I am currently trying to figure out what is the best way to run the wiring for 21 Sharp 224U1F panels. (These are the panels I really had to get.) According to the XW sizing tool, there should be 3 strings of 3 panels to one charge controller, and 4 strings of 3 panels to the other charge controller. It doesn't specifically state this on the sizing tool, but it seems the 3 panels are hooked up in series for each string.

So, after hooking up the 7 groups of 3 panels / string, is it better to run 14 wires down to the two combiner boxes (because I need 2 wires / string), or is it better to put the combiner boxes up by the solar panels, combine the 4 strings into one and the 3 strings into one and then just run 4 wires down to the XW Power Distribution Panel. And is it better to run these wires outside either in conduit or open air, or should they go into the attic and run in conduit there.

I have been trying to work with an electrician on this, but things are moving forward a bit too slowly.

It is 3 panels in series, that's the easy part

Next, regardless of where than strings are combined, each string must be fused/breaker

As for the wiring, its an ease of installation and cost that makes the rest of your decisions. Combining on the roof is less total footage of wire, but costs more as the wires of the combined current must be MUCH larger, and wire cost is not linear with gauge. Its also much much harder to pull thru conduit the low wire gauges ( thicker wires ) than 12 gauge, which would be fine for home runs on each string

On the external vs internal question, that's a code thing. on the roof, you can have USE-2 wire, but anything accessible requires conduit. If you run the wire into the attic, that MUST be emt, not pvc.

My wiring is thru the roof, into the attic, emt to the combiner panel which is next to the charge controllers in the garage. Its all TWTH 12 gauge from home depot, about 120 bucks for a 500 foot roll. You need to have this all planned ( drawings and material lists ) and approved by the electrical inspectors BEFORE you get the permit

Send 50 bucks and by the current NEC code book, I got mine of amazon. All the PV stuff is section 690 and its invaluable for planning or installing PV. The plans examiner will be doing the same thing against the plans so having the book is the only way to know what is the correct way to install.

My inspector was pleased that I had the book, took the time to understand the regulations and made approval a snap. My electrician friend, while an excellent electrician ( master electrician ) had no clue about anything PV as its not anything he was taught , tested on or ever installed. I did all the work under his license for my installation

Hope this helps

Yes, yes, it is a gigantic help and greatly appreciated.

Maybe we have the same master electrician friend??

After more research, it seems a bit more complicated because the panels require the MC3 / Solarline 1 connector extension cables. And those cables seem expensive. And then they are saying that maybe because of the new codes it might really need MC4 connectors, which of course would require another expensive MC3 to MC4 adapter cable.

But apparently some places go by the new 2008 code, some places go by the older 2005 code, and some places go by even older code. So it is very confusing to try to get things figured out. I don't see why everybody couldn't at least go by the same code book at the same time. And I don't see why the solar panels couldn't at least meet the code book for the year they are manufactured. And no matter what the code, you will have an MC3 connection on the solar panel as it is currently made, which would seem to be a violation of the code.

ask your local inspector as to what he wants and odds are he'll want the cables with the connectors the pvs were made for. if no inspector then you will still need to buy the mc cables due to warranty restrictions on the pvs for removal of the connectors will void the warranty. you won't get around this extra expense very easily.

I just received my supply of MC4 connectors and my MC3 should be here next week. When I called a few weeks ago to order more I was told by my salesperson there is a world-wide MC connector shortage. I managed to get some of the few available right now. I can custom make any size MC extension or whip from 6 inches to 500 feet. I can even put a nice ring terminal on the one end. I have not re-figured the new pricing but should be between $3-$4 per connector plus per foot for the wire (50 cents or less/ft) and I charge $1 per cable assembly charge. I have #10 AWG Copper USE-2 cable and a limited supply of #10 XHHW-2 sunlight & fire resistant. I only charge actually USPS shipping charges.

I've been doing more research, and I'm wondering if the 2008 NEC has changed from previous editions in regards to the Array to Combiner Box wiring.

It now seems that the practice of running the PV wires far away from the array to the combiner box is now frowned upon. But I liked running the PV wires at least into the house, that way the combiner box and circuit breakers are inside the house instead of baking outside on the roof.

It does seem like the new way would use much less of the very expensive MC wiring, so maybe I should be happy doing it that way in spite of the combiner box being outside.

I'm wondering what those with more knowledge and experience than I think about this.

Thanks.

You only have to use the USE2 wire to the point where the wire enters the conduit, then you can use TWTH wire , 120 bucks for 500 foot roll at home depot. NEC has changed in the fact that conduit must now be EMT where PVC was previously allowed, but that's it.

I have four (almost) 12 panel tracking arrays out on the north property. The longest run is 259 feet of #8 THHN to a combiner box in the garage. Each tracking array is arranged as two 6 panel setups with each having a home run of #8THHN in underground conduit. I run high voltage of 120VDC @ 7.4A to the combiner. If I had the combiner out at the arrays I would have to think of like 200 feet x 2 of #4/O THHN to the garage, and I don't even want to price that copper out, let alone pull it that far. The #8 THHN was expensive enough. With PVC you also have to pull a ground wire based on the largest current carrying conductor. I used a #8 THHN Green, for the #4/O I'm guessing something around #3 THHN would be required. High current DC also has magnetic losses base on how far apart the plus & minus wires are. To minimize this you would have to tape the 4/O wires about every 4" for the run, not fun for 200 feet. Just my 2c.

I can only say much about the wirings as I have just tried it a few number of times. It is really to install it and much of the consideration would have to be made with the price tag it comes along.

If you really want to combine on the roof, there would be less need for the length of the wire but you probably would have to get more in terms of bigger current capacity.