Wire Sizing

goingtoghanagoingtoghana Solar Expert Posts: 34
I am purchasing 8 of the Uni-solar PVL-128 panels. I plan on connecting them in parallel and into a MidNite Classic 150.

I just want to make sure I am correct on the wiring size. I am planning on connecting the panels together on the roof with one negative and one positive wire coming into the house.

I am attaching the calculator results from the midnite site and a gauge calculator.

I just want to make sure this is correct.

Thanks

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,696 admin
    Re: Wire Sizing

    Vmp~33 volts is too low for charging a 24 volt battery. You want, at least, Vmp~35 volts to account for Vmp depression in full sun/hot weather and charging a 24 volt battery to near 30 volts (or more for equalization).

    Since you are getting a very nice Midnite MPPT charge controller--Why are you not placing (at least) Two panels in series, then parallel connecting the rest?

    That will save wiring costs (less copper) and fusing/breaker/combiner box (4 circuits in parallel instead of 8).

    Do you have some reason you believe you want to parallel connect the panels?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • goingtoghanagoingtoghana Solar Expert Posts: 34
    Re: Wire Sizing
    BB. wrote: »
    Vmp~33 volts is too low for charging a 24 volt battery. You want, at least, Vmp~35 volts to account for Vmp depression in full sun/hot weather and charging a 24 volt battery to near 30 volts (or more for equalization).

    Since you are getting a very nice Midnite MPPT charge controller--Why are you not placing (at least) Two panels in series, then parallel connecting the rest?

    That will save wiring costs (less copper) and fusing/breaker/combiner box (4 circuits in parallel instead of 8).

    Do you have some reason you believe you want to parallel connect the panels?

    -Bill

    The batteries are 12 Volt

    I am looking at parallel because it isn't that far of a run from the panels to the charge controller. Not too expensive and less wires.

    Why do I need more then 1 fuse in what I am suggesting and why would I need a combiner box?
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Wire Sizing
    The batteries are 12 Volt

    I am looking at parallel because it isn't that far of a run from the panels to the charge controller. Not too expensive and less wires.
    If you wire them up in four series pairs it's half the wire than if they are all in parallel. You''l need as many fuses as you have strings, even if the strings are one module long, and you'll need a combiner box to combine the strings.

    Do you maybe have series and parallel confused?
  • BB.BB. Super Moderators, Administrators Posts: 29,696 admin
    Re: Wire Sizing

    In general, solar panels are rated for series protection fuses to protect against a short in one panel being fed by other parallel connected panels and starting a fire.

    Typically, the standard wiring is two or one string of panels, no fusing is needed.

    For three or more panel in parallel, each parallel string should have its own fuse/breaker in the (typically) positive lead.

    I did a quick look and did not see the series fuse rating for your panels--But it is probably somewhere around 8-10 amps maximum.

    With 4-8 panels in parallel, each parallel connection should have a fuse/breaker. Usually, people use a combiner box with breakers (or fuses) to combine the array leads into one feed to the charge controller.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • goingtoghanagoingtoghana Solar Expert Posts: 34
    Re: Wire Sizing
    BB. wrote: »
    In general, solar panels are rated for series protection fuses to protect against a short in one panel being fed by other parallel connected panels and starting a fire.

    Typically, the standard wiring is two or one string of panels, no fusing is needed.

    For three or more panel in parallel, each parallel string should have its own fuse/breaker in the (typically) positive lead.

    I did a quick look and did not see the series fuse rating for your panels--But it is probably somewhere around 8-10 amps maximum.

    With 4-8 panels in parallel, each parallel connection should have a fuse/breaker. Usually, people use a combiner box with breakers (or fuses) to combine the array leads into one feed to the charge controller.

    -Bill

    OK, so I think I understand what you are saying. I made a quick diagram. Let me know if this is what you mean.

    The first one is what I thought I was going to do. But, I get that you are saying there is a limit to the number of parallel panels you can have on one fuse.

    The second one shows what I think you mean (assuming 4 panels in parallel is allowable).

    Am I correct, that either diagram would feed the same volt/amps into the charge controller?

    Attachment not found.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire Sizing

    There is a limit to the number of parallel panels or panel strings you can have on one fuse: it is one.
    For eight parallel connects you need eight fuses.

