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Thread: PV feeder voltage drop - California

  1. #11
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    Quote Originally Posted by pv_n00b View Post
    The ANSI B range upper end is 508V. That means you get at most 11V of voltage drop to work with before you hit 529V . Based on a nominal voltage of 480V that gives you 2.29% voltage drop. Even the less conservative A range upper limit is 504V allowing up to a 3.12% voltage drop. Get even close to these and you are rolling the dice on having to maybe replace all the AC conductor and conduit. Have a fat reserve fund if you want to play that game on a 1000 ft run.
    Having trouble following your math. Think you meant "at most 21 volts to work with."

    Another way to put it is the inverter will go 10% over. How much will the utility be over and how lucky are you feeling, punk? Of course if the POCO's transformer has taps, and they are willing to change them, that would be an easy fix. My solar system would have issues if I didn't have taps (see the "my solar system thread").

    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.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  2. #12
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    We want to look at the voltage window we have to work with, so we compare the maximum operating AC voltage of the inverter with the max AC voltage the utility is supposed to limit their system to in the ANSI standard. In this case if the max AC voltage for the inverter is 529V and the upper ANSI voltage is 508V then 529-508 = 21V. Looks like my other posting I got 11V by mistake. We can drop up to 21V in the conductor and the inverter will not cut out under worst case conditions. That's if everything is perfect, I would cut that down to be conservative to maybe 80% of 21V or 16.8V. And if we want a percent voltage drop then that's 16.8/480*100 or 3.5%. It's true that some people play too close to the edge and have to ask the utility if they can lower the line voltage and sometimes the utility will.

    That article in SolarPro was interesting, but they were only looking at the financial and code aspects of conductor sizing and not any operational effects. Their idea being maybe it did not provide a return on investment to up the conductor size to get a voltage drop from 16% to 3% because the energy gain would not offset the cost of the conductor. They did not look at the system operational requirements like low and high voltage cutout.

  3. #13
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    Quote Originally Posted by jaggedben View Post
    I can't imagine why you're oversizing your inverters so much, but that's irrelevant. There's no code requirement that would have you use 125% of the inverter output for a voltage drop calculation.
    I've found a lot of variety in doing the voltage drop calc. Since it's not a code issue we are left to choose values based on experience and how close we want to get to losing everything on the project. With the high DC/AC ratios of most designs these days hitting the inverter max output current can be assumed. If the inverter is oversized then we can find the maximum operational current based on the estimated array output.

    If I were running 1000ft of underground AC conductor I would be so conservative. No way do I want to replace that run. If it's 10ft of above ground conduit I might be less conservative since it won't break the bank to pull larger conductor and I can oversize the conduit just in case.

  4. #14
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    Quote Originally Posted by pv_n00b View Post
    I've found a lot of variety in doing the voltage drop calc.
    Why is that? If you know the current it 's just V=IR.

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    Quote Originally Posted by ggunn View Post
    Why is that? If you know the current it 's just V=IR.
    I think he's saying he's seen a lot of variety in the choice of what current figure to use. However, if that includes using 125% of inverter output, well that just seems like someone not knowing what they're doing.

  6. #16
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    Quote 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.

  7. #17
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    Quote 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.

  8. #18
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    Quote 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.

  9. #19
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    Quote 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 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.
    In the OP's system it's worse than a worst case scenario.

  10. #20
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    Quote 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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