Combiner boxes; Fuses or Breakers ?

bethbeth Solar Expert Posts: 32
Why would you chose one over the other in your combiner box?

Comments

  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Combiner boxes; Fuses or Breakers ?

    I choose fuses because breakers are a lot more expensive and are unnecessary, since solar panels should not short out. The whole idea of combiner box is for safety, to make sure that high current will not flow into shorted string thus causing wiring to melt and possibly starting fire.
  • BB.BB. Super Moderators, Administrators Posts: 29,854 admin
    Re: Combiner boxes; Fuses or Breakers ?

    You also have a choice of how weather resistant the boxes are (exposed or inside a building).

    And there is always price. :roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mikeomikeo Solar Expert Posts: 386 ✭✭✭
    Re: Combiner boxes; Fuses or Breakers ?
    Why would you chose one over the other in your combiner box?
    Either fuses or breakers will work just fine in your combiner box. Fuses typically are cheaper but I like breakers so I can more easily check my panel strings for output once in a while by throwing all breakers off and turning them on in sequence one at a time and reading the input on my MX60 controller. If a string starts failing, this lets me know which string without having to pull all the fuses.
  • KeithWHareKeithWHare Solar Expert Posts: 140 ✭✭✭
    Re: Combiner boxes; Fuses or Breakers ?
    AntronX wrote: »
    I choose fuses because breakers are a lot more expensive ...

    I haven't done an exhaustive study of available equipment and costs, but on the NAWS web site, I find:

    Fuses:
    • Midnite Solar MNTS Touch Safe Fuse Holder: $9.00
    • Midnite Solar MNFUSE High Voltage DC Fuses: $6.85
    • Total: $15.85

    Breakers:
    • