Best panel configuration for Midnite Classic 150

H2SO4_guy

I have purchased a Midnite Classic 150 from the NAWS folks. (And I always tell them it is because of the forum and I vote with my dollars) It is to run the 12 volt side for now. 6 panels are about the right amperage to charge 3 Crown CR-185 batteries. (185 AH each @ 20 hr. rate purchased at 16 months old for $40 each) The batteries were in a 36 volt floor scrubber so they should be pretty close together.

Panels: Evergreen 210 watt
VOC 22.8
VMP 18.3
ISC 12.11
IMP 11.48

Putting in the different configurations into the string sizing tool yield the same amperage going to the batteries. The wiring is 55' one way using #10 THHN wire. Each panel or string will go through a Midnite 20 amp breaker (Also from NAWS). The choices would be (1) wiring 6 panels in parallel , (2) 3 strings of 2 panels, or (3) 2 strings of 3 panels. I think option 3 would be the least desirable as there is more to down convert and would produce less amperage to the batteries.

I am setting up an off-grid loft (In a barn hehe, NO Building Codes, NO Zonning, NO Code Nazi's) and trying to slum along on junk batteries so there is going to be a 12 volt system and at least 2 48 volt sets of batteries on different panels / charge controllers. This system will hopefully run a refrigerator and a chest freezer and that's about it as they will need about 2 KWH per day in Missouri where we average about 4 hours of usable sun per day. If it is not enough, I can plug in the critical loads to one of the 48 volt systems or run the generator.

Thanks in advance for everyone's help and willingness to empower the off-grid community.

Skip

Photowhit

Re: Best panel configuration for Midnite Classic 150

H2SO4_guy wrote: »

...Putting in the different configurations into the string sizing tool yield the same amperage going to the batteries....I think option 3 would be the least desirable as there is more to down convert and would produce less amperage to the batteries.

Those 2 statements make no sense. I guess there would be slightly more energy used by the charge controller, but I'd trust their sizing calculator.

Hi again Skip,

I would think option 3 would be the best, no need or reason for fusing, higher voltage over the same wire diameter should give you slightly more current to the CC over the 55' (110' there and back) with lower wire losses. The VOC at @68 volts is well within the 150V allowed for the Classic...

niel

Re: Best panel configuration for Midnite Classic 150

with 2 strings of 3 you get 68.4v voc and 54.9v vmp at 11.48a each string and 22.96a both strings combined. running this in my v drop calculator there's 3.22v dropped for 5.86%. obviously you don't want losses this bad in the wires and to keep that wire at that length you need to place all of the pvs in series. this will give the following,
136.8v voc and 109.8v vmp at 11.48a. i get 1.61v dropped at 1.46%. no combiner or fuse needed either. losses are not great, but not too bad either in the cc and would be terrible in the wires if you do 2 strings of 3 in series without changing the wire gauge or the length of wire used. to equal the 1.46% in a 2 string deal you would need to run #4 that same length.

you don't need nec rules to know you don't want to lose very much of that expensive solar power to something like wire resistance. even if you expanded in the future with another string of 6 pvs the loss % would be 2.92% and that is far better than the 5.86% on your present 2 string proposed system. of course that expansion would exceed the classic's abilities at 12v and in that kind of an expansion one should then consider a 24v or 48v battery bank and it will still work out great.

vtmaps

Re: Best panel configuration for Midnite Classic 150

H2SO4_guy wrote: »

The wiring is 55' one way using #10 THHN wire. Each panel or string will go through a Midnite 20 amp breaker (Also from NAWS). The choices would be (1) wiring 6 panels in parallel , (2) 3 strings of 2 panels, or (3) 2 strings of 3 panels. I think option 3 would be the least desirable as there is more to down convert and would produce less amperage to the batteries.

I ran the numbers for both 3 strings of 2 panels, and 2 strings of 3 panels.

You are quite right that 3 strings of 2 panels means the controller will run cooler and more efficiently. BUT... the #10 wire will have huge losses (at full power, 10.54% volt drop and 132.8 watts lost in the cable).

If you configure 2 strings of 3 panels, at full power the voltage drop is 4.68% and there will be 59.0 watts lost in the cable.

Neither is a good solution. You need heavier cable if you want to push 1260 watts through 55 feet of cable.

Also, 1260 watts is very near the Classic's limit with a 12 volt battery. The combination of pushing the classic to its limit on the output and starting with a very high input voltage means the Classic will be running hot!

--vtMaps

Edit: Niel, I wonder why my numbers are a bit different than yours???
the calculator at http://www.calculator.net/voltage-drop-calculator.html agrees with my spreadsheet numbers (actually they are very slightly different because in my spreadsheet I use the AWG maximum resistance values. The calculator.net uses AWG nominal resistance values.)

mtdoc

Re: Best panel configuration for Midnite Classic 150

Not sure what calculator you guys are using.

Using the Southwire calculator and using 10 awg copper, I get 2.42% drop using VMP of 54.9 and 11.48 amps at 55 feet.

BB.

Re: Best panel configuration for Midnite Classic 150

There are different assumptions for wire temperature (hot copper has higher resistance vs cold copper). Also, SAE gauge is slightly smaller diameter vs NEC/Electrical gauge.

-Bill

vtmaps

Re: Best panel configuration for Midnite Classic 150

mtdoc wrote: »

Using the Southwire calculator and using 10 awg copper, I get 2.42% drop using VMP of 54.9 and 11.48 amps at 55 feet.

