Wire Type from Roof Solar Panels to Controller

[CA_Tallguy]

Hi folks - I've been scouring for ideas on what wire type to use from solar panels that are going in on the roof down to the solar controller inside the cabin. I'm looking for suggestions on what is suitable for outdoor use, first and foremost.

Home Depot has a good selection of wire but the folks there don't seem to think any of it is UV rated. From souring the web, I can't get a very good picture on what to use and people seem to be using all kinds of stuff.

Some options I've read about/considered...
-- "tray cable" (not sure exactly what that is)
-- welding cable
-- marine cable
-- something else from home depot
-- order something from a solar store
-- go to local electrical supply and ask for something

I will be installing about 200 watts of panels and can hook them up as 12, 24 or 48 nominal voltage. The Outback MPPT controller I purchased would prefer 24 or 48. (I read one post somewhere that suggested that 24 might be better than 48 since it isn't as drastic as a drop for the controller -- but I have not seen other info to back up that claim.

I'm inclined to go 48 volt since the higher the voltage, the less power is lost in transmission. I am thinking that I will use a minimum of 10 gauge cable for the short 1 foot jumpers between panels, and maybe that would be sufficient for the drop down to the controller. Some of the options in the list above, such as welding cable, would certainly provide a much heavier gauge for that run but seems like overkill if I am sending 48 volts across it.

The run from the controller to the batteries will likely be welding cable so the main question I have is what to use from the roof down to the controller?

[DanielB]

Good stuff here..

from The RV Battery Charging Puzzle « HandyBob's Blog

[CA_Tallguy]

Thanks - I was checking out that site last night. Have not had a chance to read it in total yet but in scanning it I can't see specific cable recommendations except "the main thing to worry about is a UV rated shield on the roof" -- and that is the difficulty I'm having right now. He also talks about using welding cable in parts of installations, and that is my plan for the 12 volt side of things from the controller to the batteries (and I am assuming that is what he is talking about too). It isn't clear to me from what I have read if you can find welding cable that is UV rated. I'm starting to think my best bet is to go to an electrical supply and tell them I need some #8 or larger UV rated cable.

I've found a few other sites that are detailed like that guy but unfortunately I can't seem to find explicit information on what UV rated cable to ask for and from where. But at the same time, TONS of people are installing their solar panels and I don't believe that they are stressing out over the cable as much as I am so I am thinking I am either missing something or people are going for other stuff.

Another route that may be promising is someone posted on here about using marine cable and that it is rated for just about everything. That sounds like an interesting lead.

From what I understand, most cable sold for solar installations is #10 and smaller. But maybe I should look at the vendor websites a little closer to be sure. I've just been trying to find something local so far.

[DanielB]

So this is what I have found:

At Wire and Cable they say "The single most important component of a wire or cable is its insulation" and "2. All outdoor wiring should be of type XLP/XHHW, TC (Tray cable), USE-2 or similar outdoor UV (sunlight) resistant insulation."

This link says what all the Xs, Ls and whatnot stand for: http://lib.store.yahoo.net/lib/wind-...decorner66.pdf

[CA_Tallguy]

Fantastic! Thanks so much... now I know what to ask for! Maybe home depot even has some of it and they just don't know that it is UV rated as that is part of one of the greater classifications. They do have underground cable, so maybe it is the USE-2 and they just don't know anything about the UV that might be part of that.

That PDF you posted says, "USE: Underground Service Entrance • 75°C, wet insulation rating • Heat and moisture resistant • Sunlight resistant, but not marked as such."

And "USE-2: As above, but with a 90°C, wet insulation rating. The most commonly recommended cable for PV module wiring."

Thanks again for the help!

[JamuJoe]

I've purchased solar cable and connectors from Solar Penny SolarPenny Renewable Energy Equipment in the USA.

[CA_Tallguy]

Thanks, JamuJoe - I looked at that site just now and I think they only have #10 cable. They have MC3 and MC4 but I think they are still 10 gauge. That is the minimum size cable I would use but if I can find something a bit thicker that would allow me to avoid losing some of the juice in the feed to the controller.

[BigAl]

If you are running 200 watt panels at 48 volts that's a maximum of about 4 amps.
the 10 guage solar wire with mc4 connectors from panel to controller would be plenty.
The controller should be as close to the batteries as possible to minimize the length of heavy battery cable required.
Just a question for you guys; I assume that there should be a disconnect in the line from the panels to the controller?
Al

[steverino]

I did install a disconnect on the feed from the panels to the controller. As the feed comes down the refrigerator flue, I found a convenient place to mount it behind the refrigerator access panel.

Steve

[CA_Tallguy]

From what I have read, a disconnect/breaker should be installed on both panel and battery legs. I may just put a heavy fuse in the lines for now though. I'm not sure where I could put a mini breaker box to serve both except for next to the controller. I'm even having some issues deciding where to put the controller as there are not a lot of places to put it close to the batteries ( if I keep the front location for them).

Back to the wire question, I swung by Home Depot and asked about 5 or so options on the list posted previously and they didn't have any.

And on #10 size, yeah that would be OK I just wouldn't mind springing for something better for the main panel run. 10 is the absolute smallest I'd use.

I'll be making a run to some electric supply houses tomorrow.

[ddruker]

48 volts is the way to go. Higher voltage, lower amps are always better, up to the limit of your charge controller.

There are plenty of calculators available on the internet that will tell you how much power loss you will see for the length of your run at various cable gauge sizes. Up to you how much you want to spend to wring out the very last watts out of your panels.

I think you would be fine with rubber SOOW cable from home depot - that's what I installed for my 290 watt, 70 volt system. Are you really expecting your install to last 20 years? Here's some language about SOOW -

Type SOOW Flexible Cords are designed for extra hard usage on industrial equipment, heavy tools, battery chargers, portable lights welding leads, marine dockside power, power extensions and mining applications.

