combine dc & ac

gino45
gino45 Registered Users Posts: 12
I have a grid tie system with a sunny boy 3000 inverter which is connected to the grid.

I would like more photovoltaic production, but my inverter is maxed out.

What I would like to do is a a few panels with individual ac inverters which would connect in on the grid side of the sunny boy.

Can this be done?

Thanks!
Gino in Hawaii

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,412 admin
    Re: combine dc & ac

    More or less, this is a "simple" task of looking at your main panel (how many amps it s designed for), how big is the main breaker, and what your local building inspectors/power company require to add on.

    One of the issues may be a simple matter of costs... There may be a new permit, possibly a new main service, and certainly mounts, new branch circuit wiring, panels, and some sort of micro or central inverters.

    If you try to add "small amounts" of panels at a time--The other fees might be too high (minimum charge) to make it worth while. If you are doing the work yourself, you still may have building permit and other fees.

    You may be better off saving your money and adding another 3-5kW minimum at a time (more cost effective). Your area may have a limit on home solar power too (many places it is ~10kW maximum rated inverter before things get "strange").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gino45
    gino45 Registered Users Posts: 12
    Re: combine dc & ac
    BB. wrote: »
    More or less, this is a "simple" task of looking at your main panel (how many amps it s designed for), how big is the main breaker, and what your local building inspectors/power company require to add on.

    One of the issues may be a simple matter of costs... There may be a new permit, possibly a new main service, and certainly mounts, new branch circuit wiring, panels, and some sort of micro or central inverters.

    __________________________

    Thanks for the response. Already installed is a main panel with 2 100 amp breakers on the meter side, and 2 20 amps on the incoming (from the inverter) side. There are also 2 double empty 'sockets' on each side. Could another 20 amp be added from say, 6 250 watt panels and then connected to one of the 100 amp, or would it need an additional 100 amp also?

    Now, I'm not planning on doing this myself; however, I am thinking, that I could up my system by a third or 50% fairly cheaply.

    FYI, our electric use is low because we have no heating, no air conditioning, and a solar water heater; however, grid electricity presently costs over 40 cents per additional kilowatt.

    Thanks again,

    Gino

    Now I am confused. Looking at the pv disconnect box, how is it if I have 14 panels at 7 or 8 amps each, that the dc to ac transfer only has 2 20 amp breakers. I admit to not knowing much about this but say 7 x 14 = 98, and I have 40 amps of circuits breaker? I don't understand.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: combine dc & ac
    gino45 wrote: »
    Now I am confused. Looking at the pv disconnect box, how is it if I have 14 panels at 7 or 8 amps each, that the dc to ac transfer only has 2 20 amp breakers. I admit to not knowing much about this but say 7 x 14 = 98, and I have 40 amps of circuits breaker? I don't understand.

    I guess that you have two strings, each string is 7 panels in series. In a series string volts add up but current does not. Thus each string puts out 7 or 8 amps of DC current. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.
    BB. Super Moderators, Administrators Posts: 33,412 admin
    Re: combine dc & ac
    gino45 wrote: »
    Thanks for the response. Already installed is a main panel with 2 100 amp breakers on the meter side, and 2 20 amps on the incoming (from the inverter) side. There are also 2 double empty 'sockets' on each side. Could another 20 amp be added from say, 6 250 watt panels and then connected to one of the 100 amp, or would it need an additional 100 amp also?

    I am not a code guy or professional installer--But here is the issue, as I understand.

    You have a 100 amp main panel (120/240 VAC split phase power--typical North American Home). And you have a pair of 20 Amp breakers for a 120/240 VAC circuit to your 3kW GT inverter (really 240 VAC only, the neutral carries no useful power).

    The NEC allows you to put up to 20% of the main panel's rating (the bus bars specifically) to support GT solar power.

