Which is better .....

Chris11

I presently have 3 strings of 4 one hundred watt panels going into a combiner then into the Outback Flex 80.  I'm adding 8 more 100 watt panels.  

I can either do 4 strings of 5 or 5 strings of 4.  The VMP of each panel is 18 volts, and the Imp of each panel is 5.56.  I'm leaning towards 5 strings of 4 so I can turn off 2 strings during the winter when I don't use my Plug In Prius at the property where this array is located.

Thanks in advance, Chris

MichaelK

I think either choice is a mistake.  I would recommend you move away from 100W, 12V panels completely.  Once you get past an application requiring 200-300 watts, you get more bang for your buck utilizing grid-tie panels.  What I would recommend is switching to adding 72 cell panels putting out 36-37VDC.  Your costs per watt will go down.  There are many resellers right now on Craigslist, and I just made another purchase a few weeks ago.  Four grid-tie 245W Astronergy panels for 55$ each.  That's 1kw for 220$.  In the last few months I've made three purchases for myself and neighbors at these reduculously low prices.  You don't even have to throw anything away.  Let's assume you have a string of four 18V panels putting out 72V.  Just wire together two 36V panels (72 cell) to get the same 72V.  I'm going crazy now, upgrading my owns systems with 1000s of new watts.

BTW, I would not turn off extra strings in winter because you think you won't need them.  Your insolation in winter is lower, so output will be down, and also, if you get snow and the panels get covered, you'll get no charge for days.  When the snow does melt, you'll want the batteries to charge back up as fast as you can.

Chris11

That's not going to happen.  I have 30 100 watt panels ( 8 spares ) and I'm too old to do all the work again.  We only use the full solar on a few weekends per year for family get-togethers .... I'm already waaaay overpaneled for what I use for these get-togethers.  However I love charging my Prius while I'm there  for some weird reason so that's why I upgraded to an array of 20 panels.  Some people love guns, some people love cars, some people love gardening..... I love being able to charge my car while in the middle of a lectric free zone. 

As for the winter, I seldom go to the property so use zero battery juice.  I even considered turning off most panels in the past years just to keep what's needed for float.

Photowhit

Chris you don't discuss system voltage. 

I'd shoot for 5 strings of 4 if a 12 volt system or even a 24 volt system, If you have all the needed parts. I might go with 4 strings of 5 if I was having to buy parts (wires, breakers) on the theory that it was a bit cheaper and not a huge difference in how hard the charge controller had to work to convert. 48 volt systems really NEED at least 5 in series

I likely wouldn't turn anything off in the winter either. No harm in having extra potential with a good charge controller. 

Chris11

Yep...sorry about that.  It's 24 volts.  4 brand new Costco GC bats....not even put into the system yet since my old ones still work almost OK....lol.  (They no longer put out enough voltage to add  to what the array puts into the inverter, without turning off the inverter many time to put a slight charge back into the bats.)

I was going to ask this question later but maybe it's  time now.  What gauge wire do I use from the combiner to the CC with the new higher wattage?  I "think" my present wire is 8 gauge but this old memory can't really remember.  And what size breaker would I use?  Its only about 2 feet from combiner to CC.

Thanks for answering the 4 or 5 strings question.

Chris

softdown

Sorry but I just had to inform folks that Costco sells Johnson Controls/Interstate GC batteries while Sam's sells East Penn/Duracell GC batteries. I'm not aware of any difference in quality though I may choose East Penn myself - more of a US company while Johnson seems to prefer Mexico. I also seen a deterioration in the quality of Johnson 8D's after 2011. 

How cold can it get Cris11? Some of us made our panel decisions based on a concept called "HyperVOC" as I recall. 

How many years did you get out of your present batteries? 

I likely would not disconnect anything over the winter. I might consider changing the charging parameters of the Flex 80 over the winter season - depending on settings. 

BB.

Your question about wiring... if you are using 2,000 Watts of solar panels on a 24 volt battery bank, the basic equation for peak current is:

• 2,000 Watts * 1/21.0 volts charging (of a "dead" battery bank) = 95 Amps (nominal worst case)

Your charge controller is rated at 80 Amps output, and MPPT type charge controllers "regulate" and limit their peak output current to max rated current (how MPPT controllers work). So, for you, it is 80 Amps.

Personally, I like to "up size" wiring and circuit breakers to 1.25x planned max continuous rated current:

• 80 amps * 1.25 NEC derating = 100 Amp rated branch circuit wiring and breaker rating

Using the NEC table (more conservative vs the Marine Wring ratings):

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Depending on what insulation type you have for your wiring, 2 AWG (copper) is probably what you would pick.

