Expanding system, need advice about combining, disconnects and wire sizes

I'm adding a new pole-mounted string facing that is located 40 feet from the other two existing strings. All panels are identical. Each string is going to be fused. Combined power will be sent to a Tristar MPPT charge controller. A few questions:

1. To save time digging trenches, can I enter the house at a different point than the existing array? I would then run the cable inside to the same charge controller.
2. Since the new array consists of only one string, can I use 12 gauge wire instead of the 10 gauge I used for the other two strings that were combined?
3. Regarding disconnects, I will have at both of the two locations. And do I need to only be able to disconnect the positive? What about the negative cable?
4. Where is the best place to "combine" the new array to the existing system?

Thanks for you help.Attachment not found.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Expanding system, need advice about combining, disconnects and wire sizes
    Kgelles wrote: »
    I'm adding a new pole-mounted string facing that is located 40 feet from the other two existing strings. All panels are identical. Each string is going to be fused. Combined power will be sent to a Tristar MPPT charge controller. A few questions:

    1. To save time digging trenches, can I enter the house at a different point than the existing array? I would then run the cable inside to the same charge controller.

    Yes. So long as the AHJ doesn't have an issue with this (can't imagine why they would).
    2. Since the new array consists of only one string, can I use 12 gauge wire instead of the 10 gauge I used for the other two strings that were combined?

    Wire size will depend on current and Voltage drop over the distance. You want size it so that you have the same Voltage from both arrays by the time the wire reaches the controller.
    3. Regarding disconnects, I will have at both of the two locations. And do I need to only be able to disconnect the positive? What about the negative cable?

    One disconnect per array will suffice. In electrical terms a single breaker on the positive lead is all that is needed. Again the AHJ may have a different opinion. If there is GFDCI on the system it will have a dual (common trip) breaker to disconnect both (+) and (-).
    4. Where is the best place to "combine" the new array to the existing system?

    Wherever you want to that can meet the requirements both electrically and mechanically. When you add in the new array the current from that point on will be higher and the conductors must be able to accommodate it.
  • BB.BB. Super Moderators, Administrators Posts: 30,638 admin
    Re: Expanding system, need advice about combining, disconnects and wire sizes

    Is there a risk of nearby/direct lightning strikes?

    If so, I would think hard about were the power enters the home, grounding rods, surge suppression, and grounding in general.

    Depending on the sizes of the arrays... You may need a distribution breakers at the house where the arrays combine to one MPPT controller input (if using one MPPT controller) to protect the arrays/wiring (i.e., a short in one solar string "somewhere--always worst case location" is feed by on or more other arrays into the short).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Expanding system, need advice about combining, disconnects and wire sizes
    BB. wrote: »
    You may need a distribution breakers at the house where the arrays combine to one MPPT controller input (if using one MPPT controller) to protect the arrays/wiring (i.e., a short in one solar string "somewhere--always worst case location" is feed by on or more other arrays into the short).

    -Bill
    Implied in the OP's drawing is that both the two old strings and the one new string will connected to the same MPPT input. As long as the panels are identical (or very closely comparable) and there will not be partial shading on any of the strings (probably true for ground mount, but the drawing does not tell us that), then they can be combined.
    SMA SB 3000, old BP panels.
  • BB.BB. Super Moderators, Administrators Posts: 30,638 admin
    Re: Expanding system, need advice about combining, disconnects and wire sizes

    The can be combined--But still will probably need some sort of Series Protection Fuse/Breaker per string, depending on the exact wiring configuration may need a "branch circuit" distribution protection too (i.e., back feeding breakers to protect the wiring from array combiner to final combiner at MPPT charge controller).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KgellesKgelles Solar Expert Posts: 25
    Re: Expanding system, need advice about combining, disconnects and wire sizes
    When you add in the new array the current from that point on will be higher and the conductors must be able to accommodate it.

    Question: If I combine the new (say, 12 gauge) array wires with the existing (10 gauge) ones "just before" they enter the charge controller is there any way that the existing panels could back-feed too much amperage towards the new string? Or does the power only flow "downstream" towards the charge controller?

    So, in short, each string produces a short-circuit current of 8 amps. So the existing two strings wired together = 16 potential amps. Does the new string wiring (as shown in my diagram) need to be able to handle 24 amps? Or does the power generated by the other two strings stay "downstream" as long as the cables terminate at a point near the charge controller?
  • KgellesKgelles Solar Expert Posts: 25
    Re: Expanding system, need advice about combining, disconnects and wire sizes

    I may just dig the extra trench and tie the new array into the existing disconnect using the appropriate size wire. That way I can use the existing surge suppressor, disconnect, etc. And it will keep things more simple in the long run. I need the exercise of shoveling a little bit more anyway. Thanks for everyone's input. This forum is filled with helpful, professional people.
  • BB.BB. Super Moderators, Administrators Posts: 30,638 admin
    Re: Expanding system, need advice about combining, disconnects and wire sizes

    Solar panels, because they are essentially "current sources" don't have the same problem as batteries with current sharing... So in itself, you can mix different gauge wire. And other than voltage drop (and more losses), wire size is not the same issue.

    However... You do need to think of failures (short circuit any wire in the solar array/wiring back to the home) and figure out the maximum current that will flow (from the rest of the panel strings/arrays).

    For a simple example. You have 3 or more solar panels (or series strings) of solar panels in parallel. You short one string, and the two more parallel strings can feed more than the series protection fuse rating (usually around 10-15 amps--depending on exact solar panel) into that short.

    Similar, if you get a short in the wiring from the arrays back to the house, where will the "short circuit" current come from.

    A 12 AWG wire is rated by NEC as 20 amps maximum Breaker/fuse protection. And 10 AWG is rated for 25 amps maximum fuse/breaker. If you have several independent arrays, you need to see if they can exceed the maximum rating of the wiring you are using.

    Sorry--Understanding and designing Fault Protection is not always easy to see.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Expanding system, need advice about combining, disconnects and wire sizes
    Kgelles wrote: »
    Question: If I combine the new (say, 12 gauge) array wires with the existing (10 gauge) ones "just before" they enter the charge controller is there any way that the existing panels could back-feed too much amperage towards the new string? Or does the power only flow "downstream" towards the charge controller?

    Panels are semi-conductors. The only time you get in trouble this way is if one set is shaded, then the other may back-feed it. There are instances of this with mismatched panels on higher Voltage arrays, but most of the time it simply isn't a problem.
    So, in short, each string produces a short-circuit current of 8 amps. So the existing two strings wired together = 16 potential amps. Does the new string wiring (as shown in my diagram) need to be able to handle 24 amps? Or does the power generated by the other two strings stay "downstream" as long as the cables terminate at a point near the charge controller?

    This is why each and every string should have its own circuit protection when you have more than two strings. In the event one of them should short and have the combine current potential of the others put to it the CP will trip first, rather than allow the current to overheat the panel circuit and start a fire.
  • KgellesKgelles Solar Expert Posts: 25
    Re: Expanding system, need advice about combining, disconnects and wire sizes
    This is why each and every string should have its own circuit protection when you have more than two strings. In the event one of them should short and have the combine current potential of the others put to it the CP will trip first, rather than allow the current to overheat the panel circuit and start a fire.

    Life was simple when I only had the two strings! I like to learn new things though and this has been a good challenge. Thank you for your help guys.
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