Niel and I were calculating for 2 strings of 3 panels. Vmp of 54.9 and 22.96 amps. --vtMaps

mtdoc

Re: Best panel configuration for Midnite Classic 150

vtmaps wrote: »

Niel and I were calculating for 2 strings of 3 panels. Vmp of 54.9 and 22.96 amps. --vtMaps

Ok - using those numbers I get 4.83%

H2SO4_guy

Re: Best panel configuration for Midnite Classic 150

There are 20 of the Evergreen 210 watt panels. 6 are going to be on the Midnight Classic 150 as that is about all it will handle for the 12 volt side. 2 are going to go on a Morningstar TS-60 (Purchased from NAWS) for a secondary 12 volt system, and 12 Evergreens are charging the 48 volt batteries through a Trace C-40. (Circa 1998 and still working) #10 THHN was the best I could afford for wire, I do have more panels than batteries for now. 1 set of 48 volt batteries are 4 of the 8d batteries at approx 225 AH @ 20 hour rate (Purchased at 18 months old for $40 each) and a 2000 set of Telco Lucent 125 AH at 10 hr rate. There is an Exeltech XP-1100 on each 48 volt battery system and an Exeltech XP-1100 on the 12 volt side. Kind of an odd setup, but trying to scrape by on junk batteries. Plan on moving in by first of year.

Thanks for everyone's help!

Skip

niel

Re: Best panel configuration for Midnite Classic 150

vtmaps wrote: »

Edit: Niel, I wonder why my numbers are a bit different than yours???
the calculator at http://www.calculator.net/voltage-drop-calculator.html agrees with my spreadsheet numbers (actually they are very slightly different because in my spreadsheet I use the AWG maximum resistance values. The calculator.net uses AWG nominal resistance values.)

i temperature compensate for the wire to be at 90 degrees c. a hot wire has more resistance and i put it to the nec standard of 90 c in my calculator. (it's programmable for different temps)

mtdoc

Re: Best panel configuration for Midnite Classic 150

I know this is getting off topic but for my own education:

I realize resistance goes up with temperature - so how does one decide on the right temperature for calculating voltage drop? Looking at various resistance vs. AWG tables it's obvious that there are large variations depending on the temperature assumptions.

If one is running a wire run from PV underground from a ground mount array the ambient temperature is obviously going to be lower (and more consistent) that running from a roof mount through a hot attic.

Current running through wire will obviously heat the wire - but how important is that relative to ambient temperature?

90 degrees C (194 F) seems awfully hot and using this number will give a much different result than the 60 degrees C that Southwire uses (and implies is NEC standard for circuits less than 100 amps). If my calcs are right a 30 degree difference in temperature gives a 12% difference in resistance for copper conductors!

I'm currently deciding on copper for my system expansion and assuming 90 degrees will significantly increase my wire expense. I'm running copper underground in conduit in a very cool climate where the air temp is never above the mid 80s (farenheit) and soil temp stays in the low 50s or below.

BB.

Re: Best panel configuration for Midnite Classic 150

There is calculating maximum wire temperature based on current, insulation, conduit, conduit fill, air/buried, etc... That is to, in worst case design, to make sure your insulation/wiring does not fail.

The other is voltage drop and wire diameter. You can pick any temperature that makes sense for your installation. Wire in conduit on a roof/or under a roof--Is going to get pretty hot and increase voltage drop.

In most cases, we size the wiring 2+ gauges higher than "NEC" would require. So the self heating from wire/current/I²R losses are not going to be enough to worry about (heating wire in middle of day). Ambient is (probably) going to be the worst case.

-Bill

mtdoc

Re: Best panel configuration for Midnite Classic 150

BB. wrote: »

In most cases, we size the wiring 2+ gauges higher than "NEC" would require. So the self heating from wire/current/I²R losses are not going to be enough to worry about (heating wire in middle of day). Ambient is (probably) going to be the worst case.

-Bill

Thanks you! - that's helpful.

Yes, I realize that max ampacity ratings and NEC requirements are one thing - but it's the performance issue - potential for loss of power that drives voltage drop concerns for PV systems and keeping to a less than 2% drop should always result in wire that is well above NEC requirements for ampacity.

Good to know that one can rely on max ambient temperature for calculations and that self heating is relatively less important.

niel

Re: Best panel configuration for Midnite Classic 150

one does have to watch that the temp is often higher than one realizes. most basic calculators are being based at 25 degrees c or 77 degrees f. a wire on a roof at 110 degrees f ambient passing a huge current could easily be upwards of 150 degrees f and if confined in conduit it could go much higher. i did take it to an extreme at 90 c, but that is the same extreme the nec came up with so it's a worst case scenario. on the bright side, the winter v drops should look much better as long as it isn't going through a chimney or near a heated duct or plumbing.

ywhic

Re: Best panel configuration for Midnite Classic 150

In my research for decent wire I found 105C rated 'boat wire' is pretty good for hot areas.. << food for thought..<< usally high starnd count and tin annealled coated (TC) very flexible compared to similar wire..

I'll be using 90C THHN/THWN 4 AWG for my Midnite Controller.. I got lucky and the stuff from my local HD actually went right into the Midnite controller 4AWG terminal without issue or fight.. I will be only pumping max of 72-75 amps about 3 feet to my copper battery bank bussbar.. :cool:

niel

Re: Best panel configuration for Midnite Classic 150

al,
we aren't talking of a wire's ability to withstand high temps even though that is a good thing to have. we are talking of the higher resistance that same wire will exhibit under those higher temps. this will happen to any wire with high temperatures so a temp rating on wire jackets does not mean it won't exhibit a higher resistance when hot.