.BB. Super Moderators, Administrators Posts: 32,097 admin
    Welcome to the forum NJ,

    First, have you a understanding/agreement with the utility about what they will accept for Solar Wattage and Billing Plan? This stuff is all highly dependent on your location (city/county/state), the engineering (can the local distribution system take a 20 kWatt solar system?), the "approved" billing plans, and any utility utilities.

    There are lots of issues going down the road... Some states have outlawed new installs, made new rules that made new installs financially unreasonable (40% taxes, reduced rates of payments/credits)... In Canada/Quebec(?) there have been permitted installs where the utilities have refused connections after the systems have been built. And some states are "adjusting" the plans--In California, they have raised the rates (to make up for green power requirements--Now $0.20 to $0.40+ per kWH costs) and adjusted plans that increase power costs after 5-8-10pm--Can't generate solar energy in the dark. In California, they will give a "grandfather" of rate plan, but after 10 years--They promise nothing (my plans are now changing every 6 months).

    You may also need a Solar Lockout that a utility person can pull the knife switch and padlock. In some states (such as California), they no longer required solar lockouts (the original threat was the utility may pull the meter to ensure that the GT solar does not back-feed energy to the grid--Which is not an issue with UL/NRTL Listed GT inverters anyway).

    I forgot the term *getting old), but there can be the ability to connect two meters to one utility drop. But that can be controversial (may be limited to commercial properties and such). And you get into the issue of two meters (one for the home, a second for solar--Some states do require two meters anyway)...

    At this point you may need to hire a (good) Power Engineer to do the design and wet stamped engineering drawings.

    If you put two Enphase branch circuits on each of your 200 Watt boxes (the GT solar does not have to all go to one box)... then you are looking at 120% * 200 Amp = 240 Amp solar rated--Or 40 Amp GT Solar for 2x 20 Amp Enphase circuits on each box (if I am following your setup/reasoning). That should work...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • njschwartznjschwartz Registered Users Posts: 2
    Bill,   thanks for all that info. As far as the utility,  in Wisconsin we have net metering, so overall I'm good there. My utility coop generally allows 10kW systems,  but will make exceptions for large homes etc. up to 20kW where you need major engineering reviews.  I'm waiting for verification of that approval but can adjust the size as needed to meet their requirements. It does not appear they will require the lockout as long as there's the emergency shutdown which is built into the newer inverters.

    I have considered splitting the back feed into the two panels 40amps each. That could be doable, but the panels aren't close,  opposite ends of a big house,  which means a long expensive wire running through. I guess it's another option. Thanks!!

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