Advice on PV string and shut off box

I am installing my PV panels (pole mounted). I have 6 (12 v) and 2 (24 v) all totaling 1100 watts. I will be combining the 12v in series to get the 24v. So now I have 3 strings of 12v in series/24v and I have one string of 24v. Can I daisy chain all and then have a shut off box with single fuse. eg. Midnite junction box?

If so what size box do I need? and what kind?

I'm new to the forum so thanks.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Advice on PV string and shut off box

    Welcome to the forum.

    You will have three "24 Volt" strings paralleled, so you should have fuses or breakers on each string. One of the 4 position Midnite combiner boxes is your friend here. :D
  • VicVic Solar Expert Posts: 2,998 ✭✭✭✭
    Re: Advice on PV string and shut off box

    Hi scheek,

    So you have 6ea 12 V panels. You are going to wire two of these PVs in series to make three 24 V strings.
    The way I read what you posted, you also have 2ea 24 volt panels. These need to be REAL 24 volt panels, with a Vmp of about 35 to 36 volts. You will need to connect these real 24 V PVs individually into the Combiner, and NOT wire them in series.

    As Coot said, each of these strings must have a breaker (or a fuse) in the Combiner. So if there are really five strings, then a six position MidNite Combiner should be fine:
    http://www.solar-electric.com/mnpv6.html

    Perhaps I have misunderstood what you want to do. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • scheekscheek Registered Users Posts: 26 ✭✭
    Re: Advice on PV string and shut off box

    Vic,
    You read it right. 3 strings of (2) 12V panels and (2) 24 volt. So I need to get a combiner with 5 fuses I suppose? Too, what size fuses do I use?

    Thanks
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Advice on PV string and shut off box
    scheek wrote: »
    Vic,
    You read it right. 3 strings of (2) 12V panels and (2) 24 volt. So I need to get a combiner with 5 fuses I suppose? Too, what size fuses do I use?

    Thanks

    1.56 X Isc for the string and then up to the next standard fuse size is the rule.
  • VicVic Solar Expert Posts: 2,998 ✭✭✭✭
    Re: Advice on PV string and shut off box

    Fuses do work in PV combiner boxes, BUT, circuit breakers are more convenient, as they can be used as switches. In today's market, breakers are little more expensive than a fuse holder + the fuse IMHO:

    http://www.solar-electric.com/mnepv.html

    These are excellent breakers and fit the MidNite Combiners.

    Just to be clear, please make certain that your 24 Volt PVs are not the 60-cell variety, with a Vmp of about 30 - 31 Volts. Real 24 V PVs have Vmps of about 35.5-ish volts, and have 72 cells. FWIW. Have Fun, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • scheekscheek Registered Users Posts: 26 ✭✭
    Re: Advice on PV string and shut off box
    ggunn wrote: »
    1.56 X Isc for the string and then up to the next standard fuse size is the rule.

    Thanks for the input. Here is one more question. I will have 3 strings (each with double 12v) to make the 24 v. As you saw I stated that I have two 24v panels. Can those 2 be run in parallel with one fuse or do they too need separated.:confused:
  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: Advice on PV string and shut off box

    Ideally, each panel would need its own series fuse/breaker. Does not matter if one 24 volt panel or two 12 volt panels in series.... It is about safely limiting over current flow in each "branch" circuit (solar panel string).

    It gets a bit more complex if you have different Isc or Series Fuse requirements (100 watt and 250 watt panel strings). You would need different fuse/breaker rating "per string" (smaller panels need smaller fuses, larger panel would use larger fuse values).

    Also, check the data sheet for the panels. Most UL/NRTL rated panels used in modern grid tied systems will call out the series fuse rating.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VicVic Solar Expert Posts: 2,998 ✭✭✭✭
    Re: Advice on PV string and shut off box

    AND, scheek,

    PLEASE answer one or two QUESTIONS for me;

    Regarding the "24 V panels", What is the Vmp spec for them,

    OR, what is the exact model number of these PVs??

    Some 30 - 31 Vmp PVs are called 24 volt nominal PVs, and are not REAL 24 V PV Modules.

    This seemingly small point is important. You probably do not have a problem, but am just trying to make certain, by asking this question so many times.