There is also voltage drop to worry about... But for short cables with the NEC derating, you should not have an issue. To calculate voltage drop: Assume for a 24 volt battery bank, you want 0.10 to 0.20 volt max recommended drop, and a generic voltage drop calculator. Guess at 3 foot of wiring from charge controller to battery bus:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.5127&voltage=24&phase=dc&noofconductor=1&distance=3&distanceunit=feet&amperes=80&x=46&y=31

Result for 3 feet, 80 amps, 2 awg copper cable

Voltage drop: 0.075
Voltage drop percentage: 0.31%
Voltage at the end: 23.925

So, that is good... And for the combiner to charge controller (solar) wiring,

• 20 panels / 5 series panels per string = 4 parallel strings
• 100 Watt panels / 18 volts Vmp = 5.56 Amps Imp per string
• 4 parrallel strings * 5.56 Amps Imp = 22.24 Amps Imp-array
• Guessing Isc ~ Imp * 1.25 (rough estimate) = 5.56 Amps per string * 1.25 conversion = 6.95 Amps Isc (short circuit current)
• 4 parallel strings * 6.95 Amps per string = 27.8 Amps Isc
• 4 parallel strings * 5.56 Amps Imp per string * 1.25 NEC cable/breaker deratings = 27.8 Amps (because I used 1.25 in Isc conversion guess)

NEC table: 10 or 8 AWG cable is fine. Using 30 Amp (array shutdown breaker). You have to check the Series protection fuse/breaker for your 100 Watt panels... Assume it is around 10-15 amps per string (one breaker per string) to protect a shorted panel/string/wiring from being feed by the other 3 parallel solar strings (overheating wiring/panels and causing a fire). A breaker between the combiner and the MPPT charge controller is "optional"--Since you have a combiner box already (4 breakers, one per string), another breaker between the combiner and the MPPT charge controller is really redundant.

Not too worried about voltage drop between the combiner box... But to do the calculation for 22.24 Amps combiner output, 2 feet, 8 AWG copper cable, and 18Vmp*5=90 Volts:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=90&phase=dc&noofconductor=1&distance=2&distanceunit=feet&amperes=22.24&x=53&y=20

Result

Voltage drop: 0.056
Voltage drop percentage: 0.062%
Voltage at the end: 89.944

One thing I don't know is the wire AWG from each series string (10 to 14 AWG)? And what is the length of the wiring. Using the Voltage drop calculator, check that voltage drop too... Ideally, you want around a 1% to 3% total voltage drop (from panels to MPPT controller solar panel input). Your system can easily tolerate a higher voltage drop (you only need around 50 volts Vpanel input for a 24 volt battery bank)--But voltage drop is wasted energy.

And the expected maximum "typical" battery charging current from a 2,000 Watt array (cool/clear day, high noon, discharged battery bank) will be:

• 2,000 Watts * 0.77 panel and controller derating * 1/29 volt charging point = 53 Amps

You can get more current (charging into a "dead" battery bank at 21.0 volts, sub freezing weather, edge of cloud events), but normal maximum charging current (panels hot from sun, battery at ~80% state of charge, dust on panels, etc.), 53 amps is the typical (few times of year) maximum "sometimes) charging current.

If you have a 50% charged battery bank, heavy DC bus / AC inverter loads, and lower Vbatt voltage, on a cold/clear day, you can see higher charging current.

-Bill

Chris11

Thanks.  Softdown….it "can" get very cold.   20 below American measurement.  (I'm one of those who gets pissed when some decide we have to change our culture and systems to match theirs.)  My old bats were also Costco GC and lasted 6 years.  Some time ago Bill took the time to explain that I was discharging my bats at the C1 rate by charging my Prius without enough wattage from the array.  Hence my decision to increase my array.  

The settings I use in my Flex 80 are straight from Interstate who I phoned to get their suggestion.


Bill, thanks also.  I always appreciate your in-depth explanations.  
I don't understand why a breaker between the combiner and CC is optional.  Doesn't the breaker protect the wire?  And I use my present one to turn off the bats first if I have to shut down the complete system, then turn it on first to restart everything. 

The panels come with 12 gauge wire.  The original array is very close to the combiner but the new addition will be about 20 feet away.  I don't think I'm good enough to go into the lil box thing on the back of each panel to upgrade the wiring so 12 gauge will have to do.

Since I just ordered a new combiner from our host, plus more 10 amp breakers, I think I'll use a 100 amp breaker between the combiner and the CC.  And go to 2 gauge wiring as you suggest.     

