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Author Topic: PV panels' cable question  (Read 2104 times)
Iceni John
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« on: June 23, 2013, 11:17:19 AM »

I'll soon be installing the main feed cables for my PV solar panels, to bring their power down from the roof to the charge controllers.   There will be two banks of panels, each bank feeding its own controller that will charge half the house batteries  -  this way I'll have essentially two separate systems for redundancy, so if anything goes wrong I should still have half my system working.   (I'll be 100% dependent on solar, and my little generator is for absolute emergency use only.)   Each bank of panels will be four 240W, nominal 12V, i.e. theoretically 80A total output going to each charge controller.   In practice there probably won't be 80A, because panels don't produce their rated output most of the time and because they produce more than 12V most of the time.   I'm guessing that real-world output to each controller will probably be closer to about 50 or 60A.   Each cable will be about 12 feet long.

Ordinarily I would look up ampacity charts to choose a cable that gives the least voltage drop for that current over that distance, but there are two complications:  A) the cables will be inside conduit, and B) they will get hot at times due to their location.   They will be inside the 1.5" gap between the outer and inner roof panels, then they will run next to some blanked-off windows that will probably get very hot on the outside.   I haven't found any information for temperature correction factors, or for running cables inside conduit.

I looked at some Ancor marine cables at West Marine  -  they have 4 gauge for $3 per foot, and 2 ga for $5.   Ouch.   Someone suggested using heavy jumper cables instead, but they're not intended for continuous loads or for use in enclosed spaces that could get hot.   Would 600V welding cable work instead?   Blue Seas chart for USCG-recommended ampacities suggests 2ga or 4 ga, but are those sizes applicable for what I'll be doing?

Help!   I'm confused!   I want to do this right, and if I have to spend $5 a foot so be it, but I want to be sure I'm doing it right the first time.   Any electricians out there who'd know?

Thanks, John   
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« Reply #1 on: June 23, 2013, 01:21:37 PM »

THHN or THWN are good for 90C or 194F.  That should be fine.  2Ga will give you about a .2 volt drop.  I wouldn't go any smaller.  If you are going to have all four cables in the same conduit, I think I would go up to 1Ga or 1/0.
Welding cable in conduit is probably not a good idea.
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Iceni John
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« Reply #2 on: June 23, 2013, 01:59:00 PM »

Thanks, Len.   There will be only two feed cables, not four, and I plan on running each one inside its own conduit.   Is 2 gauge still good for that?   Just out of curiosity, is the Ancor marine tinned stranded cable equivalent to THHN or THWN?   Does it make any difference to the ampacity if the conduit is metal or plastic?   Plastic will be easier to install inside the curved roof, but I'm worried about the heat doing bad things to it (and I can't inspect it when it's in place).

Thanks, John   
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« Reply #3 on: June 23, 2013, 02:08:13 PM »

Why not welding cable one of the largest seller of solar supplies in the US Northern AZ Sun and Wind welding cable is their first choice ?
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« Reply #4 on: June 23, 2013, 04:04:58 PM »

 What is your estimated cost of your solar?
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Scott 
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« Reply #5 on: June 23, 2013, 05:50:42 PM »

         I have 840 watts and a Morningstar MPPT controller, connected all in series, using welding cable from the roof. 2 separate combiner boxes on the roof, 1 for + and 1 for --,,,Also I fused input and output from controller,
        Got all my stuff from Northern Wind and Sun in Flagstaff
        One major item IF you do not wire heavy and loose any Volts it will affect total system!!
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« Reply #6 on: June 23, 2013, 06:16:50 PM »

Why not welding cable one of the largest seller of solar supplies in the US Northern AZ Sun and Wind welding cable is their first choice ?

The only question is welding cable in conduit.  If it is in open air or a cable tray there would be no problem.
Because most welding cables do not have a UL type number, and the ones that do are SJ or similar, they are not approved for conduit.  On the other hand, THE NEC does not really apply to low voltage so it's probably not an issue.

I would be concerned about heat dissipation in a conduit. It's really kind of nit picking and for 12 feet likely wouldn't make any difference.
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Iceni John
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« Reply #7 on: June 23, 2013, 07:53:28 PM »

What is your estimated cost of your solar?
Large panels are now less than $1 per watt, cheaper than ever before.   My gut feeling is that PV will soon start climbing in price, as either anti-dumping tariffs are enforced or as demand picks up generally.   I'll buy the panels this year, even if I don't install them for another year or three.   Two MPPT charge controllers and all the other associated wiring and so forth may end up costing as much as the panels themselves.   Mind you, once I have it installed I'll have free electricity until I'm a doddering grizzled old fart.   Oh wait, I already am.

