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

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  • #16
    Originally posted by solarix View Post
    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.
    I don't think it'd be done as a line tap.

    My *guess* at how it would be setup would be a 400A main panel with a pair of 200A breakers. One of those would feed a 200A subpanel which would have a 150A breaker that'd go to a subpanel for the loads, and a 80A for the breaker for the solar.

    Or it'd be one 200A breaker to a subpanel for generation and the other 200A breaker to all loads.

    My *guess* is that it'd be the first choice - and it'd be 200A for barn (including solar production) and 200A for the house.

    But it is just a guess.

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    • #17
      My question is who has performed the electrical engineering design for this 400amp panel and service connection?

      The OP should be able to find all of the details on the submittal drawings that go to the POCO otherwise they are taking a chance on not getting what they want.

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      • #18
        This is all complete nonsense. 400 -amp service to a house. Your house must be in excess of 10,000 ft/2. If that is the case you should not be asking anyone anything. You can afford to have a licensed engineer to design this monster size system. You gotta be nuts to ask a DIY forum for advice on a Commercialized system. Add batteries and you are a fool. Your gonna spend $50K every few years replacing batteries. Hope you are filthy rich.
        Last edited by Sunking; 09-15-2017, 03:12 PM.
        MSEE, PE

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        • #19
          Originally posted by Sunking View Post
          This is all complete nonsense. 400 -amp service to a house. Your house must be in excess of 10,000 ft/2.
          It's a barn + house in the country.
          400A service for a property like that would not be unheard of.
          Since he's planning on 15kW of solar, I'd assume he's planning on something that'll consume a lot of electricity. In a farm setting that often means some good sized motors (air compressors, AC compressors, fans, other mechanical items for moving feed etc etc.

          have a licensed engineer to design this monster size system ... a Commercialized system.
          I would call 400A a residential system.
          I can buy a standard 400A main panel at the big-box store. And an electrician who does residential wouldn't see it as really much different than other residential installs (while doing a commercial building often means something more than 400A and different style of equipment.)
          I don't see a need for a licensed engineer for this - it wouldn't hurt - but I think there are lots of electricians who will have done similar sized residential service entrances and would be capable. And wouldn't charge an arm and a leg.
          IMO - talk to your architect about getting someone to come up with high-level electrical plans for the property (I am assuming you have an architect for the planned house)

          Add batteries and you are a fool.
          Can't really argue with that...
          A generator would almost always be a better/cheaper choice.

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          • #20
            Originally posted by foo1bar View Post
            Since he's planning on 15kW of solar, .
            Here is some food facts. A rural 200 amp service from the POCO uses a 12 to 17 KVA transformer for a 200 -Amp service and a 1/0 AWG copper feeder or 4/0 aluminum.
            MSEE, PE

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            • #21
              Originally posted by foo1bar View Post
              It's a barn + house in the country.
              400A service for a property like that would not be unheard of.
              Since he's planning on 15kW of solar, I'd assume he's planning on something that'll consume a lot of electricity. In a farm setting that often means some good sized motors (air compressors, AC compressors, fans, other mechanical items for moving feed etc etc.


              I would call 400A a residential system.
              I can buy a standard 400A main panel at the big-box store. And an electrician who does residential wouldn't see it as really much different than other residential installs (while doing a commercial building often means something more than 400A and different style of equipment.)
              I don't see a need for a licensed engineer for this - it wouldn't hurt - but I think there are lots of electricians who will have done similar sized residential service entrances and would be capable. And wouldn't charge an arm and a leg.
              IMO - talk to your architect about getting someone to come up with high-level electrical plans for the property (I am assuming you have an architect for the planned house)


              Can't really argue with that...
              A generator would almost always be a better/cheaper choice.
              Excuse me but a 400 amp service is not considered to be a residential panel size. And while most electricians probably can wire up a 400 amp panel I have found they usually follow directions from an installation drawing and not just wing it.

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              • #22
                Originally posted by SunEagle View Post

                Excuse me but a 400 amp service is not considered to be a residential panel size. And while most electricians probably can wire up a 400 amp panel I have found they usually follow directions from an installation drawing and not just wing it.
                I'd expect in a countryside this wouldn't be so unusual if they run some of their equipment from electricity. I'd agree though this needs to be planned and executed as planned, this forum is poor source for recommendations at such scale. My only concern from the beginning was transformer/service mismatch- why go to trouble to wire 400A service/MSP and put 25kW transformer?

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                • #23
                  I encountered a similar situation that the OP described. I installed a 17.25kW DC system that produced 15.0kW maximum AC output to the grid. I had a 15kva transformer on the pole. My POCO felt that the 15kva would handle the load...but to be on the safe side upgraded the can to 25kva. No cost to me.....out of the goodness of their utility heart. Sometimes its good to be on the bleeding edge.