Most people suggest I don't turn off any panels during the winter …. OK, you've convinced me .... but if I have to trudge through 19 feet of snow each winter to check the water levels, and find them low,  y'all will be hearing from me …. lol.

Thanks again, Chris  

mcgivor

Because each string has its own breaker, the rating of which is below the rating of the main combiner to CC conductors, they are intrinsically safe and therefore the 100A breaker would be superfluous, othere than being a master shut down. Only if the combined current could exceed the rating of the conductors would that be required, though that would be poor design.Chris11 said:

Thanks.  Softdown….it "can" get very cold.   20 below American measurement.  (I'm one of those who gets pissed when some decide we have to change our culture and systems to match theirs.)  My old bats were also Costco GC and lasted 6 years.  Some time ago Bill took the time to explain that I was discharging my bats at the C1 rate by charging my Prius without enough wattage from the array.  Hence my decision to increase my array.  

The settings I use in my Flex 80 are straight from Interstate who I phoned to get their suggestion.


Bill, thanks also.  I always appreciate your in-depth explanations.  
I don't understand why a breaker between the combiner and CC is optional.  Doesn't the breaker protect the wire?  And I use my present one to turn off the bats first if I have to shut down the complete system, then turn it on first to restart everything. 

The panels come with 12 gauge wire.  The original array is very close to the combiner but the new addition will be about 20 feet away.  I don't think I'm good enough to go into the lil box thing on the back of each panel to upgrade the wiring so 12 gauge will have to do.

Since I just ordered a new combiner from our host, plus more 10 amp breakers, I think I'll use a 100 amp breaker between the combiner and the CC.  And go to 2 gauge wiring as you suggest.     

Most people suggest I don't turn off any panels during the winter …. OK, you've convinced me .... but if I have to trudge through 19 feet of snow each winter to check the water levels, and find them low,  y'all will be hearing from me …. lol.

Thanks again, Chris  

BB.

Basically, your short wire from the combiner to the charge controller is large enough to carry 4x Isc of the four parallel strings (Isc-array ~ 27.8 Amps--Again, guessing without your Isc per panel information).

8 AWG cable is good for 40-55 Amps (depending on exactly which insulation type you have)--So, even if there was a dead short on the controller side, the solar array outputs 27.8 Amps (again, guess). You can add a 1.25 NEC solar up rating (in the case of a "solar flare" or other strange event):

• 27.8 Amps * 1.25 NEC "over rating" of solar circuits = 34.75 Amps

Still under the 40-55 Amps of a typical 8 AWG copper cable rating.

So, while a 40-55 Amp breaker is also a great switch--It will probably never be tripped from over current (with the present configuration).

Since the combiner is close to the charge controller, and you are using breakers (rather than fuses and fuse holders???), you just switch off the 4x breakers instead.

NOTE: Most fuse holders are not "safe" or rated to use as switches... Touch Safe fuse holders, if just Popped Open can sustain an arc, metl, or even catch fire. So if you (or somebody else) is using fuse holders for per string array fuses, then a "master breaker" or other "rated for 40 amps or so, plus > 90 VDC" rated switch would be used).

You do not have to "replace" the 12 awg cable on the panels... You just use the 12 AWG pigtails into a 10 AWG (or whatever wire)... You have the 4 foot 12 AWG pigtails and the 10 AWG xx foot run back to the combiner... But, probably not a big issue at all.

Say, 12 AWG 5.56 Amps per string (Imp), and 20 feet, at 90 VDC array voltage (note: this voltage drop calculator uses "one way" wire run lengths... Other use "round trip" lengths):

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=90&phase=dc&noofconductor=1&distance=20&distanceunit=feet&amperes=5.56&x=0&y=0

Result

Voltage drop: 0.35
Voltage drop percentage: 0.39%... 
Voltage at the end: 89.65

So, add that 0.4% drop plus the drops of other segments, and you are still under 1% end to end drop (3% or less total drop is usually what we aim for).

-Bill

Chris11

Thank you Mr. McGivor and Bill.

I got the new 10 panels mounted today …. so I'm now at the wiring stage.  I'm going to order the MC 4 connector with the 10 gauge wire to run from the panel connectors to the combiner.  I measured the distance instead of approximating....it's 30 feet not 20 so I'll order a 100 feet just to have some spare on hand.  And while I do appreciate you telling me I don't need a breaker between the combiner and CC I'm going to get one since I do like being able to turn off the connection.

Thanks again.  Chris