Solar Array Wiring question

Ski66

Hello, I have been running a small off grid PV system that started with 2 - 140 watt panel wired in series and parallel, to charge a 24 volt battery bank.
I recently incresed this to 8 - 140 watt panels ( each pair is wired in series then to the combiner box) this is giving me 8 strings to tie into the fused combiner box ( 4 positive , and 4 negitive ) My question is would I be able to hook the panels up in series and parallel so that I would only have 4
strings, 2 on the positive , and two on the negitive bus bar ? My concern is if the amps from the first set of panels would burn out the second
set of panels ( as shown in the drawing ) Do I need a fuse inbetween each set of panels ? Or could these be wired in a more efficient way , perhaps increasing the voltage , and let the Converter control the voltage to the battery bank ? The panels / Combiner box is about 120' away from the house , I use overhead 6/6/6 triplex wire to tie into the converter ( The wire is rated for 60 amps ) an this is about the best wire I found with the minimal loss of electric for the distance. I appeciate any and all sugustions, and / or evaluations on my system. It has been up and running for about 3 years , however I am trying to make this run more efficient , and believe I maybe able to start with the solar panel array wiring. I have listed the specs on the panels below. Any information is greatly appreciated. Thank you, Paul

PV Panel Power: 140 watts
Maximum Power Voltage Vpm : 18.2 Volts
Maximum power Current Ipm : 7.89 Amp
Open Circuit Voltage VOC : 21.4
Short Circuit ISC : 8.1 Amp
Module Tolerance : 5 %
Maximum system voltage : 1000 V DC
Series fuse rating: 12 Amp
These are connected with MC4 connectors using # 10 awg PV wire / or 5.26 mm PV wire
Plese see the pics below.

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niel

Re: Solar Array Wiring question

welcome to the forum.
it seems that you have a misconception of what a string is. it is not each wire that is present. it is a series arrangement of pvs. if there are 2 pvs in series it is 1 string. if there are 3 in series it is 1 string. if there are 4 in series it is 1 string. if you take 2 in series and parallel them to another 2 in series it becomes 2 strings. you have 4 strings in your proposition.

now anytime there is more than 2 strings in parallel (and for that matter it applies to single pvs paralleled too) you must fuse each strings output before being combined. this would entail 1 fuse (yes, you can use a circuit breaker if you would like) for each string and not a fuse for every wire.

you did not state what you are using to regulate the pv voltages as a converter is not what we use. we use a charge controller or cc for short. maybe you got the terminology mixed up? not sure. anyway, if you would like to get an mppt style charge controller this can be fed higher voltages and down converted to your battery voltage. you could go 2 strings of 4 pvs in series to give you 72.8v vmp and being there are 2 strings it would be 15.78a imp. when it outputs it transforms it as it ups the current while lowering the voltage. this is usually the watts/batt nominal volts x efficiency = output current. be aware that not all mppt controllers can take this high of voltage or current so you must do your research. efficiencies do vary, but around 95% would be a good assumption give or take a few %. with 140 pvs x 8 = 1120w. 1120w/24v=46.66a. 46.66a x .95 = 44.33a. this would be cutting it very close for an mppt cc like this one rated at 45a,
http://www.solar-electric.com/motr45ampmps.html
you will have more choices of mppt ccs at 60a and up and of course the price goes up too. may be good to get a bigger one if you plan any expansions.

as to your wires you have a great deal of voltage drop losses going on with setting up each string to be only 2 pvs for a 36.4v vmp. i figure in the neighborhood of about 2.62% for 1 string of 2 so this you should not ignore. 4 strings through #6 is going to be 4x the current of 1 string for 31.56a. v drop losses are now 10.48%. at a minimum it should be under 3% for all wire losses with 2% preferable.

rearranging with an mppt cc for the higher voltage lower current from the pvs will give 2.62% loss through that 120ft run of #6 wire. the losses in the other wires will add to the losses in the main run, but the 2x4 arrangement is far better than the 4x2. losses can be further cut in half if you were to use a run of #6 wire for each string to go back to the combiner. as you can see the wire rating for higher voltage ac differs a bit when applied for lower voltage dc. if you want to play with different arrangements on the wire yourself then you can use a voltage drop calculator. i have one listed in my signature that you can download. bb also has used a good one and it escapes me right now where it is, but maybe he can link to it for you.

hope i covered everything without confusing you.

Ski66

Re: Solar Array Wiring question

Thank you for such a quick response. My apologies for the terminology on the charge controller, or cc. Having so many questions
I have to realize the importance to be as specific as I can. With your suggestion, and additional research I now understand how to wire a string of panels.
I intend to go 2 strings of 4 pvs in series giving me the 72.8v vmp. In doing this I will have to purchase a good MPPT CC , as I am currently using a
Xantrex C60 PWM , that will handle VOC of 55 vdc. ( but it is a PWM) I do plan on expanding the system. In looking the MPPT CC's that NAWS sells. I am
reviewing the specs on the Outback Flexmax 60 MPPT or the Midnite Classic. From my understanding the Outback seems to have good reviews, however
the Midnite Classic seems to have some additional options, including the wireless dashboard, and easy set up ? Please advise if there is any pro's or con's
on either of these, or would you consider these to be about he same in quality and would ultimalty be the end users preference.

Your response answered my questions , and I was able to follow your mathematical sequence , that has allowed me to re-evaluate, and plan on making my system more efficient. Also Thanks for the " voltage drop calculator " this is a great tool !
Paul

Vic

Re: Solar Array Wiring question

Paul,

While the OB FM CCs are good, that design is getting a bit long in the tooth. And do not expect much if any FirmWare updates for it, as there appears to be no room left for the code, as I read it. Furthermore, any update will require a ride to Arlington, WA -- updates cannot be done in the field.

The MidNite Classic is the hot setup right now, IMHO. The folks at MidNite have been doing Solar hardware for decades. The Founders of MidNite Solar were also the Founders of Outback Power and spent years at Trace/Xantrex as well. The Classic CC has been out in the market for over two years so it is maturing, but not old. MidNite does a great job in supporting all of their products, and they have an active Forum, where the Engineers are daily contributors -- answers right from the source. I DO gush about MN!

The Classic will produce more power than its competitors, It can endure higher input voltage when it is not producing power (HyperVOC), and there are three different input voltage rated Classics, and there are Classics without the graphic display built in available for less money. This should be the link to the Comparison Chart for various CCs including the Classic:
http://www.midnitesolar.com/pdfs/comparisonChart.pdf

This page has a lot of info about the Classic, for your study:
http://www.midnitesolar.com/pages/pages.php?article_ID=14

I am very pro MidNite and the Classic, but am just another happy user of Classics and a number of other MN products. Top Notch all the way IMHO. Good Luck, Vic

niel

Re: Solar Array Wiring question

i have to agree with vic as the classic has the best bang for the buck and they are still innovating aspects to it that you can download yourself.

don't get me wrong here as there's nothing wrong with using a straight pwm, but when facing long wire runs the mppt is the way to go and the classic is tops.