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PV feeder voltage drop - California

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    #16
    Originally posted by electrofelon View Post

    I recall some papers on VD in PV systems (solar pro?) And the thesis was that keeping VD low is often not the most economical when you weigh conductor cost with losses. Of course keeping your inverters from dropping out from overvoltage is a separate issue.
    I recall one paper that argued that upsizing DC conductors would not pay for itself. But AC side is a whole different ball of wax.

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      #17
      Originally posted by ggunn View Post
      Why is that? If you know the current it 's just V=IR.
      There is no guidance on what current to use, and the output of an inverter is highly variable.

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        #18
        Originally posted by pv_n00b View Post
        There is no guidance on what current to use, and the output of an inverter is highly variable.
        Well, OK, but I use the nameplate rating of the inverter for that when calculating Vd. Sure, it's a worst case scenario, but most of the systems I design have at least the potential to get near that AC output current. On the DC side I use Imp.

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          #19
          Originally posted by ggunn View Post
          Well, OK, but I use the nameplate rating of the inverter for that when calculating Vd. Sure, it's a [COLOR=#ff0000]worst case scenario[/COLOR], but most of the systems I design have at least the potential to get near that AC output current. On the DC side I use Imp.
          In the OP's system it's worse than a worst case scenario.

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            #20
            Originally posted by jaggedben View Post
            In the OP's system it's worse than a worst case scenario.
            How can something be worse than a worst case?

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              #21
              Break apart into strings

              You can always break your array into multiple strings, with one line to each, to reduce your voltage drop.

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                #22
                Originally posted by ESolar View Post
                You can always break your array into multiple strings, with one line to each, to reduce your voltage drop.
                Well, yes, but then you have a lot of parallel wires. It adds up to a lot of wire one way or the other.

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                  #23
                  Originally posted by ggunn View Post
                  How can something be worse than a worst case?
                  You're asking how can a number be larger than the largest number that represents real world possibility? I think you should reread the thread. There's always a number that's larger (or smaller) than the one you should be using.

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                    #24
                    Originally posted by jaggedben View Post
                    You're asking how can a number be larger than the largest number that represents real world possibility? I think you should reread the thread. There's always a number that's larger (or smaller) than the one you should be using.
                    I'm not sure what you are driving at. The maximum current that an inverter can produce is documented, and if you use that number to calculate Vd that's the worst it can be. Higher numbers of course exist, but who cares? What am I missing?

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                      #25
                      Originally posted by ggunn View Post
                      I'm not sure what you are driving at. The maximum current that an inverter can produce is documented, and if you use that number to calculate Vd that's the worst it can be. Higher numbers of course exist, but who cares? What am I missing?
                      You're missing that the OP's array will never deliver that much power in his case. (Maybe they are planning for future expansion, that's the only reason I can figure for the inverters being so oversized. )

                      It's a pithy point, but it underscores pv_noob's observation that there's no firm guidance on the number to use.

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                        #26
                        Originally posted by jaggedben View Post
                        You're missing that the OP's array will never deliver that much power in his case. (Maybe they are planning for future expansion, that's the only reason I can figure for the inverters being so oversized. )

                        It's a pithy point, but it underscores pv_noob's observation that there's no firm guidance on the number to use.
                        Ah. I use the inverter nameplate current as a worst case scenario irrespective of how loaded the inverter is. When I said that I was responding to the general comment that there isn't any guidance for AC current, not specifically to the OP's system.

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                          #27
                          There was a time in the dark past, 20,000 PV years ago (10 real years) when PV modules were so expensive that inverters were always sized to prevent clipping and therefore were under producing 99% of the time. Under those conditions there was more flexibility in doing voltage drop calculations using less than the max inverter AC output. Now that people are using 1.5 DC/AC ratios we can assume the inverters will be putting out 100% most of the time. So using the max AC output current is more of a given.

                          But we still run into the odd ball once in a while. I saw a design a few months ago that was using 3kW out of a 30kW inverter.

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                            #28
                            Originally posted by ESolar View Post
                            You can always break your array into multiple strings, with one line to each, to reduce your voltage drop.
                            I don’t think so..

                            lowering the voltage on the string will lower the volts dropped but the VD% will stay the same.

                            The only way to lower the VD% is to lower the impedance (bigger or shorter wire) or lower the amperage (smaller modules)

                            at 10 amps, the VD% over a given wire will be the same at 1 volt or 1,000 volts.

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