two strings same voltage different current

FreeWatts
FreeWatts Registered Users Posts: 14 ✭
in Solar Beginners Corner #1
Can i parallel two series strings with same voltage but one has 30% more current
ie. more panels on one string.

thanks in advance
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #2
    Re: two strings same voltage different current

    Yes, you can parallel "same voltage" (ideally, Vmp within 10%), with higher or lower Imp strings without any day to day functional problems.

    HOWEVER--There is one safety concern. Normally, when you have one string, or two parallel strings, you do not need series protection fuses per string. But that is based on the assumption that the two strings have identical panels. (with three or more parallel strings, you need to have a series protection fuse/breaker per string--A "combiner box" is typically used).

    If you, for example have a 24 volt 240 watt 10 amp panel and a 24 volt 48 watt 2 amp panel--you would need a series protection fuse on the 50 watt panel/string to prevent a short from being fed by the much larger 240 watt panel.

    Check the data sheets, but the series protection fuses are about 2x Imp/Isc of the panel's rated current.

    In your case, a 30% difference in Imp between string A and string B (assuming two paralleled strings)--you do not need the series protection fuse/breaker (they may even be rated the same size due rounding of standard fuse sizes).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • FreeWatts
    FreeWatts Registered Users Posts: 14 ✭
    #3
    Re: two strings same voltage different current

    Thanks BB

    here is my dilema. i have two roofs, house and garage 150' wire length. both have same properties such as tilt and athzmuth. i need to fit 20 panels total. house can fit 12 and gararge 8 and inverter must be in garage. ideally 2 strings of 10 but with 8 on one and 12 on the other what should i do?

    1. two strings of ten one string having 8 modules on the garage and two 150' away.
    2. wire the 12 on house with 4 in perelell so it has the same voltage as the 8 on the garage and have 2 strings.
    3. only use 16 modules and waste roof space
    4. use emphase

    p.s. this is a great site:D
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,023 ✭✭✭✭
    #4
    Re: two strings same voltage different current

    FW,
    Have a look at the Power One Inverters. They can split the inputs (2 inputs per inverter) for just such eventualities...two slightly different facing roofs.

    Other products may do likewise, but I don't know for sure. Me? I like Enphase.

    Ralph

    http://www.power-one.com/renewable-energy/index.php

    edited: put up common link for power-one
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #5
    Re: two strings same voltage different current

    Ground mounted out of the question? Do something "interesting" like a car port/patio shade/etc. near the meter using the solar panels?

    Not mounting the panels on a roof if I had available land would, to me, seem to be a good option--Avoids the issues of roof mounting (potential for leaks, roof structure, easier re-roofing decades later, etc.).

    And/or you could use the roof for solar hot water/heating panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • FreeWatts
    FreeWatts Registered Users Posts: 14 ✭
    #6
    Re: two strings same voltage different current
    BB. wrote: »

    And/or you could use the roof for solar hot water/heating panels.

    -Bill

    this is why i have no room for PV as roof being used for solar thermal water


    how high does the shading structure have to be? i was under the impression that it has to be 10' high or fenced off per NEC
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #7
    Re: two strings same voltage different current

    You're also going to have to pay attention to voltage if one array will be next to the inverter and the other 150'(?) away. The long run will have more voltage drop so you have to size the wire accordingly.
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