Another incidental benefit to covering most of my roof with panels is shade  - there'll be much less sunlight reaching the roof.   I'll also make some solar water heater panels to at least preheat the water before the water heater, and in the summer I may not need to use the WH at all.   The main reason for me to have PV is simply to not have to run a generator every time I need power  -  I really HATE the sound of generators!

John
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« Reply #8 on: June 24, 2013, 07:31:41 AM »

Im buying and picking up my first bus in Phoenix, and would like to have solar installed before I start fulltiming in it. Can anyone tell me of someone that can install it for me? I have rewired my harleys, but this solar stuff is all new to me, and truthfully I don't understand how it works?
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Scott 
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Iceni John
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« Reply #9 on: June 24, 2013, 07:56:16 AM »

Im buying and picking up my first bus in Phoenix, and would like to have solar installed before I start fulltiming in it. Can anyone tell me of someone that can install it for me? I have rewired my harleys, but this solar stuff is all new to me, and truthfully I don't understand how it works?
As Clifford also suggests, Northern AZ Sun & Wind in Flagstaff is well respected for honest business practices and fair pricing, not something one can take for granted in the solar industry!   See what they have to offer  -  I think they have complete packages which would take a lot of the guesswork off your shoulders.   Their price for 25 feet of 2 gauge welding cable is way less than I can buy 4 ga marine cable for locally (even with S&H), so they'll be getting some of my business.   Their prices on panels are also pretty good, not quite as low as some companies, but what recourse does one have with some fly-by-night outfit if a panel is damaged or defective?   Their website has lots of useful info.

John
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« Reply #10 on: June 24, 2013, 08:23:29 AM »

             I agree contact Northern Wind n Sun for install advice, several in Az. do the work but from what I have read most do not wire properly! And the systems do not work to the max.
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« Reply #11 on: June 24, 2013, 08:39:42 AM »

Wire the panels in series up to the limit of a good MPPT controller or two, say 24 or even 48 volts (or whatever).

The controller will then do it's thing and input the voltage the battery's are configured for. Personally, I use a 24 Volt battery bank because there are converters, inverters, and such readily available in 24 Volt Batt banks.

The smaller cabling is a lot easier in the higher voltages!


Ed
 
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harleyman_1000
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« Reply #12 on: June 24, 2013, 09:35:21 AM »

I'll soon be installing the main feed cables for my PV solar panels, to bring their power down from the roof to the charge controllers.   There will be two banks of panels, each bank feeding its own controller that will charge half the house batteries  -  this way I'll have essentially two separate systems for redundancy, so if anything goes wrong I should still have half my system working.   (I'll be 100% dependent on solar, and my little generator is for absolute emergency use only.)   Each bank of panels will be four 240W, nominal 12V, i.e. theoretically 80A total output going to each charge controller.   In practice there probably won't be 80A, because panels don't produce their rated output most of the time and because they produce more than 12V most of the time.   I'm guessing that real-world output to each controller will probably be closer to about 50 or 60A.   Each cable will be about 12 feet long.

Ordinarily I would look up ampacity charts to choose a cable that gives the least voltage drop for that current over that distance, but there are two complications:  A) the cables will be inside conduit, and B) they will get hot at times due to their location.   They will be inside the 1.5" gap between the outer and inner roof panels, then they will run next to some blanked-off windows that will probably get very hot on the outside.   I haven't found any information for temperature correction factors, or for running cables inside conduit.

I looked at some Ancor marine cables at West Marine  -  they have 4 gauge for $3 per foot, and 2 ga for $5.   Ouch.   Someone suggested using heavy jumper cables instead, but they're not intended for continuous loads or for use in enclosed spaces that could get hot.   Would 600V welding cable work instead?   Blue Seas chart for USCG-recommended ampacities suggests 2ga or 4 ga, but are those sizes applicable for what I'll be doing?

Help!   I'm confused!   I want to do this right, and if I have to spend $5 a foot so be it, but I want to be sure I'm doing it right the first time.   Any electricians out there who'd know?

Thanks, John   

So you will have 2 banks of 960 each, or a total of 1920 watts? Is that 160 amps going to the batteries?
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Scott 
St.Louis Missouri

1958 GM 4104 Extended 2 feet, with a 6v92 and 5 speed automatic

http://s783.photobucket.com/user/harleyman_1000/library/Gm4104%20bus?sort=3&page=1
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« Reply #13 on: June 24, 2013, 12:51:42 PM »

So you will have 2 banks of 960 each, or a total of 1920 watts? Is that 160 amps going to the batteries?
There will be two separate banks of almost 1,000 W each, each bank feeding its own charge controller which will charge four of the eight golfcart batteries.   Each bank will be independently tiltable to improve performance, especially in winter when the sun is lower.   The more I think about this, the more I realize that it's going to be a lot less than the nominal 80-ish amps going down each feed cable, but everyone stresses the critical necessity of minimizing voltage loss.   While I can probably get away with 4 ga cables without a problem, I think I'll use 2 ga  -  in the big scheme of things an extra few dollars for heavier cables is peanuts compared to everything else I'm doing!

