String size advisable?

zozomike · January 2020

So I thought I was posting this question before, but evidently no one really thought I was asking for suggestions. I am tomorrow ordering 20 of the following, and I think I can parallel wire two strings of 10,each, one string east facing one west at 20 degree tilt, seems that should bring them into spec for two Outback FM80 controllers. Am I missing something? East west will supposedly cut output 15 or more percent. 3 % wiring loss and 2500 watts gets very close to 2000. Available location is the reason for venturing into east west, and the panels are very inexpensive used.
Sunpower 250 black
72 cell
VOC 48.8
VMP 40.5
ISC 6.56
Coldest annual -10f
80 foot from farthest panel to cc.
Latitude 33, longitude 105,altitude 7500ft.
I think based 66 amps ,48 volts I probably need at least number 6 from combiner to cc? 50 foot run.
And will need a 2 string combiner, with of course, 2 runs to 2 cc, in 1- 2inch underground conduit. Any suggestions?

Estragon · January 2020

You might want to try various strings in the OB tool online. My guess is 5 strings of 2 per array should would.

I'd err on the large size for conduit. Trying to squish big wire into small pipes isn't worth the marginal cost saving IMHO.

Photowhit · January 2020

zozomike said:

I think I can parallel wire two strings of 10,each, one string east facing one west at 20 degree tilt, seems that should bring them into spec for two Outback FM80 controllers. Am I missing something?

Any suggestions?

FM80 has an open voltage hard max of 150 volts, in a string they are in series. In series the voltage adds so a string of 10 panels has VOC of 488 volts. Too much to handle.

Link to Manual;
http://www.outbackpower.com/downloads/documents/charge_controllers/flexmax_6080/specsheet.pdf

BB. · January 2020

As solar panel voltages increase (Voc/Vmp), you run into a difficult position when running at 140/150 VDC Vpanel max for typical higher end MPPT charge controllers and 48 VDC battery bank operating voltages.

2x 48.8 Voc panels in series is about the maximum you can run in a cool region... 3x 48.8 = 146.4VDC @ Voc standard temperature (~75F/25C), and will go way over that in subfreezing weather.

And you have 2x 40.5 volts Vmp = 81 volts Vmp-array standard temperature... And it will drop a lot in hot/sunny weather (down towards ~65 volts @ hot panel).

And your battery bank needs about 60 Volts to charge... That leaves 5 volts for wiring drop and controller drop (almost all MPPT charge controllers can efficiently "drop" voltage--They cannot increase output/battery charging voltage higher than Vpanel input voltage).

Say you will allow 2 volt wiring drop from array to charge controller (~3% drop). If you assume 250 Watt panels * 20 panels = 5,000 Watts

5,000 watts * 0.77 panel+controller derating * 1/58.0 volts charging = 66 amps "average best case charging" current to battery bank
5,000 watts * 0.77 panel derating * 1/80 Volts Vmp-array std = 48 amps (roughly)

Using a simple voltage drop calculator: 80 feet, 48 amps array current, 2.0 volt max drop:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=80&phase=dc&noofconductor=1&distance=80&distanceunit=feet&amperes=48&x=57&y=22

4 AWG wiring from array to charge controller (just two wires total):
Voltage drop: 1.91
Voltage drop percentage: 2.39%
Voltage at the end: 78.09

4 AWG is good for ~70-95 Amps, depending on insulation type, operating temperature, conduit fill, etc.

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

-Bill

zozomike · January 2020

Thanks folks. Yes I thought about 5 strings of 2, after posting. That's 97.6 vdc with 32.8 amps correct per array? Each string on a15 amp breaker? Just means more spaces in combiner box, or 2 boxes. But yields #6awg runs to controllers which is more reasonable. I was planning to lay #6 bare ground in the ditch alongside the conduit to tie to the single point ground of the entire system. Is this appropriate?

BB. · January 2020

If you use 2x parallel strings (or divide the 5 kWatt array on two parallel charge controllers), then 6 AWG should be fine (each 3 AWG drop in wire size, is ~2x heavier cable/cross sectional area is 2x more per 3 AWG steps). So 2x 6 AWG parallel runs is the same as a single ~3 AWG cable run.

The 15 Amp fuse/breaker, is one per 2x parallel connected panels. So, 5x panels "strings" in parallel would require 5x breakers/fuses.

-Bill

zozomike · January 2020

Thanks Bill. I'm not sure why you guys say my 24 volt battery wants 60 volts to charge. But I'm just going to plunge ahead with two arrays of net 2000 watts each. And two fm80s.
I appreciate everyone's time.

BB. · January 2020

Sorry, I missed the 24 volt battery in your signature (or thought you were building a new system). Or thought the 66 amps was "total charging current" @ 48 volt bank vs 1/2 the total charging current per FM 80 on a 24 volt bank.

-Bill

Estragon · January 2020

60v would be for a 48v nominal bank. 30ish would be the min on a 24v.

Note that 24v nominal bank could potentially charge at ~22-23v at low SOC. At 7500' in cold, you could potentially see something close to STC output on the arrays. 2500w STC / 22 = ~113a vs 80a controller. This is all just potential and the overpanelling may work out fine, but something to be aware of.

zozomike · January 2020

Got it, and like everything sometimes you gotta just leap. So thanks for helping me look first, hehe.
My clumsy number crunching tells me a 10 degree tilt at 180 and 270 degrees for 20 ( 10 per array) of these would yield just a few more ( like 800) watts per solar day, per array. vs 8 panel arrays at proper 90 degrees in a space made necessary sawtooth configuration. And the east west tent look would be a much easier racking setup in this particular case.

mike95490 · January 2020

30V minimum for charging a 24V bank. How much "overhead" your charge controller needs to be able to output that 30V, depends on the mfg's specs. Some PWM merely need 30V Some MPPT want to have at least 45V to be able to downconvert and current multiply,

So setting your array voltage becomes more of a project than slapping some panels together

Dave Angelini · January 2020

I would say 31v minimum, and really more if you have to do a super eq someday

zozomike · January 2020

Hmmm, on the array suggested of 5 strings of 2, I thought I was looking at ( per pv watts) giving up almost 500 watts in the 2500 watt array for the proposed east west orientation. But I calculated that would still yield 40 volts at 49 amps. Is that incorrect?

Estragon · January 2020

In strings of 2 assuming array output of ~2000w, that would be something like 2000 ÷ ~70vmp = 28ish amps per array into each CC.

That gets bucked to ~80a into a 24v bank.

The array vmp will vary a bit with temperature (eg from near or above string voc ~100v on a cold morning to maybe 65-70v in hot sun).

zozomike · January 2020

Thanks, I am always so grateful that you folks take the time to herd the clueless into better directions. Or at least less dangerous ones.

Dave Angelini · January 2020

Next time look at the FM100. You have 300 vdc max on the array to work with and can make a much faster installation, and 100 adc.

zozomike · January 2020

Thanks Dave. I already had one FM 80, and picked up 3 more along with the buy of the slightly used FlexPower 2. Otherwise the 100 would be a better buy. And I will go that direction if these wear out before I do.

String size advisable?

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