Solar Array Wiring

Hi, I have 6- 230 watt sun ele. panels,each rated at 30.3 voltage at peak power, 7.6 amps at peak power. I have a 12 volt off grid system. I plan on using an Outback mppt charge controller. My distance from batteries to combiner box is app. 85' one way. What is my best options for wiring the arrays and wire size from batteries to combiner box. Is the Outback charge controller a good choice? Thanks for the help. First time on this site.
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Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring

    Welcome to the forum.

    We also need to know the panels' open circuit Voltage, as it figures into keeping the input Voltage to the charge controller below the max.

    As a general rule, the best plan is to run a high array Voltage to the controller. This reduces the Voltage drop over the lines and keeps the current low, meaning you can use smaller wire over the longest distance with the least amount of power drop. 85 feet is quite a long run. If you experience cold temps in your area, the MidNite Classic controller line might offer a better choice as they have higher input V max so more panels could be in series (I'm thinking the Voc is around 38, in which case three in series in cold weather could exceed the Outback's 150 Volt limit).
  • jthalejthale Posts: 6Registered Users
    Re: Solar Array Wiring

    The Voc is 37. and the short circuit current is 8.24, what would be the wire size you think I might need? Are you thinking 2 parallel runs of 3 in series? Thanks, Jim
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring
    jthale wrote: »
    The Voc is 37. and the short circuit current is 8.24, what would be the wire size you think I might need? Are you thinking 2 parallel runs of 3 in series? Thanks, Jim

    Yes: 37 * 3 = 111 Voc for a string. At a maximum 1.3 cold weather factor that would be 144 Volts, so unless you live in a really cold climate (like here :p ) it should work with the Outback FM80. One caveat being that the panels totaling 1380 Watts is a bit large for a 12 Volt system; it has the potential to produce more power than the FM80's 80 Amps peak output current. OB recommends an array of 1250 Watts for a 12 Volt system. Basically, the extra power potential will just be "clipped".

    You can use the Voltage drop calculator here http://forum.solar-electric.com/showthread.php?29-voltage-drop-calculator to get an exact number for your system. I believe you are looking at 10 AWG for 85 feet from the array to the controller using two parallel strings of three panels (array Voc 111, Vmp 90.9, Imp 15.2, Isc 16.48).
  • jthalejthale Posts: 6Registered Users
    Re: Solar Array Wiring

    That would be both parallel circuits going into the #10 to go back to the controller? Are you recommending the Fm80 because of the 1380 watts , I could go with just 4 panels? Thanks, Jim
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring
    jthale wrote: »
    That would be both parallel circuits going into the #10 to go back to the controller?

    Yes; the two strings formed of three panels each would be parallel connected at the array (no fuses needed here) and feed #10 to the controller.
    Are you recommending the Fm80 because of the 1380 watts , I could go with just 4 panels? Thanks, Jim

    If you need the full array, you need the 80 Amp controller capacity. Otherwise you'd be wasting even more power (a 60 Amp controller would be clipping up to 216 Watts, so to speak: 88 Amp panel potential - 60 Amps controller maximum = 18 Amps @ 12 Volts; 216 Watts).

    So the question now is: what size battery bank (in Amp hours) are you recharging? As a rule-of-thumb, the 80 Amps would work nicely for 800 Amp hours (up to 1600) which is pretty big. At 50% DOD you have a DC potential of 4.8 kW hours.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Solar Array Wiring

    the higher voltage helps keep the v drops into a good area, but your problem is a different one being you have a 12v battery bank. the problem is that the controller is current limited at 80a and at 12v this translates to 12v x 80a = 960w capability. i can see a bit on going near the max, but in the case of 6 pvs there would be much power not realized. if you went with a 24v battery bank minimally the controller could then easily handle the power of 6 of those pvs for 1380w stc. 24v x 80a = 1920w capable. if you stay at 12v for the batteries then 4 of those pvs would work as roughly 4 x 230w = 920w at their stc power rating. a fifth pv may even work considering the 77% efficiency factor, but going higher than 5 pvs there most likely will be times power will be wasted in that it would be held back from the batteries due to current limitations in the controller.
  • jthalejthale Posts: 6Registered Users
    Re: Solar Array Wiring