    100 Amp panel * 20% = 20 Amp maximum for GT solar power.

    You have to check your installation manual for the Inverter--But in round numbers, a 3kW inverter will output:

    3,000 Watts * 1/240 VAC = 12.5 amps nominal maximum current

    The maximum allowed continuous current on a 20 amp branch circuit is:

    20 amps * 0.80 NEC derating = 16 amps

    So, at this point, in theory (if allowed by your inspector), you could add ~3.5 more amps of GT solar power or:

    240 VAC * 3.5 amps = 840 watts

    So--roughly 840 watts (or 3.5 amps) of enphase inverters to your home without installing a new main service (there are some other options--but lets stop here for a moment as they may not be allowed by most inspectors for homes).

    It is not a lot of solar panels that you can add right now. And you cannot add another branch circuit to your existing main service (legally/to code).
    Now, I'm not planning on doing this myself; however, I am thinking, that I could up my system by a third or 50% fairly cheaply.

    You have to check the micro inverter(s) you wish to use--They cannot exceed 3.5 amps in total (so you may have be less, depending on the specific rated output of the micro inverter you wish to use).
    FYI, our electric use is low because we have no heating, no air conditioning, and a solar water heater; however, grid electricity presently costs over 40 cents per additional kilowatt.

    Conservation is still, usually, your best "first" investment of time and money. A Kill-a-Watt type meter can show an old stereo, TV, fridge, desktop computer, etc. using a lot more power than you first expected.

    For example, if you want to get down to ~200 kWH per month (possible if you don't use electricity for cooking/heating/etc.), the average load would be:

    200,000 WH per month * 1/30 days per month * 1/24 hours per day = 278 Watt average load 24 hours per day, 30 days per month

    It is not unusual for a desktop computer to use 100-200 watts just by itself... use it 12 hours per day and that could be almost 1/3 to 1/2 of your electric bill (in a small home/apartment).

    -Bill
    Now I am confused. Looking at the pv disconnect box, how is it if I have 14 panels at 7 or 8 amps each, that the dc to ac transfer only has 2 20 amp breakers. I admit to not knowing much about this but say 7 x 14 = 98, and I have 40 amps of circuits breaker? I don't understand.

    Remember Power=Voltage*Amperes

    Those 14 panels may be organized into 2 parallel strings by 7 panels each... That could be around Vmp-array of 210 volts * 16 amps = 3,360 Watt solar array.

    Taking efficiency into account and the 3,000 watt limit by your inverter--The AC output would be:

    3,000 watts * 1/240 VAC = 12.5 amps (at 240 VAC)

    Amps, without knowing the voltage is only part of the answer... Amps*Volts which equals Watts is the "total" answer...

    More or less the power at some Varray*Iarray=Power-array=Vac*Iac (less losses). The GT inverter just takes the DC solar power and "inverts it" to AC power that can be fed back to the utility grid. It is a "power conversion" process (the GT inverter). No energy was created or destroyed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    Re: combine dc & ac

    bill
    I love your quote below you name. "Just some guy", begets trust not pride.

    Thanks
    gww
  • Rick1
    Rick1 Registered Users Posts: 24
    Re: combine dc & ac
    gino45 wrote: »
    __________________________

    Thanks for the response. Already installed is a main panel with 2 100 amp breakers on the meter side, and 2 20 amps on the incoming (from the inverter) side. There are also 2 double empty 'sockets' on each side. Could another 20 amp be added from say, 6 250 watt panels and then connected to one of the 100 amp, or would it need an additional 100 amp also?

    Now, I'm not planning on doing this myself; however, I am thinking, that I could up my system by a third or 50% fairly cheaply.

    FYI, our electric use is low because we have no heating, no air conditioning, and a solar water heater; however, grid electricity presently costs over 40 cents per additional kilowatt.

    Thanks again,

    Gino



    Now I am confused. Looking at the pv disconnect box, how is it if I have 14 panels at 7 or 8 amps each, that the dc to ac transfer only has 2 20 amp breakers. I admit to not knowing much about this but say 7 x 14 = 98, and I have 40 amps of circuits breaker? I don't understand.

    Hind sight is great. If you had used one 200 amp box with a 225 amp buss bar you could have added that third 20 amp circuit.
  • gino45
    gino45 Registered Users Posts: 12
    Re: combine dc & ac

    Hind sight is great. If you had used one 200 amp box with a 225 amp buss bar you could have added that third 20 amp circuit.

