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  • Simple Sunny Island question

    I'm a bit confused on some terminology in the Sunny Island 6048 manual. Under the AC1 terminal connections, it has this statement:

    ***************
    Cable lengths in single-phase, parallel, split-phase, double split-phase and three-phase systems
    The AC cables between the Sunny Island and the sub-distribution of a system must have the same wire size and the same cable length for all parallel connected devices.
    ***************

    My setup is using two Sunny Island 6048 inverters for 120/240 split phase home power.

    The Master AC1 goes to Line1
    The Slave AC1 goes to Line 2

    Are these inverters considered to be operating in parallel? To my understanding, parallel would mean I had 4 inverters with two of them on L1 and two of them on L2. Under such conditions, I would understand the need for wires to be cut to the same length.

    The reason I'm asking is because if the cable lengths of both phases (L1 and L2) have to be the same length, then I need to coil up some wire in a box somewhere. Since each inverter is operating on its own line (one inverter on L1, the other on L2) and since L1 and L2 will probably never ever see a balanced load, I can't imagine why I would need the wires to be the same length.

    Just thought I'd check to see if my reasoning is correct.

  • #2
    I would guess they mean each pair (L1 &L2) need to be the same length, not the other pair from the slaved unit.

    I am however confused by your statement " The Slave AC1 goes to Line 2" . These are parallel connections, all L1s should be connected and all L2s should be connected if I am understanding your setup correctly. I don't think the issue is the balanced loads, I think it is the phase timing between L1 and L2 which are 180 degrees from each other at 60 Hz.
    Last edited by Ampster; 09-28-2019, 12:03 AM.
    9 kW solar. Driving EVs since 2012

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    • #3
      Originally posted by Ampster View Post
      I would guess they mean each pair (L1 &L2) need to be the same length, not the other pair from the slaved unit.

      I am however confused by your statement " The Slave AC1 goes to Line 2" . These are parallel connections, all L1s should be connected and all L2s should be connected if I am understanding your setup correctly. I don't think the issue is the balanced loads, I think it is the phase timing between L1 and L2 which are 180 degrees from each other at 60 Hz.
      I didn't think of that. In HAM radio, we sometimes have to calculate signal speed through different coax types, but we are working with signals in the megahertz range, not 60hz. Why would a few nanoseconds of timing difference have any effect on standard household current? If I run 25 feet from one inverter and 30 feet from the other, could that actually have any detrimental effect?

      Thanks for your input, very helpful. Now I just need to figure out if I really need to add a box to coil up five or six feet of wire.

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      • #4
        I am only guessing here but it doesn't have anything to do with the current. I believe it has more to do with the phases being exactly 180 degrees from each other because each phase has to clock with the signal coming from the grid.
        I wouldn't bother with the wire length to the other inverter. For what it is worth I have copied what I have seen others do and that is to use a gutter above and/or below my sub panels. That makes changes easier and would leave room for a longer loop of wire if needed. It is also a convenient place for current clamps and associated interfaces.
        9 kW solar. Driving EVs since 2012

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        • #5
          At 300 million meters per second, and perhaps 90% of that in wiring (somewhat less
          in coax) the electrons bobble some 2.25 million meters per half cycle. Unless the
          difference in wire length is a substantial fraction of that, phase angle is not really
          bothered. Voltage drop in the wiring resistance is a different story, could make the
          neutral look off center in a 240 circuit. Bruce Roe

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          • #6
            Thanks for the advice.

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            • #7
              Originally posted by bcroe View Post
              At 300 million meters per second, and perhaps 90% of that in wiring (somewhat less
              in coax) the electrons bobble some 2.25 million meters per half cycle. Unless the
              difference in wire length is a substantial fraction of that, phase angle is not really
              bothered. Voltage drop in the wiring resistance is a different story, could make the
              neutral look off center in a 240 circuit. Bruce Roe
              But are we talking differences of 5 feet with this kind of stuff?

              Its funny because I built a double-cross dipole antenna designed to receive NOAA satellite images direct to the HAM radio and I had to cut two of the four coax feeds by a 1/4 wave length so the cross would be out of phase. It ended up being something like 14.25 inches at 140 something Mhz. Significant at those frequencies.. not so much I suspect at 60hz.

              Now I want to know exactly what SMA says about this and why.

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              • #8
                Originally posted by bcroe View Post
                ....... Voltage drop in the wiring resistance is a different story, could make the neutral look off center in a 240 circuit. Bruce Roe
                Good info. So my guess about phase angle is not an issue.
                I am trying to understand this issue better. My question is, would unbalanced loads make the neutral look off center?
                Last edited by Ampster; 09-28-2019, 07:56 PM.
                9 kW solar. Driving EVs since 2012

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                • #9
                  Originally posted by Ampster View Post

                  Good info. So my guess about phase angle is not an issue.
                  I am trying to understand this issue better. My question is, would unbalanced loads make the neutral look off center?
                  Off balance loading will tend to make voltages unsymmetric. That could make it easier to trip
                  the inverter neutral voltage monitor, which may be as sensitive as the line to line monitor.

                  At 390 mhz (6.5 million times higher than line) my garage door radio colinear antenna was
                  measured per the reduced propagation rate (66%) in the coax sections. Bruce Roe

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