    I could reconfigure my bats. for 24V. since I only have 4, I will be adding more with the pv array. I would have to go with a different inverter though. How would the 5th pv be wired into the array if I only go with 4 pv's, 2 each in series, like you are suggesting? Also, am I correct in assuming from what is being said that the controller will hold back amps to protect itself from too high of output amps? Thanks for the help, Jim
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Solar Array Wiring

    for a 24v battery setting the pvs will be required to be at least 35v (or preferably better) vmp output or they will need to be placed into series to make it work. for you this would mean multiples of 2 pvs be bought as you need more than the 30.6v available from 1 pv. there is of course the option of series for all pvs, but you run into the max voltage input ceiling on the controller (based upon open circuit voltage from the pv string or string voc). with your initial intent of 6 pvs, this works out well in a couple of configurations. 1> 2 pvs in series paralleled to 2 other strings of 2 in series or 2> 3 pvs in series paralleled to 1 other string of 3 pvs in series.

    note that the v drops will be less with 3 pvs in series and could possibly allow a smaller wire size. this would need to be calculated to be sure it is good for each of your proposed setups. just to be sure what is the open circuit voltage of the pvs you are addressing?
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring

    Okay, differences between input and output and how the two relate.

    The thing about using a 12 Volt system is the issue with "clipping" power if the full array is used. This problem goes away if you increase to 24 Volts. Basically the controller can handle its maximum current out at either Voltage. which roughly translates to: 12 Volts * 80 Amps = 960 Watts maximum vs. 24 Volts * 80 Amps = 1920 Watts maximum.

    Niel was referring to the possibility of using either only four of the panels (920 Watts) or at most five all in series (1150 Watts). (my goof so disregard. niel) To do this you would need a MidNite Classic 200 charge controller because the panel string would have a Voc of 185 Volts - way over the limit for the Outback unit. The Wattage "discrepancy" between calculated and "real world" is a matter the controller's ability to handle slightly more than its rating coupled with the inevitable system losses (the 77% factor).

    This is why off-grid systems are based on loads determining battery bank size which governs what system Voltage is best and the necessary array & controller to recharge with.
  • jthalejthale Posts: 6Registered Users
    Re: Solar Array Wiring

    Hi, I don't understand why you say I don't need fusing on the pv's in series wiring. Don't I want to fuse that wire size? I think I am going with 2 panels in series, paralled back to the controller. I installed #2 sec yesterday from pv location to controller location. Can you also tell me what fusing I need and where does it need to be? Disconnects? This is a simple system that I am putting together because I enjoy working at things that will save me money and I do it myself. I also have access to electrical supplies, new and used(electrician). I said it was an off grid system, it is but I started with 2 windmills and 4 golf cart bats new, 12 v. and I manually switch from grid power to bat power when I have full bats to power some 120v lighting in my home. I have some questions on that at a later time. Thanks, Jim
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Solar Array Wiring

    when dealing with 1 or 2 pv strings fusing isn't required. fusing is required when 3 or more strings are employed.

    i forgot whether it was on the imp or isc, but i think it's the imp to be multiplied by 1.5 and round up to the nearest commercial value for the fuse or circuit breaker value of individual strings. couldn't hurt using them.
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: Solar Array Wiring
    jthale wrote: »
    Hi, I don't understand why you say I don't need fusing on the pv's in series wiring. Don't I want to fuse that wire size?

    In my opinion you should fuse the wire from the combiner to the charge controller. When you have more than two strings of panels in parallel, each string must be individually fused.

    Suppose you have three strings and one is defective (short circuit). The other two strings can put enough current through the defective string to cause a fire. If you have only two strings, the good string cannot put enough current through the defective string to cause a fire.

    If you use DC circuit breakers (rather than fuses) the polarity of the breakers may not be obvious. If the breaker was to be used as a way to disconnect the string, it would have its line (or plus) side connected to the plus side of the panel string. But that is NOT the purpose of the breaker. The breaker in each series string of panels is designed to break reverse current flow through a defective string, therefore its polarity is opposite what a disconnect breaker would be.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • ggunnggunn Posts: 1,973Solar Expert ✭✭✭
    Re: Solar Array Wiring
    niel wrote: »
    when dealing with 1 or 2 pv strings fusing isn't required. fusing is required when 3 or more strings are employed.

