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What size Transformer and Line doesElectric company need to install for a 15kw system

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  • What size Transformer and Line doesElectric company need to install for a 15kw system

    My power company is running brand new service to my barn which will have a 15kw worth of panels on the roof, with a battery back up system. They want to know if 350 MCM aluminum conductor and a 25 KVA transformer will be enough. I will have a 400 amp service. After reading your forum, I also wanted to confirm the info I have read. I am installing 15kw worth of panels on the Garage, and have convert to DC and have my batteries in the barn, and run the power DC to the house which will be about 450 feet from the house. Thanks in advance for your help.

    Matt

  • #2
    Originally posted by mrrrkva View Post
    ... installing 15kw worth of panels on the Garage, and have my batteries in the barn, and run the power DC to the house which will be about 450 feet from the house ....
    HUH?

    The PV panels are on the garage (house).
    The batteries are in the barn.
    Then you are running 450' of DC cables between the house and the barn?
    Then what?
    Where is the Inverter and what type?
    Your description of your project is very difficult to understand.

    Don't you already have approved electrical drawings?

    You can start with this ...
    62.5 Amps = 15,000 Watts PV / 240 Volts

    How does a 25KVA transformer allow for a 400 Amp Service?

    Show us your electrical drawing and your math ...
    Last edited by NEOH; 09-14-2017, 11:32 AM.

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    • #3
      To clarify my Garage is my Barn and its 450 feet from the house and the Barn will have the solar panels (I am flexible on everything else) ... I don't have approved drawings as I wont install the solar panels until next summer. I have an idea of what I want and I am in the planning stages now but need to run power to the barn now. I don't want to come back and "buy" a upgraded transformer. The power company gives us an "allotted budget" when they First install it. I need an idea of How big of system I can have with a 25KVA transformer. I have NO idea what I should be telling the power company. What size transformer should be on a 400 amp service? I am the only house on this transformer, as the line I am pulling from is a transmission line going thru the back of my property. I will have 60 260 watt PV panels on my Barn, and the batteries and invertors will in the barn. ( I am open to change, if it makes sense to put the batteries and invertors in my house vs In my barn. But my wife doesn't want the panels on "HER" house unless we can do Tesla's panels ( I am concerned with cost) I know I want basically a 15kw system and liked this package. https://www.wholesalesolar.com/18910...rgy-260-panels The link will also have some drawings but they aren't mine. Basically I would like the Solar panels and all the equipment in the barn and feed the power AC (not DC as I first said) to my fuse box on the house IF this is the SMART way to do it. Again, I am open to moving the equipment (EXCEPT the solar panels) to the house If sending DC from the panels to the house 450 feet way is smarter.

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      • #4
        Before we get too deep in how to execute your plan, please write more about your objectives. Why solar panels at all? Why batteries?
        CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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        • #5
          Originally posted by mrrrkva View Post
          To clarify my Garage is my Barn and its 450 feet from the house and the Barn will have the solar panels (I am flexible on everything else) ... I don't have approved drawings as I wont install the solar panels until next summer. I have an idea of what I want and I am in the planning stages now but need to run power to the barn now. I don't want to come back and "buy" a upgraded transformer. The power company gives us an "allotted budget" when they First install it. I need an idea of How big of system I can have with a 25KVA transformer. I have NO idea what I should be telling the power company. What size transformer should be on a 400 amp service? I am the only house on this transformer, as the line I am pulling from is a transmission line going thru the back of my property. I will have 60 260 watt PV panels on my Barn, and the batteries and invertors will in the barn. ( I am open to change, if it makes sense to put the batteries and invertors in my house vs In my barn. But my wife doesn't want the panels on "HER" house unless we can do Tesla's panels ( I am concerned with cost) I know I want basically a 15kw system and liked this package. https://www.wholesalesolar.com/18910...rgy-260-panels The link will also have some drawings but they aren't mine. Basically I would like the Solar panels and all the equipment in the barn and feed the power AC (not DC as I first said) to my fuse box on the house IF this is the SMART way to do it. Again, I am open to moving the equipment (EXCEPT the solar panels) to the house If sending DC from the panels to the house 450 feet way is smarter.
          as #2 post pointed out you have some mismatch between 400A and 25kVA transformer. All things perfect 25kVA transformer can't produce more than 25,000W / 240V = 104A Basically you can have wires thick enough and service panel rated for 400A but transformer will never pass more than 100 A. For 400A service you'd need at least 100kVA transformer.

