Joining parallel strings using MC4 Y connectors with a Schneider XW 6848 and MPPT 80 charge cntrls


I am a solar engineer in training and loving it so far.  I have a good understanding of electricity but I'm relatively new to understanding the NEC.

I'm designing a system with:

2 Schneider XW 6848 Inverters
2 Schneider MPPT 80 600 charge controllers
4 strings of 9 Yingli 265W panels (YL265-29b)
A long conduit run from the array to the inverters in the basement (183 feet)

My goals are:
Minimal component cost
Relatively low voltage drop in the long DC run
Keeping installation as simple as possible

Each Schneider charge controller has 3 DC + and - terminals.  Each charge controller will be receiving the power from 2 strings in the array.  I'm deciding if it is best to combine strings using a MC4 Y connectors with fuses.  I'd like to avoid using a DC combiner box to reduce costs and simplify installation.  

The questions are:

1.  Is it ok to simply use MC4 Y connectors for each junction with 30A MC4 fuses installed on the joined side of the Y?
2.  It appears that DC disconnect breakers are necessary.  Where is the ideal place to put them in this scenario?  Can they be in the basement near the charge controllers or do they have to be out near the array?
3.  Is it required to have fuses (probably 15A) on each string before the Y?
4.  Where would one install the lightning protection device?
5.  Can I get away with avoiding the combiner box?  

(I couldn't find anything in the code for my AHJ having anything to do with this topic.)

Thank you.



  • Dave AngeliniDave Angelini Solar Expert Posts: 4,199 ✭✭✭✭✭
    If you limit to less than 3 strings, per mppt, a fuse/breaker is not required.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • SolarEITSolarEIT Registered Users Posts: 3
    Thanks Dave!  That is good to know.

    I was concerned when I read this in the MPPT 80 600 installation manual however:

    • As per applicable electrical codes, fuses are required when paralleling (combining) more than two PV strings.

    • Fuses must be installed in a combiner box or in a PV array disconnect switch.

    • These items are not provided with the charge controller.  

  • Dave AngeliniDave Angelini Solar Expert Posts: 4,199 ✭✭✭✭✭
    I don't get your "however"  If you don't use more than 2 strings none of that is required.
    I would use a disconnect outside the building as that is the most often thing some inspectors want for fire and all the other things they may or may not want. You are welcome to email me but it sounds like you are on the right track.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • SolarEITSolarEIT Registered Users Posts: 3
    Ok.  It makes sense.

    Could you or anyone recommend a DC disconnect for this application?  Schneider also recommends a lightning arrestor.

    According to my calcs, I need a disconnect rated only at 500V and 12A per string.  Does anyone know of one disconnect box with 4 poles and close to that rating so we can buy just 1 instead of 2?

  • Raj174Raj174 Solar Expert Posts: 650 ✭✭✭✭
    Attached is a PDF of various photovoltaic DC disconnects made by Eaton.


    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
  • MichaelKMichaelK Registered Users Posts: 79 ✭✭
    edited October 2017 #7
    After spending 12,000+$ for the inverters and charge controllers, it seems almost absurd to complain about not spending ~100$ for a combiner box.  I'd like to suggest you still need one, not necessarily for safety issues, but for convenience during maintenance.  You may find that you want to shut off one or another string for testing, repair, or experimentation, without completely shutting power off.  If you ever find the need to separate a panel cable, that is not something you ever want to do under load.

    In terms of positioning the DC disconnect breakers, the proper site is in the Power Distribution Panel (PDP) which if using the Schneider version, mounts on the right side of the XM.  For my own system I've blended both Midnight and Schneider components, and my Midnight E-panel mounts directly underneath the XM.  I have this one.
    I have my DC panel and battery disconnects mounted on the right hand side of the E-panel.  I have my combiner box mounted outside, closer to the arrays.

    15 Renogy 300w panels,  Midnight 200 CC, 8 Trojan L16 batteries, Schneider XW6848 NA inverter, AC-Delco 6000w gen.
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,199 ✭✭✭✭✭
    A quality installation is just conforming to requirements. You can do what you want. If you are being inspected you need to do what the AHJ wants. There should not be any maintenance of  2 strings of panels, but there are ways to shut off each string in a dual pole 600vdc disconnect.

    Fuses in this 2 string case are just another failure source and no matter what you are doing, you never disconnect under load!

    Many would tell you the last place you want to run high voltage components is in the Power Distribution box where there is AC power and battery power. The best place is  (in my opinion) is in the Mppt-80-600. It is your choice though. After close to 100 installs of this system I am very biased and always concerned about what can happen with lightning.

    I typically stick with one brand as all of the components are listed as a system and some inspectors require this. Not always but I don't need the stress. Lucky there is much less of this offgrid :)

    Is it running?

    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

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