Any problems with this setup?

cmdurancmduran Registered Users Posts: 3


I am about to install (6) 180W Grape Solar panels with a Magnum PT-100 controller on my motorhome (9.15 amp, 24.06 VOC). I was going wire them all in series. Anyone see a problem with that configuration? I decided on wiring them all in series after talking with a Magnum technician who said that a 2X3 or 3X2 configuration would be delivering minimum voltage to the controller which might cause error codes. Does that make sense?

Also after reading a jillion posts on it, I'm still uncertain about what gauge wire would I need.




  • BB.BB. Super Moderators, Administrators Posts: 33,265 admin

    Welcome to the forum Mike,

    What is the voltage of your battery bank?

    It is sort of difficult to find the Vmp/Imp rating of these panels... They appear to be 36 cell panels which would put Vmp~18 volts. A perfectly good "12 volt" panel if used with a PWM supply on a 12 volt battery bank (or higher voltage if put in series).

    For MPPT type charge controllers, you need to look at the ratings needed to operate your MPPT controller... Specifically, the MPPT operating range and the Vpanel never to exceed voltage.

    Solar panels are "imperfect batteries"... Their operational voltages (Voc voltage open circuit and Vmp voltage max power) fall as their cell temperature rises, and increases as the cell temperatures fall.

    So, you need to look at the Vmp-array range for the controller... Typically Vmp-array-hot --- As Vmp falls as the array is in noontime sun in a hot/windless day.

    For for Voc which rises in cold weather, you need to make sure your Voc-array-cold does not exceed the the Vpanel-max for the controller.

    The Magnum manual is here:

    And you need to look at page 17 (PDF page 26) and there is a chart at the bottom. So, first we need to have your battery bus voltage. I.e., 12 volt battery, Vmp-controller runs from 60-187 VDC. Very roughly, in a hot climate, your 6x panels in series would be:

    • 6 * 18 volts Vmp = 108 Volts Vmp-array-standard-temperature (cells ~75F)
    • 108 volts * 0.81 high temperature derating = 87.5 volts

    Which is still over the 77 volts Vmp-min needed for a 48 volt battery bank. All is good there.

    You also need to look at the Voc-cold for these panels... On the next page of the manual, are the calculations needed for both Vmp and Voc vs Temperature calculations. But just from looking at your Vmp and Voc panel ratings, you do not have any issue with Voc-cold and the 188 Volt shutdown voltage.

    The wiring question... There are two questions. One is the maximum current on the wiring from the array... And that (appears to be) the Ioc (?) ~9.15 amps

    The NEC (national electric code) allows 15 amps on a 14 AWG cable (with proper insulation, etc.). And if the cable is exposed to sunlight, it needs to be UV/Weather rated cable too.

    The other calculation is how far is it from the array to the charge controller/battery? Longer wire runs, you want heavier cable so you have less losses. Typically we design for 1% to 3% maximum voltage loss.

    Note that placing all off your panels in series allows you to run much smaller (cheaper) copper cable from the array to the battery bank--One of the major reasons to "like" MPPT controllers and running "higher voltage" solar arrays.

    So, how far is it from the array to the controller?


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭

    One potential issue is if you have any vent stacks, antenna, etc on the RV roof that could cause any shading. Even a thin shadow from an antenna or overhead wire near one panel can substantially reduce output for the whole string.

    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭

    Panels in series is great - for fixed installs with no shade

    If anything on the roof, shades any of the panels at any time, even a small shadow (like from a pencil) will shutdown that segment of the panel (usually bypass diodes split the panel into thirds) and your output drops off. With enough voltage from series panels, harvest will slow, but not stop

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • BB.BB. Super Moderators, Administrators Posts: 33,265 admin

    Does anyone have information on these Grape panels?

    They seem to be missing the standard ratings and information that we see on solar panels (no Voc/Vmp/Ioc/Imp, no NRTL, etc.). The use of a blocking diode (nobody does that any more, unless is translation issue and they are using by-pass diodes--Which are very much needed in this series array application).

    At this point, I would be buying another brand/model of panels unless there is a data sheet out there with the missing information/clarifications on diodes.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cmdurancmduran Registered Users Posts: 3

    Wow. Great info! That is so helpful. And it's good to hear that I seem to be on the right track. It is a 12v system. The cable will be about 20-25' (say 30 to be sure).. I was looking at some wiring calculators and that seems to come out to about an 8 AWG cable?


  • BB.BB. Super Moderators, Administrators Posts: 33,265 admin

    8 AWG (one way run for this calculator), 30 feet, on a Vmp~108 volt array and 9.15 amps:

    Voltage Drop Calculator


    Voltage drop: 0.34

    Voltage drop percentage: 0.32%

    Voltage at the end: 107.66

    Actually, way overkill.... 14 AWG:


    Voltage drop: 1.39

    Voltage drop percentage: 1.28%

    Voltage at the end: 106.61

    Would be OK too.

    If you ever add more panels (Larger array), you could install the heavier cable now, and just add another parallel string (or many, with a combiner box) down the road.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2littleharbor2 Solar Expert Posts: 1,966 ✭✭✭✭✭

    Here's the specs.


    Solar Cell

    Monocrystalline silicone 156x156mm

    Number of Cells

    36 (4x9)


    1480mm x 675mm x 35mm (58.27” x 26.57” x 1.38”)


    12.0 kg (26.4 lbs)

    Front Glass

    High transmittance, low iron tempered glass with enhanced stiffness and impact resistance.


    Anodized Aluminum

    Junction Box

    IP65 Rated


    MC-IV comparable

    STC Maximum Power (Pmax)

    180 W (0%, +3%)

    STC Optimum Operating Voltage (Vmp)

    19.67 V

    STC Optimum Operating Current (lmp)

    9.15 A

    STC Open Circuit Voltage (Voc)

    24.06 V

    STC Short Circuit Current (Isc)

    9.77 A

    STC Operating Module Temperature

    -40 ºC to +85 ºC

    STC Maximum System Voltage


    STC Maximum Series Fuse Rating

    15 A

    STC Power Tolerance

    0%, +3%

    Nominal Operating Cell Temperature (

    46 ºC ± 2 ºC

    Temperature Coefficient of Pmax

    -0.38 %/ºC

    Temperature Coefficient of Voc

    -0.28 %/ºC

    Temperature Coefficient of Isc

    +0.04 %/ºC

    Selling Unit



    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.BB. Super Moderators, Administrators Posts: 33,265 admin

    Thank you LH2,

    Anything about the diodes?


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cmdurancmduran Registered Users Posts: 3

    It does have bypass diodes. It's this one:

  • BB.BB. Super Moderators, Administrators Posts: 33,265 admin

    Thank you CMD,

    Bypass diodes good. Blocking diodes not needed and usually not wanted.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.