Does current add up in a parallel connected Solar Array?

st4rgutst4rgut Registered Users Posts: 17 ✭✭
I have a 3S8P solar array part of which is shown in the picture. I measured the amps across each parallel connection of the array and noticed that the current was getting higher along each leg. The measurements confused me because I had read that the current outside the branches in a parallel circuit is the same, but it appears to accumulate according to my measurements. 

According to this website
 In a parallel circuit, charge divides up into separate branches such that there can be more current in one branch than there is in another. Nonetheless, when taken as a whole, the total amount of current in all the branches when added together is the same as the amount of current at locations outside the branches
My measurements seem to contradict this, with the parallel connections closer to the controller/load reading higher amperage. So if this is the case, then is it safe to use smaller wire sizes for parallel connections farther away from the load?

Comments

  • mike95490mike95490 Solar Expert Posts: 9,244 ✭✭✭✭✭
    Well, it's "terminology"  The measurement you didn't record, at the top bit of red wire, would be 4A (in your example) and
    it is the Branch.   You were measuring each Leg

    In parallel, Volts are the same, and amperage is additive
    in series, volts are additive, and amps stay the same  ( the blue interconnect wires in your sketch )
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • PhotowhitPhotowhit Solar Expert Posts: 5,543 ✭✭✭✭✭
    st4rgut said:
    I have a 3S8P solar array part of which is shown in the picture. I measured the amps across each parallel connection of the array and noticed that the current was getting higher along each leg. The measurements confused me because I had read that the current outside the branches in a parallel circuit is the same, but it appears to accumulate according to my measurements. 

    According to this website
     In a parallel circuit, charge divides up into separate branches such that there can be more current in one branch than there is in another. Nonetheless, when taken as a whole, the total amount of current in all the branches when added together is the same as the amount of current at locations outside the branches
    My measurements seem to contradict this, with the parallel connections closer to the controller/load reading higher amperage. So if this is the case, then is it safe to use smaller wire sizes for parallel connections farther away from the load?

    Your diagram appears to have panels in series and parallel. 

    In series, like panels, the voltage adds and the amperage(current) remains the same. In parallel the voltage remains the same and the current adds.

    So if you have a string(series) of 3 panels that are each 20 volts and 1 amp, the strings has a voltage of 60 volts and 1 amp.
    If you put 3 strings of these in parallel, you have an array(collection of panels) that has 60 volts and 3 amps.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • st4rgutst4rgut Registered Users Posts: 17 ✭✭
    thanks, but I'm still wondering whether its safe to use smaller gauge wire closer to the load
  • MichaelKMichaelK Registered Users Posts: 192 ✭✭✭
    Here is an ampicity chart to help select correct wire gauges.  Let's assume that each string of your panels produces 8.0A.  If there are four parallel strings of panels, then the combined amperage is 8+8+8+8=32A.  Looking at the chart, the smallest gauge that would carry 32A at a temperature less than 60C would be 8 gauge.  Keep in mind that is rating is for brand-new, uncorroded wire with no broken strands.  For overkill, I'd recommend at least 6 gauge.  For my own system, I went with 4 gauge.  Keep in mind though what is the largest gauge your controller's terminals will accept?


    System 1) 15 Renogy 300w + 4 250W Astronergy panels,  Midnight 200 CC, 8 Trojan L16 bat., Schneider XW6848 NA inverter, AC-Delco 6000w gen.
    System 2) 8 YingLi 250W panels, Midnight 200CC, three 8V Rolls batteries, Schneider Conext 4024 inverter (workshop)
  • PhotowhitPhotowhit Solar Expert Posts: 5,543 ✭✭✭✭✭
    st4rgut said:
    thanks, but I'm still wondering whether its safe to use smaller gauge wire closer to the load
    For safe wire carrying here's a NEC wire ampacity chart;
    What is wire ampacity and how do you find ampacity rating

    For the voltage drop, here's a link to a calculator, 2-3% loss is considered acceptable;

    Voltage Drop Calculator

    I don't know the panels or charge controller, but you should figure out if the voltage and current are within the range of the charge controller. Understand you should use VOC for max voltage on an MPPT and 130% of the max charging voltage for acceptable input current for a MPPT. 


    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • st4rgutst4rgut Registered Users Posts: 17 ✭✭
    I feel that I haven't been clear enough about my intended application. Let's say I had to ad a separate circuit with just one string of panels connected to the same solar controller as the 3*4 array due to space constraints. Could I use heavier gauge for the 3S4P array vs the 3S1P array? Or would this system still be considered as 3S5P system, and the same gauge matching cumulative current (5 amps) should be used throughout?

  • BB.BB. Super Moderators, Administrators Posts: 31,425 admin
    Yes--You are correct "under normal conditions" you could "scale" the wiring for the actual current flow...

    HOWEVER

    Say there was a short circuit in the "1A" string... The other strings would fee >4 amps into the "1A" string you designed--And start a fire in the shorted "1A" string.

    This is back to the old parallel (or series/parallel) connected solar array. With only two (or one) string of panels, a short circuit anywhere in the system will only pull 1x current.

    If you have (for example) 3 parallel sets of panels (strings), then the two panels could feed 2x current into the 1 shorted panel. That is why we use "combiner boxes" that have a fuse/breaker per string...

    https://www.solar-electric.com/search/?q=combiner+box

    For example, say each panel can output 9 amps... And the panels have a requirement for a 15 amp series protection fuse. Then if there is a short circuit, the 2x 9 amp panels in parallel for 18 amps total will trip the 15 amp series protection breaker and help reduce the chance of a wiring/panel fire.

    This is especially critical for larger arrays with >>3 parallel strings of panels.

    There are lots of ways to start fires with off grid power systems... Loose connections, not using circuit breakers, something metal falls on a battery bank, a genset fuel line comes loose and sprays gasoline on the genset, "boiling" batteries dry/over charging/etc. ....

    You really need to do everything "correctly" to avoid the more common sources of fire.

    Looking at your system and doing a "fault analysis" (what could happen, and making sure that nothing really bad can occur) and failure analysis (see what did happen and make sure it does not happen again)--It is why the NEC and various other codes are so big. They are there to protect everyone by having standards of how things are done. Both to prevent failures (like fire), and to ensure that the next folks that work on/add to the existing system know how the first system was installed (per code) and their repairs/additions will also be safe.

    Anytime you deviate from "Code"--You better have a very good reason for doing that... Or just don't do it in the first place--There is probably a reason that code required that in the first place.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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