Seems to me that if it's good enough for dockside power and mining, it will last for plenty long enough on top of my trailer...

[CA_Tallguy]

Thanks Dan! I'll look into the SOOW. I figured some of the stuff would be OK but the people at the store really had no clue about the ratings. It sure would be nice if they had some of the descriptions like that at the store.

[CA_Tallguy]

I'm becoming discouraged. I've been back to Home Depot, Lowes, and also to two electrical supply houses. I am shocked it is so hard to find outdoor cable. But it may be a California thing as they say that most outdoor cable is run in conduits here. I ran though 5 or 6 options listed and nada. One supply house normally stocked USE-2 in #10 but they were out.

I'm currently in the Palm Springs area so the selection isn't as good as what I might find in LA. Heading out that way is probably my next step, after researching online options after this post. I need to head to LA anyway for something else. Or, I might have my father pick up some marine cable since he is down in San Diego today.

I am posting photos of the cable charts from Lowes, which I think are identical to the Home Depot charts. They did have THWN-2 but I have not been able to tell if it is UV rated based on the PDF posted earlier in the thread and a few other quick web searches. I'm feeling like USE-2 is what I am after, or some other #6 or #8 cable that is UV rated.

Lowes did have SJOOW, but not SOOW. The PDF says that the SJ is "junior" hard service, vs "hard service" for just the SOOW. They only had #10 though. I may fall back to that as I'm getting desperate and I am itching to hook these panels up! I would really like to go with heavier gauge though to wring every last watt out.

[AWCHIEF]

If you have a West Marine nearby check them for Marine Cable. It is UV protected. I have used it exposed on the deck for years on boats and have never had an issue with cover breakdown. I have about 50 ft that I salvaged from my last boat and plan on using it when I get around to installing my panels on the Bambi II.

[CA_Tallguy]

Just talked to another electric supply and they also said it would be hard to find outdoor UV cable and he said "it ain't legal" here in CA. It is hard to imagine that is so, as I know that I've seen overhead cables strung many times here. Maybe that was only done under older codes though. They did say they special order some other stuff he couldn't remember what it was called for some solar guys though.

I've just checked West Marine's website and they have some stuff that looks promising -- they have 6-2, 8-2, and 10-2 "safety marine cable" for $5, $3, and $1.63 per foot respectively. Separately, the have just "duplex cable" that is about the same price. I don't know what the difference is between that and "safety" cable. They also have single conductor cable in #8 and #10 for $1.20 and $0.74 respectively.

The local store says they have a good selection in stock.

The cable is made by Ancor and I can't find specs that say UV resistant but I've seen some marketing copy on some websites that says it is....

"Ancor produces a wide line of specially designed marine grade electrical products engineered for the demands of marine environment and elements faced out on the water. It is important to remember that marine electrical systems are much different than standard electrical wiring. All parts in the wiring process must be waterproof, UV ray and corrosion resistant, as well as strong enough to stand up to extreme vibration and flexing. Ancor wiring components exceed these standards."

This is starting to sound like a good option that I might be able to get locally.

[ddruker]

Wierd. All three of the Home Depot stores near Palo Alto where I live carry SOOW cable, in lots of different gauges and conductor configurations.

[CA_Tallguy]

Yeah they only seemed to have the SJ variety here in Palm Springs and then only down to 10 gauge I think.

I ended up with marine cable as described from West Marine. (Trying to attach pic of selection from phone.) I'm very happy with it though it was expensive. Everything they have is uv rated. I got the 8-2 cable for about $3 per foot. In reality, only 6 feet or maybe less will be exposed so it would be possible to have a junction/combiner to transition to other cable just inside or right before the inside drop.

I have also been thinking about using trempro to attach it to the roof and another cable type might have worked fine if it were embedded and covered in it. That sounds potentially messy so I'm unsure about the idea.

Tomorrow will be running the cable and not sure how that will go and if trempro is the right way to adhere... And not sure what I am going to face with the fridge vent. Hope it all makes sense when I start poking around. If anyone has advice, please share!

[CA_Tallguy]

Just wanted to correct myself about the wire types at Home Depot. The chart there is in fact different than Lowes and a pic is attached. They do have the SO type wire. It looks like it is 3 and 4 conductor but does go down to 6 guage. Didn't check the price though since I already went with marine cable.

[CA_Tallguy]

One note about the marine cable. It is finely stranded, much more finely than what I think was some 8 gauge THHN cable I bought previously at Home Depot for my truck. That makes it pretty flexible. I would guess that the SO cable is pretty flexible too since it is kind of a power cord type application.

I vaguely recall reading in my research that electricity flows on the outside of the conductors or something so I wonder if the fine strands are better for that? I suppose there may be a purpose to choosing heavy strands over fine but maybe not and it is just a quality thing. [EDIT: upon further research it seems it may just have more to do with how flexible the cable is.] Maybe there is also a difference between AC and DC in how it travels on the wire and if fine vs thick strands matters. Of course, household wiring is generally not stranded for common romex runs and that is AC. [EDIT: I researched this concept of power flowing on the outside a bit more and it is called the "skin effect" and applies more to AC than DC]

The welding cable I got for the 12 volt run between the solar controller and the battery bank is also finely stranded.

[CA_Tallguy]

One more little note -- although the Ancor brand #8 cable from West Marine was about $3 a foot, it was two conductor so it looks like it wasn't too much more expensive than the #10 single conductor wire sold by solar merchants online ($1 to $2 per foot for single conductor). Northern Arizona Solar has an outdoor rated #8 two conductor for $2.26/foot.

And West Marine had #8 single conductor cable for a bit over a buck a foot too.