    Thanks to you & the others for good information.

    RE: the above. If my connecting box has a double 100 amp breaker on the side that says 'house main disconnect', am I wrong to assume that is a 200 amp box? Also, that box has a slot(s?) for another double breaker, as it does on the right side which says 'pv disconnect'. I was thinking add another 100 on the left, and a 20 on the right and I'm good for another 7 250 watt panels.

    I do appreciate you guys time and effort, I have low watt lights and go around trying to limit unnecessary usage. Still we come out to 14+ kw a day. It's freezers and frig and electric stove, not to mention tvs and computers. And I'm not getting as many watts as hoped for from the initial install.

    Thanks!
    Gino
  • BB.
    BB. Super Moderators, Administrators Posts: 33,412 admin
    Re: combine dc & ac

    No--100 amps main breakers is still 100 amp @ 120/240 VAC service. The two breakers are in series at 240 VAC (100 amps in one breaker and 100 amps out the other--They do not add).

    I am a bit surprised at 100 amp service with the electric stove and such... In our area, they seem to do 125 amp minimum service for an all natural gas home (don't know if it is code or not--My home from ~1955 was a 60 amp service).

    It is possible that you have larger bus bars in the main panel--You would need an electrician to be sure (or look for the box id plate and see what it says/look up part number on web).

    Even if you have a larger box and could put in 200 am service--Then you would need a new permit (I am guessing) and a call to your utility to make sure your service can manage a (much) larger main service.

    You have to follow code in how they calculate adding more sources to the panel--But, I don't think you can add another 20 amp PV breaker at all (20 amp for your first pair is already maximum). You might be able to add another ~3.5 amps of Micro Inverters to the existing 20 amp branch circuit--But you would have to work closely with the building inspector to see what he would allow (he could just as easily say no too about adding more micro inverters).

    There are other possible options--But probably will not be allowed for residential install anyway (line side tap).

    The only option left would be to install a larger main panel. You can install a 200 amp panel and use 175 amp service disconnect--and you will get:
    ....200 amps * 20% solar allowance + (200 amp - 175 amp service breakers) = 75 amps maximum of 240 VAC solar power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: combine dc & ac
    gino45 wrote: »
    RE: the above. If my connecting box has a double 100 amp breaker on the side that says 'house main disconnect', am I wrong to assume that is a 200 amp box?
    Yes, you are wrong. It's a 100A panel.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: combine dc & ac
    BB. wrote: »

    It is possible that you have larger bus bars in the main panel--You would need an electrician to be sure (or look for the box id plate and see what it says/look up part number on web).

    Even if you have a larger box and could put in 200 am service--Then you would need a new permit (I am guessing) and a call to your utility to make sure your service can manage a (much) larger main service.
    -Bill

    If the original panel has 125 amp or higher rated bus bars (which is possible for a panel originally sold with a 100 Amp main breaker, often the 100 and 125 amp panels use the same bus bars), then the OP would not need to change his service or change the main breaker. With a 125 amp bus (pair), he could go as high as 50 amps by 240 total backfeed.

    But even with no change to the service equipment, you are correct that technically the upgrade, even to just add panels with microinverters to the existing system with no other wiring changes ,would normally require a new permit from the AHJ and a new agreement with the POCO.

    And it is possible that the POCO has a policy which limits the backfeed to a fixed maximum percentage of the service amperage, above and beyond what is required in the NEC.
    SMA SB 3000, old BP panels.
  • gino45
    gino45 Registered Users Posts: 12
    Re: combine dc & ac
    inetdog wrote: »
    If the original panel has 125 amp or higher rated bus bars (which is possible for a panel originally sold with a 100 Amp main breaker, often the 100 and 125 amp panels use the same bus bars), then the OP would not need to change his service or change the main breaker. With a 125 amp bus (pair), he could go as high as 50 amps by 240 total backfeed.

    FYI
    Checked out the main box (the one with the meter), and it has a sticker that reads 125 amp max.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,412 admin
    Re: combine dc & ac

    You may be lucky there--You need to verify that the bus bars (which all the breakers plug into) is also rated at 125 amps (and not just the meter socket).

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