John
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Lee Bradley
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« Reply #14 on: June 24, 2013, 01:09:05 PM »

I don't know how your banks are configured but I would configure them so I had each 12 volt series separately cabled down to the controller and parallel them at that
point on a large bus bar.  Smaller wiring and easier to drop a leg if you a cell.  Just a thought but keep us updated I can see this in my future. About how big is a 1,000 watt panel? 
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« Reply #15 on: June 24, 2013, 02:47:47 PM »

 Hey John when your done can I bring my bus by for you to start on?
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Scott 
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1958 GM 4104 Extended 2 feet, with a 6v92 and 5 speed automatic

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« Reply #16 on: June 24, 2013, 03:41:55 PM »

         Rather than golf cart batteries look at L16 batts , same foot print as golf cart just taller, but higher usable amps.
        Check them out.
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Iceni John
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« Reply #17 on: June 24, 2013, 04:15:07 PM »

I don't know how your banks are configured but I would configure them so I had each 12 volt series separately cabled down to the controller and parallel them at that
point on a large bus bar.  Smaller wiring and easier to drop a leg if you a cell.  Just a thought but keep us updated I can see this in my future. About how big is a 1,000 watt panel? 
That's an idea worth some shower-time.   (I do my best thinking in the shower.)
The 240W panels I'm interested in are about 3ft x 5ft-something.   They'll be hinged to either side of a walkway running along the center of my roof from one roof hatch to the other  -  there's 26ft of clear space between them, enough for four panels hinged on their long sides.   This way half my panels can be tilted up to face the sun, and the other half will lay down about 15 degrees below level against the curved roof.   Obviously this isn't quite as good as having all the panels tilt up, but it's a reasonable compromise.   Either side can tilt up depending where the sun is.   The walkway will be 12"-wide diamond-plate aluminum anchored substantially to the roof ribs that are only 19" apart (school buses are stout, and Crowns especially!)   To make washing down the panels easier, I'll put two quick-connect water outlets on the roof, then I can just plug in a washdown brush and easily clean bird poop etc off the panels without needing to lug heavy buckets of water or hoses up to the roof.   Whatever space I have left over on the roof will have some simple homemade water heating panels, probably something along the lines of coiled black pipe in a glass-topped shallow box, and the recirculation pump for this will run off the PV  -  free hot water in the summer!

My plan is for a bus that will comfortably support one person for at least a month, or two people for at least a fortnight, entirely off-the-grid and not using a generator  -  that's taking showers every night and living comfortably.   Water will be the limiting factor, but I've got 220 gallons of fresh and a BIG poo tank.

John 
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Iceni John
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« Reply #18 on: June 24, 2013, 04:25:18 PM »

        Rather than golf cart batteries look at L16 batts , same foot print as golf cart just taller, but higher usable amps.
        Check them out.
The only reason I'm thinking of plain-Jane golfcart batteries is that they're available everywhere and are reasonably cheap.   With constant recharging ability, plus not discharging them more than 25 to 30%, I should get a long life from them.   If one goes bad, a replacement pair is available almost anywhere.

My pull-out battery trays are 13.5" wide and 30" deep, each big enough for four golfcart, L16 or Group 31 batteries.   I'm hoping my modest intended power consumption will allow me to use golfcart batteries, but if I need more power I can use L16s.   We'll see.

I'm going to make a place for several bottles of distilled water next to the batteries, then rewatering them will be simple and quick.   I'll have a gravity-fed tube to bring water to each battery as needed  -  easy!

John    
« Last Edit: June 24, 2013, 10:51:02 PM by Iceni John » Logged

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« Reply #19 on: June 25, 2013, 10:21:46 PM »

John -

2000W of solar on a bus will be awesome - I look forward to hearing about your project as it comes together.

The one quick bit of advice - wire your panels in series, and increase the voltage dramatically running down the wire.  No need to target 12V - this is going to hurt your efficiency a lot.

My top pick charge controller is MorningStar Tristar MPPT - it can handle 150V input with a 12V battery bank output.  Here is more detail:
 http://www.morningstarcorp.com/en/tristar%20mppt

With higher solar array voltage, you can get away with 4 gauge or 8 gauge wiring to the roof - a lot cheaper and easier to work with.