    i forgot whether it was on the imp or isc, but i think it's the imp to be multiplied by 1.5 and round up to the nearest commercial value for the fuse or circuit breaker value of individual strings. couldn't hurt using them.
    It's Isc X 1.25 X 1.25 and round up to the next fuse/breaker size.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Solar Array Wiring

    thanks as i forgot which it was. for the record x 1.25 x 1.25 is the same as x 1.5.
  • ggunnggunn Posts: 1,973Solar Expert ✭✭✭
    Re: Solar Array Wiring
    niel wrote: »
    thanks as i forgot which it was. for the record x 1.25 x 1.25 is the same as x 1.5.
    For the record, it's 1.5625. ;^)
  • PhotowhitPhotowhit Posts: 4,670Solar Expert ✭✭✭✭
    Re: Solar Array Wiring

    We also take liberties with '2 strings parallel don't need fusing' it's actually 2 string of equal size don't need fusing, this is likely come up as people add new panels to old systems.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • calbikercalbiker Posts: 50Banned ✭✭✭✭
    Re: Solar Array Wiring

    I think you guys are missing the point when saying one or two strings don't "need" fusing. Fact is, it's IMPOSSIBLE to fuse one or two strings. The maximum current from a string is Isc. Isc is only 10% greater than normal operating current (Imp). You will not be able to find a fuse that conducts Imp yet opens at Isc. To make matters worse, Isc is a function of irratiation. It can vary by more than 2:1. And that's why it's impossible to fuse one or two strings. With 3 or more strings fusing is possible.
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: Solar Array Wiring
    calbiker wrote: »
    I think you guys are missing the point when saying one or two strings don't "need" fusing. Fact is, it's IMPOSSIBLE to fuse one or two strings. The maximum current from a string is Isc. Isc is only 10% greater than normal operating current (Imp). You will not be able to find a fuse that conducts Imp yet opens at Isc. To make matters worse, Isc is a function of irratiation. It can vary by more than 2:1. And that's why it's impossible to fuse one or two strings. With 3 or more strings fusing is possible.

    Welcome to the forum. It is possible to put circuit breakers in each of two strings. I have done so. It allows me to disconnect a single string for diagnostic purposes. I don't think they will ever trip because of overcurrent. If there are two strings and one is rated at higher current than the other it is advisable to fuse at least the lower current string. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Posts: 27,788Super Moderators admin
    Re: Solar Array Wiring

    Many people use breakers as On/Off switches for arrays too...

    I agree that it is not possible to "accuratly" fuse/breaker a solar array as they are current sources and Isc is only a bit larger than Imp.

    When we fuse/breaker an array, it is when there are three or more parallel strings... A single shorted string needs to be protected from over current by the 2 or more parallel connected strings (for most solar panels). With two parallel strings, it is not possible for one string to over current a shorted string.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring
    BB. wrote: »
    With two parallel strings, it is not possible for one string to over current a shorted string.

    Add to that the previously mentioned exception of when there is a significant disparity in the current potential of the two strings.

    Now, is that clear enough or shall we go over it again? Yes, I know; we will be going over it again, in another thread coming soon to this forum! :p (That's a joke, folks.)
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: Solar Array Wiring
    Now, is that clear enough or shall we go over it again? Yes, I know; we will be going over it again, in another thread coming soon to this forum! :p (That's a joke, folks.)
    Its actually not a joke. We will go over it again and again. That's the nature of an open forum that welcomes beginners (such as myself). Very different from forums that allow posting by professionals only (such as MikeHolt.com, for example).

    Even if the subject of this thread was covered in an FAQ or sticky, we would go over it again, but at least we could refer to the FAQ or sticky. By the way, I am not complaining... I go by the mantra "see it, do it, teach it". I have learned a lot here (thanks to repetition) and I'm still learning and I'm happy to do some teaching while here. Teachers (and moderators) do burn out.... I think it happens when the repetition begins to outweigh the learning.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring

    The only problem with repetition is that it gets boring, so we get lazy, and the next thing you know we start saying things ever-so-slightly wrong as calbiker correctly pointed out. And so ...

    You don't need to put circuit protection on two parallel strings of panels because there is no point: one string can never produce more than its Isc so it can't put too much current to the second string.

    However, it is possible for the two strings together to supply their combined Isc rating to a shorted charge controller input which could cause a fire. Thus you may well want to put some form of circuit protection on the combined output.

    Et cetera, ad infinitum, ad nauseum, until all possible scenarios are dealt with. :roll:

    It's a bit of a problem when you have to deal in posts that cover things generally and at the same time know there are a million little details than can skew the advice for any particular installation.