    Thank You, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Advice on PV string and shut off box
    scheek wrote: »
    Thanks for the input. Here is one more question. I will have 3 strings (each with double 12v) to make the 24 v. As you saw I stated that I have two 24v panels. Can those 2 be run in parallel with one fuse or do they too need separated.:confused:
    No. If you have enough strings to require fuses (three or more), then every string needs to be individually fused.
  • scheekscheek Registered Users Posts: 26 ✭✭
    Re: Advice on PV string and shut off box

    OK. I will check on this and get back. thanks
  • scheekscheek Registered Users Posts: 26 ✭✭
    Re: Advice on PV string and shut off box

    I checked the BACK OF THE 24 volt panels and it says
    190 max watts
    max power voltage 36 v
    open circuir voltage 43 v
    short circuit current 5.76 a
    max power current 5.28 a
  • VicVic Solar Expert Posts: 2,998 ✭✭✭✭
    Re: Advice on PV string and shut off box

    Hi scheek,

    Thanks for checking. Great that they ARE real 24 V PVs. You should not have a problem using them with strings of two of the 12 V PVs. Cannot look at the specs of the 12 Vers just now. The 190s may be slightly low in Vmp -- the 36 V spec -- but should be fine. Thanks, Good Luck, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • scheekscheek Registered Users Posts: 26 ✭✭
    Re: Advice on PV string and shut off box

    I am about to order about 9 extension cables to go from PV's to combiner box. Ranging from 15' to 6'. What gauge cable should should I be looking for. 10.12,14,16.?

    Thanks
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Advice on PV string and shut off box
    scheek wrote: »
    I am about to order about 9 extension cables to go from PV's to combiner box. Ranging from 15' to 6'. What gauge cable should should I be looking for. 10.12,14,16.?

    Thanks
    #10 is pretty much standard for these connections.
  • scheekscheek Registered Users Posts: 26 ✭✭
    Re: Advice on PV string and shut off box

    Thanks. That is kinda what I was thinking, but not sure.
  • scheekscheek Registered Users Posts: 26 ✭✭
    Re: Advice on PV string and shut off box

    I have all my Pv panels installed now ( 8 ) 2 rows of four. Is it better to aim all in one direction for morning and noon sun or split a few for afternoon?
    Thanks
  • bmetbmet Solar Expert Posts: 630 ✭✭
    Re: Advice on PV string and shut off box
    scheek wrote: »
    I have all my Pv panels installed now ( 8 ) 2 rows of four. Is it better to aim all in one direction for morning and noon sun or split a few for afternoon?
    Thanks

    It would depend on your location, obstacles in the arc of the sun's path (shadows), and your average weather conditions for the time of day. If you live in a region where morning fog hangs around until the sun burns it off, then it would be better to orient for later in the day. If you have clear skies, aim them for solar noon. REMEMBER, by splitting the panels orientation, you won't harvest their combined power; your Charge Controller will only produce charging current based upon the output of one string (at a time). Also, your calculations must be based upon the panel specs in each individual 'split'; you can't just do the math for one string and double it.
  • scheekscheek Registered Users Posts: 26 ✭✭
    Re: Advice on PV string and shut off box

    What is an ok volt meter to have to use with my battery bank. I have a 6000 watt Aims but meter is hard to see. I was thinking of something larger I could tie into with a bigger screen. Need something that does not borrow a lot of wattage either. Any help?

    Thanks,
    Stephen
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Advice on PV string and shut off box
    scheek wrote: »
    I have all my Pv panels installed now ( 8 ) 2 rows of four. Is it better to aim all in one direction for morning and noon sun or split a few for afternoon?
    Thanks
    Never split a single string between orientations. Every module in a string performs at the level of the weakest module, so if you have some pointed one way and some the other, the string will never operate at full output.
  • VicVic Solar Expert Posts: 2,998 ✭✭✭✭
    Re: Advice on PV string and shut off box
    scheek wrote: »
    What is an ok volt meter to have to use with my battery bank. I have a 6000 watt Aims but meter is hard to see. I was thinking of something larger I could tie into with a bigger screen. Need something that does not borrow a lot of wattage either. Any help?

    Thanks,
    Stephen

    Stephen, this may not be what you have in mind, but here is the manual for the MidNite Battery Capacity Meter:
    http://www.midnitesolar.com/pdfs/MNBCM12.13a.pdf

    Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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