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Help with solar setup


ctm4u2

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I am setting up a solar system for my RV, I have three 100 watt solar panels. I want to connect them to a combiner box before running wires to my charge controller. I am looking at buying a solar combiner from www.solarpowercombinerbox.com. They have 3 string combiners with fuses or circuit breakers. Im not sure if it maters what type I use. Is there any difference? Any advise or help would be great. Thanks

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There are a couple of things to be aware of:

  • Combiners that are intended for residential application generally cannot be mounted flat on the roof of an RV. MOST require at least a 45* (from vertical) mount to retain their waterproof rating. If you are bringing each line into the RV and not mounting on the roof then you would be OK.
  • For three panels most people do not find the expense of a breaker/combiner to be necessary. However, there is an advantage in that each panel can be isolated and tested at the breaker/combiner. This is not a critical factor on such a small system, however.
  • Given a choice a combiner with breakers would always be my choice - assuming was going to use this level of capability for that small a system.

Take a look at using a MidNite Solar Baby Box or Big Baby box if you want to use breakers. It will be cheaper and smaller.

 

In your installation, given the information you have provided, I'd use the AM Solar large combiner box.

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Those look like good quality boxes. They serve as junction boxes I assume???? which is something you could do without. I, like MANY RV owners here, wired my panels without the use of those boxes, while of course, you want any splices to be quality low resistance and protected against moisture and the environment.

 

In my many years as an electrical power distribution design engineer, I learned from study and experience circuit breakers are less then perfect "mechanical" devices (spring, lever, hinge etc) which can (especially if exposed to the elements or temperature or humidity changes over the years) fail, corrode, stick etc., while a fuse with no moving parts and for the most part better sealed, were fairly reliable and less age sensitive. A 10 yr old fuse is probably just as good as a new one, while a 10 year old breaker (with all its moving parts), especially in an imperfect environment, may not work as when new. HOWEVER if the fuse blows it needs replaced while you simply re set the mechanical breaker. NOTE the smaller automotive type of breakers are less mechanically complicated then the larger commercial breakers.

 

BOTH (fuse versus breaker) have advantages and disadvantages, there's NO right or wrong answer here, its a matter or preference and the environment in which its used and how hard it is to get to it. In many many applications I specified Breakers while in others I chose Fuses. IT DEPENDS ON SEVERAL FACTORS. USE WHICHEVER YOU PLEASE.

 

John T NOT a Solar Expert so no warranty, and listen to those more experienced in the solar field.

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First question is what voltage controller are you using? If you are using a good MPPT controller, and these are 12V panels you can just connect the panels in series. The higher voltage means less power loss in the cabling to your voltage controller and thus greater efficiency for the system. Any good MPPT controller should be able to handle the 60 or so volts the panels will produce in a series configuration and convert to 13-15V to charge you battery. I have a Morningstar TS-MPPT-30 controller and I run two 190watt 24V panels in series to charge my 12V battery bank. You can use inline fuses in case anything shorts out, or use a circuit breaker before taking the string into your controller

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The only thing I would add to Jack Mayer's post is the cite to his site discussing these matters. http://www.jackdanmayer.com/Wiring.html. There are places to save in equipment cost and this may be one of them. Essentially you just need a waterproof enclosure and your choice of electrical parts to establish a secure connection. Like Jack said, breakers may be convenient but they are NOT necessary.

Dave and Lana Hasper

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I just went to Lowe's and bought a PVC waterproof junction box and a couple of bus bars and made my own combiner box.

<p>....JIM and LINDA......2001 American Eagle 40 '.towing a GMC Sierra 1500 4X4 with RZR in the rear. 1999 JEEP Cherokee that we tow as well.

IT IS A CONTENTED MAN WHO CAN APPRECIATE THE SCENERY ALONG A DETOUR.

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I just went to Lowe's and bought a PVC waterproof junction box and a couple of bus bars and made my own combiner box.

 

X2. All told.. under $10. If you think you might toss up another panel or two in the future, installing a C-box would make sense.

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I just went to Lowe's and bought a PVC waterproof junction box and a couple of bus bars and made my own combiner box.

 

If it's one of those gray plastic ones, I hope you didn't put it on your RV roof. I've stopped using them because the UV would warp the covers.

SKP #79313 / Full-Timing / 2001 National RV Sea View / 2008 Jeep Wrangler Rubicon
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Dave H "But, remember the downside that when panels are in series, shading on any single panel, decreases output of all the panels"

 

Good point, that's why it depends on your roof real estate and if potential shade sources are in the way and if all panels are exposed to fairly equal sun light. If there was nothing in the way and all panels get the same equal sunshine I might prefer series, but if there are restrictions, some parallel connections may be best. I have four panels in series but they are pretty well open so shading shouldn't be too much of a problem. Of course, MATCHED PANELS is very important ESPECIALLY if in parallel.

 

Fun chat

 

John T

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But, remember the downside that when panels are in series, shading on any single panel, decreases output of all the panels.

I'm not sure I get this Daveh. If one panel of a group in series is shaded then obviously that panel will not be contributing to the output of the group but that would also be true if they were in parallel. I dont see how one panel will affect the individual output of the other panels

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In series a panel without sun becomes a fairly high (in solar terms) resistor and will kill the power from that string.

 

Some panels include bypass diodes so you can still get some power out of the string but it will be very little.

 

Series setups are usually not considered if shading is an issue.

First rule of computer consulting:

Sell a customer a Linux computer and you'll eat for a day.

Sell a customer a Windows computer and you'll eat for a lifetime.