    What is being suggested is that since the Classic 150 can take 150 VDC on its input you put some of the panels in series, increasing the array Voltage while decreasing its current (overall power remains the same).

    So you would have four parallel strings of two in series:
    (-)PANEL(+)----(-)PANEL(+)
    (-)PANEL(+)----(-)PANEL(+)
    (-)PANEL(+)----(-)PANEL(+)
    (-)PANEL(+)----(-)PANEL(+)

    All negatives on the left connected in parallel, all positives on the right go through one fuse each and then combine.
    This would give your array a Vmp of 66 and an Imp of 15.52 in which case 10 AWG would handle the 30 foot run.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Wire Sizing

    goingtoghana, you are doing the right thing by putting all 8 panels in parallel. Your charge controller will be much happier converting 33 volts to 12 volts than converting 66 volts to 12 volts. You need to put a combiner box near the panels and the combiner box will have 8 circuit breakers. From the combiner you have just one pair of wires carrying the combined current of the 8 panels. If you do decide to have 4 strings of 2 panels per string, you still need a combiner, but there will be only 4 circuit breakers. Neither of your diagrams is correct.

    When you have multiple parallel strings it is very useful to be able to turn a string on and off with a circuit breaker. Sooner or later you will wonder why you are not getting as much power as you think you should. When that time comes you can turn on the strings one at a time and figure out if one string is lagging.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire Sizing

    Hint to boB if you're reading this: could we have a chart or graph of the MidNite Classic's conversion efficiencies at different PV to battery Voltage ratios please?

    I suspect the whole range from 18 VDC PV to 150 VDC PV on a 12 Volt system is about 7%.

    I would not worry about efficiency differences between a 66 VDC array and a 33 VDC array on a 12 Volt system.

    But whether you use the 8 in parallel on larger wire or the series/parallel arrangement with the smaller wire you should have circuit protection per each parallel connect of the array.
  • goingtoghanagoingtoghana Solar Expert Posts: 34
    Re: Wire Sizing

    If I have each panel coming into a combiner box that I connect parallel, do I have to size the wire on the combined amperage or the amperage of each individual string coming into the combiner box?

    Now, one last thing. Since the combiner is going to combine them all to one positive and one negative, can I use one blocking diode between the charge controller and the combiner?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire Sizing
    Thanks guys. I am 100% clear on this now.

    Now, one last thing. Since the combiner is going to combine them all to one positive and one negative, can I use one blocking diode between the charge controller and the combiner?

    There is no need for any blocking diode on this system.
    The charge controller will prevent the batteries from draining through the array at night, and the array Voltage is not so high (in either configuration) to force reverse current through a shaded panel.
  • goingtoghanagoingtoghana Solar Expert Posts: 34
    Re: Wire Sizing
    There is no need for any blocking diode on this system.
    The charge controller will prevent the batteries from draining through the array at night, and the array Voltage is not so high (in either configuration) to force reverse current through a shaded panel.

    You beat my edit. :)

    If I have each panel coming into a combiner box that I connect parallel, do I have to size the wire on the combined amperage or the amperage of each individual string coming into the combiner box?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire Sizing
    You beat my edit. :)

    If I have each panel coming into a combiner box that I connect parallel, do I have to size the wire on the combined amperage or the amperage of each individual string coming into the combiner box?

    Going in to the combiner it is the current per string. Coming out it's the combined current of all strings.
  • goingtoghanagoingtoghana Solar Expert Posts: 34
    Re: Wire Sizing

    Thanks Cariboocoot
  • PhotowhitPhotowhit Solar Expert Posts: 5,022 ✭✭✭✭
    Re: Wire Sizing

    I prefer going with 'Coot's suggested 4 strings of 2 panels, since you are on the verge of making/wanting to go to a 24 volt system. What size inverter do you plan? If you are 1800 watt or above it might pay to look into a 24 volt system now... loads tend to grow and above the 1800/2000 watt wire sizes might well be above 0000, If you need more capacity in your battery bank, you'll need more than 2 strings of what ever battery your planning now....

    Since the array will be roof mounted and your charge controller will handle strings of 2 (even 3) might as well bring the slightly higher voltage down, your not pushing the capacity of the charge controller.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
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