    Okay, maybe not a million but there's a lot. ;)
  • calbikercalbiker Posts: 50Banned ✭✭✭✭
    Re: Solar Array Wiring
    Add to that the previously mentioned exception of when there is a significant disparity in the current potential of the two strings.

    Now, is that clear enough or shall we go over it again? Yes, I know; we will be going over it again, in another thread coming soon to this forum! :p (That's a joke, folks.)

    If you clean up your wording you won't have to go through this again and again. Not only don't you "have to" fuse two like strings, but it would be stupid to do so. The fuse will never blow.

    It seems your present "official" position on fusing two like strings is: you can if you want to. That's flat out wrong!
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring
    calbiker wrote: »
    If you clean up your wording you won't have to go through this again and again. Not only don't you "have to" fuse two like strings, but it would be stupid to do so. The fuse will never blow.

    It seems your present "official" position on fusing two like strings is: you can if you want to. That's flat out wrong!

    No, it isn't.
    You can if you want to; it just doesn't afford any safety advantage unless the strings are unevenly matched.

    Around here the term "fusing" is often used as a short-hand for "circuit protection" and may include the installation of circuit breakers rather than fuses specifically. Sometimes people use them to be able to shut off an array or string. Sometimes they install them in advance with an eye to future expansion (in which case putting it in to begin with at the combiner point makes perfect sense).

    Now let's not make me go through the explanation again in this thread just to resolve some further nit-picking over wording or terminology. If someone has an additional question about it regarding something they don't understand that will be answered gladly.
  • calbikercalbiker Posts: 50Banned ✭✭✭✭
    Re: Solar Array Wiring
    However, it is possible for the two strings together to supply their combined Isc rating to a shorted charge controller input which could cause a fire. Thus you may well want to put some form of circuit protection on the combined output.

    That logic doesn't work. It's impossible to size one fuse that's located between the controller and x-number of parallel strings that can conduct x*Imp but blows at x*Isc. The two currents are only 10% apart.

    I suppose it's possible for fire to start within the controller, but a fuse will not prevent it from occurring.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring

    Sure, fine, whatever.

    Here's another POV:

    You don't need to use any form of circuit protection anywhere in any system because once the fireworks start it's all over anyway.

    :p
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: Solar Array Wiring
    calbiker wrote: »
    I suppose it's possible for fire to start within the controller, but a fuse will not prevent it from occurring.
    that depends on the type of fuse... a DC ground fault protection or arc fault protection circuit breaker may prevent the fire. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring
    vtmaps wrote: »
    that depends on the type of fuse... a DC ground fault protection or arc fault protection circuit breaker may prevent the fire. --vtMaps

    As per NEC regs.

    Any further argument regarding this matter should be directed to Mr. John Wiles. :roll:
  • calbikercalbiker Posts: 50Banned ✭✭✭✭
    Re: Solar Array Wiring

    I would like to see how you would mechanize dc ground fault protection to a PV array.

    As for using an arc fault circuit breaker, you do realize we're dealing with a current source? You think an arc is generated when current increases by 10%?

    I think the threat of fire is blown way out of proportion. How many panels have been reported to catch fire? What will burn? Usually circuit failures that start as a short will end up as an open.

    No matter how many fuses are inserted in the array, there's no protection for the controller (from array current). If the controller shorts, it's on its own. Add a smoke detector by the controller if you're paranoid.
    vtmaps wrote: »
    that depends on the type of fuse... a DC ground fault protection or arc fault protection circuit breaker may prevent the fire. --vtMaps
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Solar Array Wiring

    calbiker, I suggest you read up on the NEC regulations regarding this subject. You won't agree with all of them, as many of us don't. Particularly the new regs on DC ground fault. But they are the rules which must be followed.

    Arc fault problems are a known failing of PV's. When you have current at greater than zero resistance you also have Voltage. If that Voltage is over 15 VDC there is potential for a sustainable arc, and resultant fire; the current need not be at Isc to create this condition.

    How many panels catch fire? Homemade ones are known to. Commercial units not so much so. The worst problems are not with the panels themselves but the wiring.

    For what it's worth, I quite agree with you regarding the NEC standard over-sizing of circuit protection being used for PV arrays. You can not expect something that is capable of a maximum of 'X' Amps to blow a fuse rated at 1.25X (+). But then I also disagree with their DC ground fault system and the derating of charge controllers current capacity because they qualify as "constant use devices".

    By the same token, I've known people who think it's okay to use under-sized wire in place of fusing in circuits "because it's the same thing". It isn't.
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