hard wiring panels in parallel

jdfnnljdfnnl Registered Users Posts: 5
I am currently mounting two 100W Renogy panels on the roof of my truck camper and would like to hard wire them to the MPPT charge controller without using MC4 connectors and the parallel couplers, etc. This is to prevent having connectors on the exterior vulnerable to tampering and also simply to streamline the wiring.

Essentially the positive and negative from one panel ought daisy chain into the box of the other with current from both panels traveling from this latter box. Has anyone gotten in these and replaced the wires? I am not sure exactly if the connections are soldered but they are pretty stuck. I would also like to find a way to have the additional wire penetration waterproof. Maybe I can get by replacing the stock 12ga wires with a two conductor 14/16 ga wire about the same OD into the existing bulkheads?

I think the max current of these two panels will rarely exceed 10A combined. According to this chart, I could even get by with 18 ga since the distance to the charge controller is <10ft.



Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,740 admin
    Welcome to the forum JD,

    I would suggest that you use the NEC wiring chart... It is a bit more conservative:

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

    And I would even suggest that for solar/battery charging, that you derate your loads by 0.8 (or 1.25x) your planned current (most loads in the home and office are less than rated circuit capacity and variable--Copier, space heater, blender, vacuum cleaner, washing machine, etc.):
    • 10 amps * 1.25 NEC derating for continuous current = 12.5 Amps minimum branch circuit fuse/breaker/wiring
    The other thing you need to watch with solar power is voltage drop. For example, you would want somewhere around a maximum of 1-3% voltage drop from array to charge controller.

    And for your charger to 12 volt battery bank voltage drop, around 0.05 to 0.10 volt drop maximum at rated current (want charge controller to "see" the battery voltage for quickest/most accurate charging). Short/heavy(ier) cable from controller to battery bank.

    For you loads, (battery bank to your DC loads), I would suggest a maximum of 0.5 volts drop (12 volt battery bank).

    You can use a voltage drop calcuator, such as this one, to estimate your drops (this calculator uses one way wire run, not round trip length).

    https://www.calculator.net/voltage-drop-calculator.html

    And each + cable that leaves your battery bank, should have a fuse/breaker to protect "that wire" from short circuits.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,875 ✭✭✭✭✭
    Often connections in the J-box behind the panel, are soldered. 
    The factory cables and MC4 connectors are pretty much the gold standard, and I'd suggest they don't need to be changed.
    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 ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • jdfnnljdfnnl Registered Users Posts: 5
    I was thinking of just cutting the wire at the terminal and just soldering the new wire on top of everything. The conductor of the MC4-terminated wire from the factory is silver in color and I figure a copper conductor would be superior. How else would one go about tamper-proofing an install if the leads from the panel aren't long enough to reach the enclosure?

    12 ga wire from a 100W panel seems totally overkill if the ideal amperage is less than 10A ..
  • BB.BB. Super Moderators, Administrators Posts: 30,740 admin
    Generally MC4 connectors are "flash gold" plated (as far as I know). They are water tight and there is a little tool for taking them apart:

    https://www.solar-electric.com/ditoformc4co.html

    A way to extend the solar panel cables without getting into MC4 parts, crimper and such... Just buy a male+female cable and cut it in 1/2:

    https://www.solar-electric.com/search/?q=mc4+cables

    And do your splicing on the cut ends...

    There is a big discussion between solder and other termination methods (crimped, bolted, etc.).

    Lots of people like to solder (use rosin flux, not plumbing/acid flux). Tape well, and mount so the joint does not flex (soldered joints can fail from flexing--right where the solder stops--acts like fulcrum).

    There are two reasons to use heavier cabling vs light weight cabling. The first is that solar charging a battery bank can go on for hours in a hot RV... Not like using a hair drier for 15 minutes. That is why the 1.25x or 0.80x derating from NEC wiring is suggested. A 15 amp load will trip a 15 amp breaker (could take many minutes, or even an hour). A 15 amp breaker * 0.80 NEC derating = 12 Amps continous current should never pop the breaker/fuse.

    Also, there is voltage drop... We like to suggest around 1% to 3% maximum wiring drop. 3% of 12 volts is only 0.36 volts drop. This is both to "save losses in wire heating" and to keep the wiring losses from causing 12 volt devices from undervolt conditions (12 volts - 0.36 volts = 11.64 volts at load, etc.).

    For example, a 12 volt @ 12 AWG cable carrying 10 amps (one way run) and 3% maximum loss, would be:

    11 feet will give you:

    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=12&phase=dc&noofconductor=1&distance=11&distanceunit=feet&amperes=10&x=0&y=0

    Voltage drop: 0.35
    Voltage drop percentage: 2.91%
    Voltage at the end: 11.65

    At 12 volts, you cannot send very much current will not go very far without "excess voltage drop".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,249 ✭✭✭✭✭
    jdfnnl said:
    ...mounting two 100W Renogy panels...//... like to hard wire them to the MPPT charge controller...
    You will need to have a voltage higher than most 100 watt panels for an MPPT type charge controller to work properly. We've had designers of some of them on here and it's suggested you need 30% higher voltage than the charging voltage. So likely you will want to run the 2 panels in series to have the 20+ volt VMP for your charge controller to work properly.

    jdfnnl said:
    This is to prevent having connectors on the exterior vulnerable to tampering and also simply to streamline the wiring.
    I've even travel with solar panels on top of a van. I never worried about people tampering with the wires, I worried about people stealing them, I worried about people breaking them...