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                  • #24
                    Originally posted by max2k View Post

                    I'd expect in a countryside this wouldn't be so unusual if they run some of their equipment from electricity. I'd agree though this needs to be planned and executed as planned, this forum is poor source for recommendations at such scale. My only concern from the beginning was transformer/service mismatch- why go to trouble to wire 400A service/MSP and put 25kW transformer?
                    Most residences are not really bigger than 3500 sf and can easily be powered by 200amps. Sure there are some very big homes exceeding 5000 sf or a place that also includes farm equipment which would need more power but estates and farms are not really considered as a standard residence.

                    IMO if you own and live at an estate or farm you should be able to afford someone to perform an engineering study and provide all of the required documents concerning the electrical part of the installation.

                    The OP sounds more like someone that just wants to install a very big solar array and really does not have a clue on what is needed and why.

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                    • #25
                      you should not be asking anyone anything. You can afford to have a licensed engineer to design this monster size system. You gotta be nuts to ask a DIY forum for al
                      I almost replied to the OP this exact comment, but didnt want to be the first post to be so harsh considering I dont have the resume to answer with any legitimate veracity. If the utility was really asking ME to spec out their equipment size, I would especially want an engineer, whom I vetted and pay to have skin in the accuracy of such a recommendation, to look over the details of the setup before I go having the utlity spend labor and equipment that I might have pay for twice if I am mistaken.

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                      • #26
                        What I know about POCO equipment at the residential level....it is very hard to burn up their equipment. Not so much at the commercial/industrial level.

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                        • #27
                          Originally posted by SunEagle View Post
                          Excuse me but a 400 amp service is not considered to be a residential panel size.
                          Square-D disagrees with you.
                          Model # SU3040D400CB
                          "Homeline 400 Amp 30-Space 40-Circuit Main Breaker Outdoor Combo Surface Mount CSED"
                          "For residential and commercial power distribution"


                          Numerous power companies disagree that it's not considered residential.
                          I could have installed a 400A service here in suburbia (I don't think theyd do larger than 400A - but they could do 400A)
                          Or a larger POCO - SCE - see page 3-25 (page 105) "Meter Panel for Single-Family Residential Underground Service

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                          • #28
                            Originally posted by Sunking View Post
                            Here is some food facts. A rural 200 amp service from the POCO uses a 12 to 17 KVA transformer for a 200 -Amp service and a 1/0 AWG copper feeder or 4/0 aluminum.
                            I don't believe that's consistent across all POCOs.
                            But lets assume it is for the sake of argument.
                            what's your point? Or what are you trying to correct/clarify in "Since he's planning on 15kW of solar, ."


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                            • #29
                              Originally posted by max2k View Post
                              I'd expect in a countryside this wouldn't be so unusual if they run some of their equipment from electricity. I'd agree though this needs to be planned and executed as planned, this forum is poor source for recommendations at such scale. My only concern from the beginning was transformer/service mismatch- why go to trouble to wire 400A service/MSP and put 25kW transformer?
                              Because a lot of the 400A service/MSP will probably still be well served by a 25KVA transformer.
                              And having a bigger transformer than needed is going to cost more - both in equipment costs up front and (probably) lower efficiency.
                              So if the POCO can do a cheaper and more efficient one, they will.

                              *If* the OP can get a 100KVA transformer and 750mcm wiring, AND not have to pay any additional, I don't see a downside for him.
                              I think other posters are indirectly saying that it's really unlikely he needs that large of a transformer.
                              And unlikely he needs that thick of wire.
                              And he should hire an expert to determine what he needs.

                              I agree with those.
                              I disagree with them on the level of certification needed to be an expert that can provide a useful/valid opinion. I think an experienced electrician is sufficient, they think a professional engineer is required.
                              And I still say the architect is a good place to get a reference for an expert.

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                              • #30
                                A 400A service has two 200A main breakers, one of which feeds a 200A buss and the other is for feeding an external subpanel. All your solar has to go through a single disconnecting means so can not be split between two backfeed taps. I see a lot of high end homes in my area that have 400A services. They don't necessarily use that much power but they have way too many branch circuits for a 200A panel. The way to handle a large solar backfeed of over 12 kW is to either dedicate the 2nd main breaker in a 400A panel to solar or to use a 200A main breaker panel on the second main in the 400A panel to isolate it. see attached 3 line diiagram.
                                3 LINE, 17kW.pdf
                                BSEE, R11, NABCEP, >1200kW installed

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