Main Panel Grid Tie Feed

Hi,
I am in the process of installing a grid tie 7,560 W enphase micro inverter system and I have some general questions about wiring the system to my main panel. Preferably I would like to make sure I can keep my main panel and not do any major changes to my current home wiring. The panel I have installed has a 200 A main breaker. All breaker slots are full althought it is a pretty small home (1,800 sqft). It is almost as if the previous owner/electrician focused on having as few loads as possible on each breaker instead of keeping the slot efficiency in the panel.
I am planning on installing the system myself and have an inspector do the final approval before I do the grid connection. Reading up on the general NEC rules related to PV panels I came across the 120% rule, which in my case would mean I can have summed up value of 240 A on each leg in the main panel. 
However currently I have a sum on the breakers of 240 A on the left leg and 270 A on the right leg which is way above the 240 A allowed according to this guideline. I know for a fact that there is no way that my home would ever utilize the 200 A.
In order to accommodate the input for my PV panels my idea was to switch four of the 15 A breakers out on the left with narrow breakers to free up two breakers slots so I can install a 40 A breaker. This would allow me to keep the PV input at no more that 120%  (40 A).
However it would make the sum of all breakers on the left leg 280 A. My questions is really if the NEC guideline is really related to the PV taking up that additional 20% and if the other breakers matter or if it is required that the sum of all breakers on each leg does not exceed 120%.

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Talk to the inspector.  See what he wants.   It doesn't matter what's in the NEC, you will do what the inspector wants :(

    Regarding your question, I believe the answer has something to do with the bus bar rating in your electrical panel... but I don't know much about grid-tie systems.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    There is a calculation in the NEC that talks about how large a panel you need vs your branch circuit total breakers. But, I believe it is "OK" to have more breakers than the rating of the main breaker (for homes, the NEC does not assume that each breaker is running at maximum rated current at the same time).

    For your GT solar circuits, you have a 200 Amp rated box+bus bar (typically, although the box+bus bars may be rated >200 amps). Say you can have a total of 200 amp main box rating + 20% more for GT branch circuits (40 amps). Remember that the GT circuits are pumping "power" back into the box and, in theory reduce the current through the bus bars and main breakers, not add to it.

    And, if you need more solar circuits, you can back down on the main breaker (say to 175 amps) and now you have 240a-175a breaker=65a rated GT solar circuits total allowed.

    It should be possible to add a sub panel (one 240 VAC branch circuit to your main panel) and you can have multiple 20 amp GT solar circuits in your sub panel--The sub panel would be labeled solar generator circuits, no load circuits allowed (to keep the inspectors happy). That way you only "burn" one pair of slots in your main panel. And depending on the brand/model of your main panel, you could use some 1/2 width breakers to get some more circuits into a smaller space too.

    I am not not very NEC knowledgeable--And recommend you follow vtMap's suggestion of talking to the inspector directly (and a licensed electrician). They will be the people you have to satisfy (bring your drawings, and any code calculations too). Just beware, solar GT is a bit "different" than normal residential/commercial power requirements. They may not understand the requirements and get a bit lost at times too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    BB. said:
     Remember that the GT circuits are pumping "power" back into the box and, in theory reduce the current through the bus bars and main breakers, not add to it.
    That is correct for the main breaker.  That is also correct for the bus bar, but only if the grid and the solar are (as required) connected at opposite ends of the bus bar.  

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • FazusFazus Registered Users Posts: 2
    Thank you for the answer. I will consult with the inspector. I have attached a picture so you can see what my panel looks like. Like I said i would prefer to not install additional panels if it is not necessary, just free up some space at the bottom by using narrow breakers and install a 40 A for the PV. It should not need more than 40 A for 7,580 W. Regards Jonas
     
  • m151m151 Registered Users Posts: 26 ✭✭
    depending on the make of your panel, you may be able to double tap some breakers. (Square D QO) and gain space. Double tap breakers are not illegal if so rated.
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    If you do "double tap" breakers... Beware, the two 180 degree hot out of phase phases (black and red) can turn into parallel "hot in phase phases" (2x black or 2x red). If you have a black+red sharing one white (neutral) return wire, that will cause 2x current flow in the return neutral and cause it to overheat/possibly catch fire.

    It depends on how the house was wired (some folks did this, others did not). You can move the connections around in the "rewired" box to properly pin out the black+red+white circuits without problem--but you need to be careful.

    Of course, if you have 240 VAC appliances (drier, electric stove, A/C, etc...), the "miss-wired" 240 VAC appliances will not work.

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