Wiring Up PV Panels

SCharles
SCharles Solar Expert Posts: 123 ✭✭
in General Solar Power Topics #1
I have 12 old Arco 2000 35 w. panels which are currently wired up as a 12 v. array. I have other, later-model panels wired up in other strings. So these old panels are separate from the newer ones.

I have another MPPT controller which I want to use with these old Arcos. So, my plan is to rewire the panels into 24 v. pairs, then the six pairs into one string. I realize I need to wire each pair in series to increase the voltage, then each of the pairs wired in parallel to the other pairs.

So far, so good. My question regards running the wires from the array once it is are rewired into the 24 v. version. Can I attach the positive and negative cables from the array to the house by simply attaching each wire to the appropriate terminal post on the last [and closest to the house] panel? Or should I, as in series battery cells, run one cable to the farthest panel? Does it make any difference?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Wiring Up PV Panels

    The parallel pairs should connect at a "common point" - as in combiner box. There each parallel pair should be fused on the (+) side before hooking to the common downlead. If you "ladder" the connections as you describe it will be difficult to properly fuse them and the wire "along the way" would need to increase in size to make up for rising current. Messy and fraught with potential problems.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Wiring Up PV Panels
    SCharles wrote: »
    So far, so good. My question regards running the wires from the array once it is are rewired into the 24 v. version. Can I attach the positive and negative cables from the array to the house by simply attaching each wire to the appropriate terminal post on the last [and closest to the house] panel? Or should I, as in series battery cells, run one cable to the farthest panel? Does it make any difference?
    Back to basics--Batteries, their output voltage is very stable and so small voltage changes cause large changes in current... Run one battery at 12.6 volts and another at 12.2 because of voltage drop/wiring--the 12.2 volt battery will output the most current--and probably take the most current when charging.

    Solar panels are "current sources"... When the panel is at Vmp~17.5 volts +/- 10% -- The change in panel voltage makes almost no difference in current flow (current flow, when voltage is in the Vmp or lower range, is mostly proportional to amount of sunlight hitting the panels).

    So--as long as you do not have too much voltage drop or differential voltage drop--the current of paralleled panels/strings will "share" based on the amount of sunlight hitting each panel.

    Since we generally try for a maximum of 3% voltage drop maximum on the whole wiring run--there is almost no effect on current flow do to individual wiring run differences in your solar array.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #4
    Re: Wiring Up PV Panels
    BB. wrote: »

    Since we generally try for a maximum of 3% voltage drop maximum on the whole wiring run--there is almost no effect on current flow do to individual wiring run differences in your solar array.

    -Bill

    I started with a #12 AWG solid core copper wire connecting panel to controller, but changed over to #10 AWG stranded wire. (run was 8' one way) The solid wire costs $3 for 25' coil, where the stranded was 50 cents/ft. The difference was an extra volt with the stranded wire. Not sure about Current, it was overcast and I couldn't get a sustained reading for comparison.
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  • SCharles
    SCharles Solar Expert Posts: 123 ✭✭
    #5
    Re: Wiring Up PV Panels
    BB. wrote: »
    Since we generally try for a maximum of 3% voltage drop maximum on the whole wiring run--there is almost no effect on current flow do to individual wiring run differences in your solar array.

    -Bill

    Thanks for the responses. Voltage drop is little issue for me, as all my panels are wired to one another with either 8 ga. or 6 ga. cabling. Or, at least I don't think there is an issue.

    I have three other strings of panels, wired 36 volts and using a combiner box. I was hoping to avoid buying another combiner box for these old Arcos 35-watters [do it a bit on the cheaper I suppose]. I don't want to add them into the other combiner box as the other panels are all BP and Kyocera in the 135 watt range. My thought was to run the Arcos at 24 volt to a separate MPPT controller, which I have sitting here, through the conduit and cabling already in place for the Arcos as they've been used at 12 v. [Actually, I could use some other voltage instead....] They have been run through an old Charge-A-Stat controller from the dark ages of the mid-80s or around there. It works great, though it must be used manually once the batteries get up to near full charge, as the old controller does not, of course, communicate with the MX60s. And is not MPPT. Very crude by today's standards. I removed it from the system a few months ago and the Arcos are just sitting up there doing nothing constructive. It has been suggested that I junk them and buy new, up-to-date panels. However, that is money out the door and I can exist ok without them but it is a matter of them working just fine and also a matter of an unused 420 watts of panels. Shame not to put them to use.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Wiring Up PV Panels

    Ideally, there should be a series fuse/breaker in series with every parallel string (three or more parallel connections). This is to reduce the chance of fire if one of the panels gets shorted/fails and gets fed power from the other strings.

    If there is no listing for a series fuse--use around 1.56 to 2x the Isc rated current.

    If the old controller works good--go ahead and use it... There is no real reason for controllers to communicate with each other. Each controller will make its own decision when the batteries are "full".

    As long as the old controller charges and does not over charge the battery bank (many older controllers do not a a "float" stage)--It should be fine.

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