Cheers!

   - Chris
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« Reply #20 on: June 26, 2013, 08:16:46 AM »

           Yes the Morningstar MPPT will handle 150Volts , But I was informed by Northern wind n Sun , you loose efficiency in the controller if above 70Volts through heat loss due to the controller dropping the Volts to battery volts.
          Also on another forum one person had series panels and their friend had parallel , on the parallel units, a power line shadow over just one panel caused a large loss in volt/amp output.     
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« Reply #21 on: June 26, 2013, 08:29:08 AM »

 First I need to say I know nothing at all about solar, but isn't there technical information on which is the proper way to install it?  I am wanting to install it on what I hope to be my first bus next month.
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Scott 
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« Reply #22 on: June 26, 2013, 08:36:28 AM »

Take Chris's advice.  If the panels are identical wire them in series to an MPPT controller.
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« Reply #23 on: June 26, 2013, 12:12:20 PM »

           Yes the Morningstar MPPT will handle 150Volts , But I was informed by Northern wind n Sun , you loose efficiency in the controller if above 70Volts through heat loss due to the controller dropping the Volts to battery volts.

Good point - I have heard the same.  70V is probably the practical max when dropping down to a 12V battery bank.  It still gives you a ton of headroom versus trying to set up a 12V solar array.

          Also on another forum one person had series panels and their friend had parallel , on the parallel units, a power line shadow over just one panel caused a large loss in volt/amp output.     

This is a great and VERY important point.  Most solar panels drop off to producing very little power if even a small part of the panel is even slightly shaded. Power lines, AC shrouds, antennas - these all can cut your power output down dramatically even on a sunny day.

When wired in parallel - one partially shaded panel can pull down all the others.  When wired in series, this is not an issue.

Cheers,

  - Chris
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« Reply #24 on: June 26, 2013, 12:53:16 PM »

I hadn't considered wiring each bank of four panels in series, but that would solve the issues of shading and of needing heavy feed cables.   With 2 ga cables close to $5 a foot I'm all for running lighter cables, providing the panels' combined voltage isn't too much for the controllers.   I'll have to research this further.   Sounds good to me.

Thanks for everyone's good advice,
John
« Last Edit: June 26, 2013, 08:11:02 PM by Iceni John » Logged

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« Reply #25 on: June 26, 2013, 04:22:08 PM »

   Boy did I get it WRONG, SERIES is where there is a big drop in volts is!!!!! 
 Chris ,, I did not reread my post, my wrong,  the series performed a little less.   
   Yes on parallel panels you drop amps on that shaded unit.
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« Reply #26 on: June 26, 2013, 05:25:50 PM »

$5 a foot is steep for 2 AWG.  Try GenuineDealz.com or WaytekWire.com  GenuineDealz is $3.25 a foot with free shipping and a bit less if you want 100 feet.  Waytek Wire will probably be less, but they will have a minimum perhaps as high as 100 feet.
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« Reply #27 on: June 26, 2013, 05:35:55 PM »

   Boy did I get it WRONG, SERIES is where there is a big drop in volts is!!!!! 

Indeed with series wiring there is a big drop in voltage when a panel is shaded, but as long as the panel array is still putting out a higher voltage than the battery bank, the MPPT controller handles it.

If your 70V array drops to 60V in partial shade, no big deal with MPPT.

   - Chris
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« Reply #28 on: June 26, 2013, 08:22:40 PM »

Ok, so series is not such a good idea?!   If one of four panels is shaded, the net voltage is reduced by 25%, but the net amperage is still the same?   For four parallel panels, if one panel is shaded the net voltage is still the same but the net amperage is down by 25%?   Am I right here?   If so, which is better  -  having a MPPT controller get 25% less voltage or 25% less amps?   Either way, isn't the total wattage reduced by 25% whether from reduced volts or from reduced amps?   Less power is less power, regardless of what causes it. and that's less power getting into the batteries.   Am I on track with this?

Who here has panels wired in series, and who has panels in parallel?   Let's hear some real-world experience from you folk.   I'm all ears.

Thanks, John
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« Reply #29 on: June 27, 2013, 02:02:26 AM »

A single power line isn't likely to cause problems with shading because the umbra part of the shadow won't reach down that far unless the line is very close to the panel. Also modern panels have bypass diodes across each cell or group of cells and so are greatly less susceptible to shadowing than older ones. I recently bought a 200W panel and was testing available short circuit current and found that my whole arm shadow made almost no difference to the output. Obviously I wasn't checking available power, but the effect was markedly less than it was on similar panels bought a few years ago.
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