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    voltage drop clarification

    Can someone please verify that I am doing this correctly. I am connecting 21 solar inverters to a 3 phase 4 wire 277/480 VAC panel. These inverters have a single pole 277 V output. 3 phases 7 inverters per phase 500 feet back to main service.

    Assuming max/output "load" of 177 amps per phase
    Here is the voltage drop formula that I am using

    VD= 1.73 . 12.9 . I . L/CM

    my inputs are
    1. 1.73
    2. 12.9 "approx" K value for copper
    3. 177 amps max output I
    4 500 feet distance back to service
    5 CM = 300,000 300mcm

    My results are this VD= 6.58 V or 2.3% vd

    The engineer I am working with on this project comes out with a wire size that is 600 MCM to achieve the same VD and his methods are cryptic at least to me.

    Please someone tell me that I have no idea what I am doing or confirm this is an acceptable result

    thanks

    #2
    I am not sure how the engineer got the results he/she got. I would have thought a 3/0 copper conductor would work but I am not an engineer. We do have engineers on this site however today being xmas you may not get a response til tomorrow. I would post his calculation and see if someone can decipher it.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky

    Comment


      #3
      I don'took handy but came up with a wire size of almost 170,000cm at 5%VD
      RichB N7NEC

      Comment


        #4
        I guess what I am asking is "Is my calculation wrong based on my inputs"

        Also what is the difference between a 277/480v formula and a 277/480 4 wire formula I see "on line " calculators offer both

        Comment


          #5
          Originally posted by ocoee View Post
          Can someone please verify that I am doing this correctly. My results are this VD= 6.58 V or 2.3% vd

          thanks
          I'm currently quite full of pumpkin pie so hope that doesn't have an impact on my reasoning ability but your calcs look correct as I got around 6.5 volts with the usual caveats on power factor etc.

          Where you need to be careful is since you have the sqrt(3) in your formula, you're calculating a voltage drop against the line voltage (480V). In other words, the ~ 6.5 V is correct but that is about 1.3% of the line voltage.

          I would still agree to ask the engineer what the rationale is behind his/her calculations.
          Dennis L. Karst, P.E.

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            #6
            My program results were 1.4% or 3.8 volts L/N for 300 kcm. Results for
            3/0 were 2.5% and 6.8 volts L/N.

            [COLOR="Red"]Merry Christmas[/COLOR]
            Last edited by bob; 12-25-10, 04:32 PM.

            Comment


              #7
              The engineer you are working with may be correct.

              More information is needed, it depends on the inverter and if the inverter can compensate for the voltage rise above utility (not likely if you are using the smaller inverters). The inverters need to stay within the allowable voltage range for the service and the 'voltage drop' causes the inverters to operate at a higher voltage than the utility. Most inverter manufacturers recommend a maximum voltage drop in the wiring of 1.5%.

              You also need to consider wire temperature as high temperature increases voltage drop.

              You do not want your inverters to start dropping out in the summer due to a high voltage error.

              Comment


                #8
                To answer the above, we are using SMA 7000 us. They will automatically shut down if the grid goes out of range in either direction. nothing we can do about voltages that are too high. At this point my concern is the voltage drop, this we can address.

                So.... Big picture We want the best possible production over time. The main variable being the wire size from the AC panel to utility interconnection, 500 feet.

                The inverters are single phase outputs @277V
                They are combined ( 21 of them) on to 3 phases of a 277/480 4 wire sub panel. Each inverter has a max output of 25.2 amps and there are 7 per phase. There is 177 amps per phase for the purposes of our calculation.

                At the point where they are combined in my panel this becomes a 3 phase circuit. No?

                Do I need to concern myself with the voltage drop from line to neutral?
                or just line to line? being that the source is a single phase output as stated above.

                We would like to limit VD to less than 2% for this run.
                To be clear my question is about the best way to figure voltage drop without overdoing it .

                Also I have done several systems this size and have others review my calcs and never a issue raised. I interest is to be better at what I do. ( not puffing chest out) If I can learn another method to figure VD for three phase please TELL ME.

                Regard Geoff

                Comment


                  #9
                  Oh yea MERRY CHRISTMAS!

                  Comment


                    #10
                    When in doubt Google.

                    http://www.csgnetwork.com/voltagedropcalc.html
                    Brian John
                    Leesburg, VA

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                      #11
                      I used an online calculator and got 250mcm for a 2.1% VD
                      Unlimited Electric Contractor/Standard Electric Inspector/Traffic Signal Inspector/Highway Lighting and Level One Traffic Signal Installer.

                      I know you believe that you understand what you think I said but I'm sure you realize that what you heard is not what I meant.

                      Comment


                        #12
                        Originally posted by ocoee View Post
                        If I can learn another method to figure VD for three phase please TELL ME.

                        Regard Geoff
                        There are a number of methods that are perfectly valid for determining voltage drop. Be a little wary with on-line calculators until you test them and make sure you understand the behind-the-scenes assumptions as they can turn out erroneous results at a furious pace.

                        Many people like to use an Excel formula which is also fine.

                        Your method above is fine assuming your line resistance dominates the inductive reactance and the power factor is reasonably good. Bob (above) got 1.4% vs. my 1.3% which for practical purposes are equivalent. In a balanced system the line-neutral drop is related to the line-line drop by sqrt(3), you just need to remember which one you are using when you calculate the voltage drop.

                        In your case, it appears you calculated the line-line drop of ~6.5 volts and divided by the line-neutral voltage to get the % which is incorrect.
                        Dennis L. Karst, P.E.

                        Comment


                          #13
                          I have used these a lot Wheres the math?

                          Comment


                            #14
                            Originally posted by ocoee View Post
                            If I can learn another method to figure VD for three phase please TELL ME.
                            Well how about using the method of Table 9? That way you don't have to wonder about some unknown internet calculator where you don't know what they are doing.

                            ice
                            I really got to go to work

                            Merry Christmas
                            Without data you’re just another person with an opinion – Edwards Deming

                            Comment


                              #15
                              Originally posted by ocoee View Post
                              ...

                              At the point where they are combined in my panel this becomes a 3 phase circuit. No?
                              Essentially yes.

                              Do I need to concern myself with the voltage drop from line to neutral?
                              or just line to line? being that the source is a single phase output as stated above.
                              [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

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