What is the best low cost solution for PV array combiners when UL listing and NEC compliance are irrelevant?
I have a project to supply a quantity of 300Wpv off-grid systems to power schools in rural Tanzania through the British NGO, SolarAid
The goal here is to find the cheapest highest quality solution. UL is not required but common sense is. There are no electrical inspectors in rural Africa. Any kind of a combiner will be better than they way they are doing it now. Take a look at my "Solar Spaghetti" photo tour of PV systems in Africa to see what I mean. I have seen some crappy homeowner installed systems here in the San Juan Islands but nothing like I have seen in Africa.
I know what the NEC says, but IMO series fusing of individual strings of PV for small off-grid systems (under 500 Watts of PV at 12 or 24 Volts) is not necessary and adds unnecessary materials cost. Circuit breaker combiner boxes are overkill at this scale. With MPPT controllers it is usually possible to keep the number of strings under two and avoid much need to parallel PV output circuits. However, PWM controllers still dominate the market in Africa and that will probably continue, AND 12 Volt systems are standard so we will be talking about multiple panels in parallel and fat wires running to the batteries.
So, specifically. say we need to combine up to 6, 12V nominal modules and transition to #2 (or larger) for the run to the battery. Systems larger than that would be at 24V or 48V and would combine multiple strings of modules.
Modules will be equipped with either junction boxes or MC4 type plug and play connectors.
For MC connectors: Right now in Africa the common practice is to immediately cut off the connector end and use the 'twist and tape' method to connect a module in parallel with its neighbor and send the combined output of those two modules to combine with another pair of modules and so on, until it splices into the larger wire running to the battery location. The T&T method is literally as it sounds, the wire is stripped about 2", twisted together tightly and wrapped tightly with CBT (cheap black tape).
My current plan is to use pre-cut one-ended MC4 extension cords of #10 USE-2 wire to get all the module outputs into the bottom of a 6"x6"x4" Nema 3R screw cover enclosure through Heyco liquid tight multi-hole cord grips. The #2 USE wire leaving the box to the battery will exit through either two Heyco single hole cord grips or a single hole strain relief of some kind (romex clamp?). In the box the circuits will be combined on red and black insulated bus bars from Midnite Solar. The plastic insulating bases will be screwed or riveted into the enclosures. The plated copper bus bars can click out of the insulators to allow for more easier wiring. Each bus has 3 holes for #6 through 14 wire and 2 for 14 up to 1/0. A third Midnite bus bar will be used as an equipment grounding bus.
For J-box modules we will provide #10 USE-2 wire and liquid tight 2-hole strain relief for 1/2" knockouts and forked terminal crimp connectors. We will provide UV resistant black nylon wire ties and encourage installers to keep all wiring tidy and under the array, or else well protected from UV and other damage.
That is my plan, to me it seems like the best cheapest way to go. There are cheaper or different ways to go that might be worth exploring, including: split bolts and rubber tape, rubber-dipped type insulated terminal blocks and bus bars, underground wire-nuts, the molded MC4 paralleling couplers, and probably more..
Anybody have any opinions or suggestions?