Combiner box placement / separate shutoff box?

jjdh Registered Users Posts: 9 ✭✭
Some background

I have a system with a Schneider SW4048 inverter, a SimpliPhi 3.8 battery and a little generator. I now want to add some solar to it.

My plan is to build up to four strings of three REC330NP, the fourth suggested configuration for the Schneider MPPT 150 60 charge controller here. Intention is to add 3 panels at a time, and add more simpliphi batteries as the system gets bigger.

The panels will go on my A-frame roof, which is super steep, 24/12 pitch (20ft rise over 10ft span..). From that steep roof there's then an existing conduit running from the ridge cap to the base of the building, and entering into the crawlspace. From there the wiring can get to the indoor location where the charge controller will go.

Combiner boxes / shutoffs

My plan so far had been to combine the four - eventual - strings on the roof on a combiner box on the roof, and run wire from there down to where the charge controller will sit, next to the already-in-place inverter. In the roof combiner box I'd fuse things to match the wires going to the charge controller and add surge protection.

But in that setup, there's no on-the-ground place to shut off power from the PV panels.


Backing up a little bit. Say you are me, you're putting - eventually - 12 REC330NP panels on the roof, and connecting them to a Schneider MPPT 150 60. How and where would you place boxes?

Things I can think of:

- One combiner on the roof with overcurrent and surge protection, a second electric box by the charge controller with shutoff and second surge protection
- No combiner on the roof, instead PV wiring through the conduit, and a single combiner at the base of the building with shutoff, overcurrent and surge protection
- Something totally different?


  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Regarding the 3x REC330NP panels in series... Check your minimum temperature for your location (around Popular Bluff Missouri?). In very cold climates, the Voc-cold could exceed Vpanel-max of ~150 volts (for the MPPT 150 controller?).

    Run the PV Array wiring on the outside of the home down to the combiner box (plus surge suppressors, with local ground rod below combiner box, and green wire safety ground from combiner box back to your main power system ground rod/cold water pipe/main panel safety ground/etc.), then bring panel wiring into the home there--You do not want to bring array wiring into the middle of the home directly. That invites lighting into your home.

    Placing the combiner box down low on the building (ground accessible) seems like a good solution vs a remote disconnect for the array... Will this be inspected by a local AHJ? Or just you? Talking ahead of time with the inspector would be a help--But make sure you agree with whatever is decided. Off grid solar (and solar in general) is not a common project for most local inspectors.

    I am sorry, I do not have any experience with array installs and the new NEC remote shutdown requirements and solutions. So take my suggestion with a grain of salt.

    In general, a properly wired and operating system should never pop/trip an over current protective device in a combiner box... HOWEVER, having access to a combiner box can be very helpful down the road when you are wondering if your solar array is working correctly or if it is outputting less than full power. Being able to turn off one string at a time (figure out if each string is supplying its share of current) is a very easy/quick debugging test--Unless the box is 20 feet in the air. And/or the ability to pull the box cover and use a DC Current Clamp DMM so you can measure the current directly in each string, at ground level, is nice too (vs at the peak of your roof). Finding a open electrical connection/bad panel/string quickly is nice... If you do have a bad roof top panel or electrical connection--That still requires a trip up (and why you want to ensure that all your roof top electrical connections are done correctly and are reliable/water proof).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jjdh
    jjdh Registered Users Posts: 9 ✭✭
    Bill thanks so much for taking the time to write that up, it really means a ton to have someone with experience check my thinking here. You are very close with Poplar Bluff (was it the temp levels that gave it away?), the system is going in near Eminence, MO. There is no inspector, so it becomes extra important to make sure I'm not missing something, since no-one will check the work.

    Keeping the wiring exterior to the building until there's been surge arrestors filtering off any lightning makes a ton of sense, and your notes on the additional benefits of having the combiner accessible, beyond providing a shutoff on the ground, also.

    If you don't mind, two more questions:

    On grounding
    With what you're describing, there's a contradiction in my head. On the one hand, that makes a ton of sense, give a fast clean path to ground outside the house for those surge protectors. On the other hand, it's been my understanding that having separate but interconnected grounding systems is a no-no. The existing DC system (the Schneider inverter and battery) are grounded through the AC system.

    Maybe that no-no only applies to stuff like subpanels in the AC system where you want to ensure a single global ground reference throughout so GFCIs and whatnot work properly everywhere?

    Incidentally, the trench for the AC grounding system is still partially open and right next to where I'd put the box on the wall, so I'd have the option of going directly from the combiner box to that.. but I'm not sure what the effects of doing that.