    I've used NC4 connecters over the past 16 years and they are very hard to uncouple easily. I wouldn't worry about this. As I stated above you will want the panels run in series, you can use wire clips to hold the wiring up under the panels. Or just zip tie to your mount, Use UL zip ties or at least black, the color helps prevent UV from penetrating and making the zip ties brittle.

    jdfnnl said:
    I am not sure exactly if the connections are soldered but they are pretty stuck. I would also like to find a way to have the additional wire penetration waterproof. Maybe I can get by replacing the stock 12ga wires with a two conductor 14/16 ga wire about the same OD into the existing bulkheads?
    I have replaced a wire in an very old solar panel, used 100% silicon to seal it, it seemed to work, but the panel wasn't moving. As I stated, I wouldn't want to replace the existing wire, but if you do you will want to use wire designed for exposure to sunlight, that's PV wire or USE-2 wire, it's not cheap and you can likely buy the single extension cable double the distance you want clip and use the MC4 ends as cheap as this is the most common way to purchase it. Look at prices with shipping off eBay or Amazon. I have needed 50 feet of PV wire and bought it this way because it was cheaper! (So info from the experienced) If you live in Missouri, 14 years later I have rolls of the stuff.

    jdfnnl said:
    Maybe I can get by replacing the stock 12ga wires with a two conductor 14/16 ga wire about the same OD into the existing bulkheads?

    I think the max current of these two panels will rarely exceed 10A combined. According to this chart, I could even get by with 18 ga since the distance to the charge controller is <10ft.
    Typically people want to conserve what power they are generating, The different in 14 gauge (if you can find PV or USE-2) and 12 or 10 gauge will be minimal. You are likely to have a bit move vibration and stress on the wire mounting as the system passes through the wind. No reason to save pennies.

    jdfnnl said:
    The conductor of the MC4-terminated wire from the factory is silver in color and I figure a copper conductor would be superior. 
    It's very likely 'tinned copper' It might even give you a code on the wire to check it out.
    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
  • jdfnnljdfnnl Registered Users Posts: 5
    edited August 1 #7
    really I am trying to hard wire the panels directly to the mppt, no mc4s involved

    so because I have an mppt that can output to a 12v battery with variable input, is a 200W series array better than parallel?
  • BB.BB. Super Moderators, Administrators Posts: 30,740 admin
    MPPT charge controller are "buck mode digital switching power supplies" and can only take higher votlage (and lower current) power in, and "efficiently down convert" to lower voltage (and higher current) to charge the battery bank.

    For various reasons, if you take your battery charging voltage (say 15.0 volts) and multiply it by 1.3 times (minimum), then the MPPT controller can operate to its best capabilities (15 volts * 1.3 = 19.5 volts Vmp array minimum using panel spec. sheet numbers).

    If you run a "12 volt panel" (nominal Vmp~17.5 volts), by the time all the "other stuff" is taken into account (hot panels depress Vmp-array, voltage drop for wiring, voltage drop across solar controller, etc.)--If you use Vmp~17.5 volt panels, you may as well use the cheaper PWM type of solar charge controllers.

    MPPT controller need higher input voltage to do their "magic". They will work with lower input voltage, just not really do much more than a PWM controller would have done in most cases.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jdfnnljdfnnl Registered Users Posts: 5
    strange when I wire the panels in parallel, the MPPT shows a current of 0.4A
    but when wired in series the voltage jumps to 40V but is only showing 0.2A
  • BB.BB. Super Moderators, Administrators Posts: 30,740 admin
    If the controller thinks the battery is full (battery at set point voltage, and battery is accepting very little charging current), then all may be fine.

    What is the battery bus voltage under charge? Do you think the battery is charged or discharged?

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jdfnnljdfnnl Registered Users Posts: 5
    oh, you're right -- the mppt is in float status and the battery is >13V
  • PhotowhitPhotowhit Solar Expert Posts: 5,249 ✭✭✭✭✭
    jdfnnl said:
    strange when I wire the panels in parallel, the MPPT shows a current of 0.4A
    but when wired in series the voltage jumps to 40V but is only showing 0.2A
    Assuming you are doing this in an enclosed area, not in direct sun light, this makes sense.
    The basic formula is watts (power) = Amps x voltage, You don't lose any power when you go from .4 amps at 20 volts to .2 amps at 40 volts. If you happen to be in direct sunlight your panels are seriously under performing or your battery is full.
    jdfnnl said:
    really I am trying to hard wire the panels directly to the mppt, no mc4s involved
    I don't understand, but go for it. As I suggested, I would use a PV or USE-2 wire which is designed for direct exposure to the sun. other wires will become brittle. Use your + and - marking on the cables (I think I can see a cable marked on the top)

    run one positive to the negative on the other panel, After this you will need to be careful with the panels as they will be linked together, shouldn't be too hard though, just flip one on top of the other for moving to the van. Likely you will want to solder the long run wires first, so there is less hassle. Once you connect the connecting wires you can take it directly to the van to attach.

    It looks like you have removed the cap of a waterproof strain relief;

     If the junction box is constructed that way, you should be able to unsolder the wire inside and pull back and remove, The strain relief and compression fitting (sometimes just an all-in-one piece) may have come off with your cap or might be still in the barrel of the fitting. They will typically look like the end of an MC4 connector; You can look at the end of the barrel and try to pull the fitting out without the wire if you want to be sure it's this type of fitting (if it has a screw down cap it likely is)


    Found a photo of a strain relief proper;
    Controall - Catlogo Lapp


    Run your new wire through the fitting, and into the junction box and re-solder. 

    I'm not sure if the vibration would be a huge problem, but you might use water proof Stain reliefs for the pass through from the outside to the inside of the van, though they could likely be placed on the hinged side of a side door and just compress the rubber moulding.

    Looks like you have some 'Y' connectors, hang onto them they have some value and you may choose to use them in the future.

    Remember the charge controller always gets connected to the battery before you connect the solar array.


    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
Sign In or Register to comment.