          Running high voltage DC (~440V) would allow to use smaller wires or have less losses compare to running 240V AC over the same 450' distance but please give us 'bigger picture' on your project goals as post #4 is asking. Do you have any shading on your barn? Try to use PVWatts to estimate your PV production at your location and azimuth: 15kW is 'rated' power, you won't actually ever see it.

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          • #6
            I am not afraid to show my ignorance. Are you saying that the power company is asking you if their proposed 25 KVA transformer and MCM conductors will be sufficient to accept the power generated by your 15 kW grid tied solar system? I think the '400A service' statement may be a red herring in this case. The statement "I am the only house on the transformer" does seem relevant though.
            Last edited by AzRoute66; 09-14-2017, 02:50 PM.

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            • #7
              Why solar panels? I would like to be able to have a Net Zero home. I know this wont run everything, but the basics would be nice. I am building a 2800 SF "ICF" home If the power goes out for 3 or 4 days, I would like to be able to get by on solar. I know a generator is cheaper...... But Eventually I would like the ability to go off grid in a catastrophe. I would like to at least be able to run my 40 foot RV systems. I would like to have battery back up to be able to go off grid. I know I would probably have to add more batteries in the future. 38.100 Lat -79.000 Long Azimuth 199 (magn) 208.5. Project goals are to keep the solar panels off the house. It sounds like it is going to be better to PV panels on roof, and put my invertors and batteries in the house. What size wires would be required to run to my house. Barn has no shade, and I am currently building. It is due south, I couldn't turn any more SW...... I will get back to you on PVWatts later this afternoon

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              • #8
                Originally posted by mrrrkva View Post
                Why solar panels? I would like to be able to have a Net Zero home. I know this wont run everything, but the basics would be nice. I am building a 2800 SF "ICF" home If the power goes out for 3 or 4 days, I would like to be able to get by on solar. I know a generator is cheaper...... But Eventually I would like the ability to go off grid in a catastrophe. I would like to at least be able to run my 40 foot RV systems. I would like to have battery back up to be able to go off grid. I know I would probably have to add more batteries in the future. 38.100 Lat -79.000 Long Azimuth 199 (magn) 208.5. Project goals are to keep the solar panels off the house. It sounds like it is going to be better to PV panels on roof, and put my invertors and batteries in the house. What size wires would be required to run to my house. Barn has no shade, and I am currently building. It is due south, I couldn't turn any more SW...... I will get back to you on PVWatts later this afternoon
                just small clarification- 'azimuth' in PVOutput means azimuth of direction you PV array facing counting from the North in clockwise direction. Straight South would be180 deg; West would be 270 deg.

                In terms of batteries and even overall system it helps a lot if you start from your loads and work your way back. PV and especially batteries are not flexible in terms of future extensions so you'd spare yourself a lot of money and headache if you do it this way.

                for your transformer it also helps to know your total loads. If you indeed have loads which would require 400A in total then you definitely would need bigger transformer.

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                • #9
                  My PV array is facing 180 degrees Due south. I am still designing the house. I wont know the loads. The issue is the power company. I have learned a lot from this discussion, and never thought I would have to figure things out for the power company. So it looks like the common consensus is a 100 kva transformer and 750 MCM aluminum wire is what they should use with a 400 amp service. If they use this set up, my "little 15kw system" wouldn't overload that size wire or transformer. I haven't determined my loads yet, but even if I had a 30kw system, it seems like it wouldn't over load their wire and transformer. I will definitely plan out my house and my loads but the Power company wants the egg before the chicken. Does anyone think differently?

                  Comment


                  • #10
                    Originally posted by mrrrkva View Post
                    My PV array is facing 180 degrees Due south. I am still designing the house. I wont know the loads. The issue is the power company. I have learned a lot from this discussion, and never thought I would have to figure things out for the power company. So it looks like the common consensus is a 100 kva transformer and 750 MCM aluminum wire is what they should use with a 400 amp service. If they use this set up, my "little 15kw system" wouldn't overload that size wire or transformer. I haven't determined my loads yet, but even if I had a 30kw system, it seems like it wouldn't over load their wire and transformer. I will definitely plan out my house and my loads but the Power company wants the egg before the chicken. Does anyone think differently?
                    I'd like to clarify something: my only issue with your original post was transformer to your service entrance/MSP mismatch, not solar system. 15kW solar system wouldn't be able to overload even 25kW transformer let alone 100kW:
                    - 15kW is less than 25kW
                    - 15kW is 'rating', it will probably never produce that much
                    - your house loads will subtract from 15kW so utility will ever see may be few kW in the middle of the day

                    Now if you're planning to have sizeable loads (otherwise why 400A service?) that's when that 25kW transformer would become a bottleneck. My understanding is you live in some rural/farm setting and 100A is nothing there- my city located house has 200A service. Any sizeable equipment and you'll be begging them for new transformer. Luckily the entrance/MSP will be 400A so all they'd have to do at that point is to swap your 25kW transformer with 100kW one and probably replace main breaker in MSP. I think utility just likes to provide you service .
                    Last edited by max2k; 09-15-2017, 12:11 AM.

                    Comment


                    • #11
                      Solar power backfeeding the Grid is a problem. The power is constant, and not intermittent like house loads. (washer pauses in the wash cycle, the thermostat on the clothes iron cycles on and off) You have to size the wires & conduit to carry 100% of load and not heat up much. Same with the loading on the Transformer.
                      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                      solar: http://tinyurl.com/LMR-Solar
                      gen: http://tinyurl.com/LMR-Lister

                      Comment


                      • #12
                        Originally posted by Mike90250 View Post
                        Solar power backfeeding the Grid is a problem. The power is constant, and not intermittent like house loads. (washer pauses in the wash cycle, the thermostat on the clothes iron cycles on and off) You have to size the wires & conduit to carry 100% of load and not heat up much. Same with the loading on the Transformer.
                        Correct me if I'm wrong but '750 MCM aluminum wire' is rated to around 400A continuous and OP's MSP sounds like is going to be 400A while 15kW solar can only produce 62.5A of backfeed. 25kW transformer surely should be capable to transform 15kW in the opposite direction even if no house loads are on and the Sun is in the sweetest spot possible. Where is the problem from the solar side?

                        Comment


                        • #13
                          A new 2800 sq ft ICF home should not be needing a 15K kW DC system IMHO.

                          And 400 amp service for a barn? What the heck are you doing?

                          I will be building next year (3000 to 3500 sq ft + barn) and after posting and reading here, I'm staying far away from off grid / battery storage. And I fully expect to be putting in a smaller array next year than my current 8K kW array (current house is 3400 sq ft and 30 years old -- fully expect new construction to be much more efficient energy use wise),
                          8 kWp gnd;SE 7600A;P400;Hanwha Q.PLUS L-G4.2 335W

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                          • #14
                            If you want a 400A service, that is all the utility should need to know. They should size their equipment accordingly. Now how you are going to back feed 15kW of solar does need to be planned out as a 400A service still only has a 200A buss which is only enough for 7.7kW (40A) of solar. Your 15kW solar needs to be a line tap before the buss, and the subpanel this tap also feeds needs to be protected by its own 200A main breaker.
                            BSEE, R11, NABCEP, >1200kW installed

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                            • #15
                              If the OP gets a "400 Amp Service" installed, would the electricians be installing two (2) @ 200 Amp Main Panels?
                              Does NEC then allow two (2) separate 7.7 KW Solar PV Arrays, with each array being backfed into its own 200 Amp Main Panel?
                              That may eliminate the need for a Line Side tap but it does limit future growth.

                              Depending upon location, a 15 KW PV Array could generate 100% of the KWHr's consumed.

                              The OP needs to make a single-line drawing today.

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