    I've read several books on DIY/off-grid solar at this point.. all of them, frankly, were really disappointing. For AC systems, Rex Cauldwells "How to Wire a House" really helped me, like a spirit guide with commentary for how to read and meet the code. Are there any good books in that spirit - rooted in code, focused on hands-on designing and installing solar systems - that you know of and recommend?

  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Since I am a moderator here--I can see the IP Address of your computer. And I use to guess at your location (pretty close about 80% of the time. With cell phone and satellite systems, usually not accurate):

    Regarding grounding--A complicated subjects with a lot of "it depends" answers. The basics...

    Grounding is not need for proper solar power (or general DC and AC) system operation. You just need the +/- and Hot/Neutral power lines to carry the electrical energy (your car is on rubber tires and works just fine). There are a few cases where grounding to earth is needed (some fluorescent tube fixtures need grounded cases to start, some automatic spark ignition system need grounding to detect flame).

    So why "ground"? There are several major reasons. First is to prevent a short circuit to a metal object (metal sink, water pipe, gas pipe, electrical "box", well pump, etc.) from being energized and shocking/electrocuting a person. By "grounding" an alternative path is provided to take short circuit current back to the source (main panel, ac inverter, dc battery bank, genset) and tripping the fuse/breaker(s) at the source and killing the power until fixed.

    Neutral bonding--The practice of connecting the white wire (neutral) of the AC power system to "safety ground" keeps the neutral lead at (or near) zero volts... A ground referenced neutral wire cannot be "short circuited" (high current flow) and does not need fuses/breakers to cut off excessive current flow--Saves money and complexity of wiring and protection.

    With DC power, we also negative ground the battery bank too... A "floating power system" (battery bank, AC transformer output) should have a fuse/breaker on both the + and - (hot/neutral/returns) not because of the first short to "ground" (no current flow on first connection), but if there is a second short to ground, then you have high current flow (because the system becomes "ground referenced"). And two breakers are needed to ensure that all the wiring (+ and -) is protected against excessive current flow (fault analysis is a complex subject... Can talk more about it if you wish).

    With DC, most of us are familiar with cars. In the case of a car, the ground is the car's frame/chassis. And it is both the "ground" and the return (negative) path for current flow. The car radio has one + lead, and rarely a - lead--Just the metal chassis connected to the frame for return current.

    For solar power systems, they have both a + and - power bus, and a "ground system" too. When we talk about single point grounding (DC and AC systems), the intent is to keep all current flowing through +/- and hot/return leads with zero current flowing through the "safety ground" (unless there is a short circuit). If you have parallel return and ground currents--Things happen. In power systems, if the return path has a shorter path/lower resistance, it will carry more current than the real "return" wiring and can overheat the safety ground wiring (especially in solar/battery systems where the scale of current is much different... 10 amps * 120 VAC = 1,200 Watts... That is 100 amps * 12 volts = 1,200 Watts -- So the DC power system may have 10x the current of the AC side--Having mixed current return paths--multi-point grounding--Can cause your 100 amps of DC to flow through your 10 amp designed AC power wiring).

    So for safety grounding--Having a single point where DC and AC grounding "meet" keeps the DC Current and AC current paths isolated from each other.

    This does get a bit confusing... Your AC inverter has a metal case. So both AC and DC are present... Suggest that the inverter case be grounded to the DC side because of the high DC currents. AC will still be "safely" grounded because at the "ground rod", both AC and DC ground systems are tied together. In large DC power systems such as telephone exchanges, this common ground point is called the "ground window".

    A major reason to ground your home power system is for lightning energy control... Get the lightning to earth as fast as you can (short/straight wire runs). For lightning control, multipoint grounding is a good thing. And tying all the metal structures/ground rods together with a minimum 6 AWG wire to provide "safety ground" for power system shorts to ground too (don't energize your solar rack mounts with 400 VDC or 120 VAC and create an electrocution hazards).

    There are other grounding issues too.. Multipoint grounding of AC neutral wiring via ground rods, cathodic protection systems for gas pipes, etc.. But most of those do not really apply to a typical home power system.

    Regarding books, in our FAQ page, I have posted information supplied by members on sources they found useful:

    The whole point of our forum here is to help people in a step by step method. Take questions one at a time from the start of a project (or debugging), and take it through to deeper subjects as needed... Vs the "fire hose" or "use the search function" answers.

    We have all been there--A first timer with no experience. Others have helped us, and we are trying to pass it one for the next generation.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset