When to fuse several series strings

I think I know the answer to this question but wanted to make sure.

I'm going to wire 8 140w kyocerra panels in 4 strings (2 panels each in series) 4 strings combined parallel.

It is my understanding that with 3 or more strings - each string needs to be fused prior to them being combined into the full array.

Is that correct?

And - if so... I've calculated the fuse size for each string as being 15amp. (7.91imp x 1.56 rounded up to the next higher size fuse)

Do the experts agree?

Thanks

Greg

Comments

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    I think you have it figured out correctly.... However, you can save yourself the trouble of calculating the fuse size by just looking at the panel specifications. Most panels have a series fuse size specification.

    One thing I would do differently is to use circuit breakers rather than fuses. It is a wonderful thing to be able to easily switch a string on or off.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • gkerlin
    gkerlin Solar Expert Posts: 27
    vtmaps wrote: »
    I think you have it figured out correctly.... However, you can save yourself the trouble of calculating the fuse size by just looking at the panel specifications. Most panels have a series fuse size specification.

    One thing I would do differently is to use circuit breakers rather than fuses. It is a wonderful thing to be able to easily switch a string on or off.

    --vtMaps

    Thanks. fuse was a generic term. My last had 3 strings and 3 circuit breakers in the 5th wheel basement. Here I am going to combine up on the roof however so I'm not certain if I'll put in fuses or breakers. I'm going to make another post on here looking for suggestions on a combiner box (wired or unwired) that would be suitable for a roof top location.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    If you have an easily accessible breaker/per string switch--It is really nice to debug if you have a bad panel/string of panels (switch on 1 string at a time and read your charge controller's input/output current--if you have a meter on the controller).

    And/or have a place where you can access each string from the array (i.e., positive wiring) with a DC Current Clamp DMM--Again, easy to check that everything is working well.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    BB. wrote: »
    If you have an easily accessible breaker/per string switch--It is really nice to debug if you have a bad panel/string of panels (switch on 1 string at a time and read your charge controller's input/output current--if you have a meter on the controller).

    And/or have a place where you can access each string from the array (i.e., positive wiring) with a DC Current Clamp DMM--Again, easy to check that everything is working well.

    -Bill

    That link is the exact Sears product that I have and recommend. But be careful not to get the slightly lower priced unit that measures AC and DC volts but only AC amps. Some online store descriptions make it easy to confuse the two.
    SMA SB 3000, old BP panels.
  • pebbledropper
    pebbledropper Registered Users Posts: 17 ✭✭
    Greg,

    I don't understand. Are you sure you're referencing Vmp. For 140W panels tha should be around 16-17. The Imp for 140W kinda fits your 7.91 number. The reason I ask is you are hooking each pair in series. That will double your V but keep your Imp the same. What I'm trying to say is have you got 7.91 Amps coming to the combiner per pair of panels?

    Jerry
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Greg,

    I don't understand. Are you sure you're referencing Vmp. For 140W panels tha should be around 16-17. The Imp for 140W kinda fits your 7.91 number. The reason I ask is you are hooking each pair in series. That will double your V but keep your Imp the same. What I'm trying to say is have you got 7.91 Amps coming to the combiner per pair of panels?

    Jerry

    It was a typo. He multiplied that value by a fudge factor to get current, so he had to mean Imp not Vmp.
    And for 140W and Imp = 7.91, the Vmp should be about 17.7.
    SMA SB 3000, old BP panels.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    inetdog wrote: »

    It was a typo. He multiplied that value by a fudge factor to get current, so he had to mean Imp not Vmp.
    And for 140W and Imp = 7.91, the Vmp should be about 17.7.

    Yes, the 7.91 is Imp, not Vmp, but the 1.56 was not a fudge factor. Fuse size should be 125% X 125% = 156% of the Imp. I believe that with some circuit breakers that are rated for continuous current, only 125% of Imp is necessary.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • gkerlin
    gkerlin Solar Expert Posts: 27
    inetdog wrote: »

    It was a typo. He multiplied that value by a fudge factor to get current, so he had to mean Imp not Vmp.
    And for 140W and Imp = 7.91, the Vmp should be about 17.7.

    Correct - I made a typo. I corrected the original post.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    vtmaps wrote: »

    Yes, the 7.91 is Imp, not Vmp, but the 1.56 was not a fudge factor. Fuse size should be 125% X 125% = 156% of the Imp. I believe that with some circuit breakers that are rated for continuous current, only 125% of Imp is necessary.

    --vtMaps

    The presence of the double 125% factor will vary with NEC version.
    But the simplest thing is always to look at the Maximum Series Fuse value listed on the panels and not worry what the Minimum value might be.
    SMA SB 3000, old BP panels.
  • pebbledropper
    pebbledropper Registered Users Posts: 17 ✭✭
    Q 1:
    I know one of the 25% bumps is for super cold, snow reflection, and just because. What's the other 25% for?

    Thinking ahead some, if it's for the MPPT boost, that hasn't happened yet at the combiner. You only care about MPPT sized wiring and protection from the controller on. Yes?

    Q 2:
    So, if I series 2 160W 12V panels, each an Ioc of 9.61, can I use a 15A breaker in that Pos conductor? And if I have 8 panels paired in 4 24V series, can I use 4 15A breakers at the combiner point where the 4 series Pos conductors go parallel? If so, using some terminal blocks and switchable breakers, I intend to make my own RV combiner box where I can get to it standing on the ground. I'll run Pos and Neg conductors from there to the controller using whatever AWG the tables call for. The Amp load for that wire would be 9.61 x 4 x 1.25 (or 1.53?) = 48.05A (or 58.8A), yes??

    Jerry
  • gkerlin
    gkerlin Solar Expert Posts: 27
    If you sized the fuse at 100% of expected maximum current it would blow. So the 125% is to allow it to not blow but still be suitable to protect the wire.

    The breaker or fuses would be before you combine the strings and in the positive conductor. If you have 4 strings then 4 fuses. Then after the breaker you can combine them and run on to the controller. As someone mentioned, if you don't want to do the calcs each panels spec shows a series fuse rating.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Q 1:
    I know one of the 25% bumps is for super cold, snow reflection, and just because. What's the other 25% for?

    The other 25% is for fuses that may blow if they carry their rated current. There are some circuit breakers that can carry their rated current indefinitely, and those breakers do not need the additional 25%.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i