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KInsey you ask "I'm not sure I get this Daveh. If one panel of a group in series is shaded then obviously that panel will not be contributing to the output of the group but that would also be true if they were in parallel. I dont see how one panel will affect the individual output of the other panels.

 

I agree with the good Stanley Man and to add a bit more theory to his answer, here's why 2 panels in parallel is less hampered then 2 in series if one is shaded.

 

If the resistance of a panel becomes high under shade, and if two panels are in parallel, then you have a high resistance (shaded panel) in parallel with a lower resistance (non shaded) voltage source, so the net result is still producing voltage into the controller even if one panel is shaded.

 

HOWEVER if two panels are in series and one develops high resistance due to shading, then you have sort of a blocking high resistor in the total series circuit.

 

An advantage of series connections is increased voltage so there's less voltage drop (can use smaller wire) from the panels down to the solar charge controller while shading is a disadvantage. I have four panels in series but all the panels are open (as far as roof location) and equally exposed unless I were to park where one may be shaded.

 

John T

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Got to apologize, Daveh was right. I have two panels free standing so shading is never an issue since I can move them around and locate them at a distance from my rig. I just tried shading one panel by about 70% and lost 70% of the array power output. I dont have bypass diodes on my panels so yes, one panel being shaded does drop power output from the whole array. Problem with lower voltage arrays tho is the power drop in the wiring12v_2p_0.gif

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In theory, bypass diodes will do just that, bypass a shaded panel to allow the unshaded panels to produce as if the shaded panel wasn't there. And that's good, but the problem is that when there's partial shade, which is almost always the case, it doesn't quite work like that, and the series string can be severely impacted.

 

I just stay the heck out of shaded areas.

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Yarome and oldman good afternoon: Can you explain your statement "In theory, bypass diodes will do just that, bypass a shaded panel to allow the unshaded panels to produce as if the shaded panel wasn't there"

 

Best I recall when I bought my panels I thought they had a diode to prevent reverse current flow so the batteries couldn't be reverse discharged overnight when in the dark. They allowed the flow of charging current OUT of the Panel INTO the batteries, but would NOT allow the batteries to reverse discharge through the panels.

 

Is there another optional diode????? Same one youre talking about???

 

John T Never to old to learn

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You can google 'bypass diode' to really get a headful. You're thinking of a blocking diode. Yes, it's different.

 

I have 6 panels in series. I can put a blanket over one of them and the array will function as 5 panels. However, if it's partially shaded, even a couple square inches, the diodes don't work as I expect them to, and I get a whopping degradation of performance. It's really not an issue for me as I hardly ever camp in shade, save for the shadow from my a/c cowling.

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Bypass diode: Allows power to flow from the rest of a series string when the panel it is on is shaded. It usually costs about 1/2 volt from the other panel's voltage if the panel is shaded, lost in the diode.

 

Blocking diode: Prevents battery voltage from flowing through the panel at night or when almost completely shaded. It also costs about 1/2 volt, all the time you are charging, which is lost in the diode.

 

Ideally you don'e want either type of diode, the blocking one in particular since it is a constant loss. It is better to have the discharge prevention in the solar controller than on each panel.

 

Paul, shading issues are easy to evaluate, all you need is a sheet of cardboard and you can experiment all you want to see just how it impacts you. The impact is different for series and parallel wiring as was mentioned above. You don't have to take our word for it, test it yourself like kinseypw did.

First rule of computer consulting:

Sell a customer a Linux computer and you'll eat for a day.

Sell a customer a Windows computer and you'll eat for a lifetime.

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If it's one of those gray plastic ones, I hope you didn't put it on your RV roof. I've stopped using them because the UV would warp the covers.

I've never had a problem with them but we dont experience the e treme heat that you guys in the south get. I think its more the heat than sun that causes the warping. If I see a problem I'll get a new cover and paint the whole works with a white UV resistant paint.

<p>....JIM and LINDA......2001 American Eagle 40 '.towing a GMC Sierra 1500 4X4 with RZR in the rear. 1999 JEEP Cherokee that we tow as well.

IT IS A CONTENTED MAN WHO CAN APPRECIATE THE SCENERY ALONG A DETOUR.

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Just to clarify a couple of things, a "combiner box" is not the same thing as a "junction box", although they can perform similar functions. A combiner box will have overcurrent protection devices (OCPD), which are typically needed in cases where three or more modules or sets of modules are connected in parallel. If the combined short circuit current of two of the modules exceeds the series fuse (or breaker) rating for any of the modules then OCPD is required. Essentially, the series fuse rating on the module namplate tells you how much current can be backfed into the module before it overheats and burns. Some early PV modules had series fuse ratings as low as 8 amps and almost always needed OCPD when three of them were paralleled. Newer modules often have 20-25 amp series fuse ratings and are less likely to need fuses or breakers for protection when paralleling three or even four modules. Inline fuse holders that connect directly to the newer MC4 PV cable connectors can be used in lieu of fuses or breakers in a traditional combiner box in many cases. The tradeoff is ease of installation vs ease of troubleshooting.

 

Yes, shading will adversely effect the output of series connected modules, and it should be kept in mind that on an RV things like air conditioners, vent covers and hatch covers can cast enough of a shadow on a module to decrease its output significantly. This is especially true in the winter months with low sun angles. Paralleling modules adds redundancy to the system and allows it to function even if one or more of the modules are shaded.

Tom - KK8M. MI Licensed Journeyman Electrician (35 years), ISA Level 2 Instrumentation Technician, UL Certified Solar Installer.

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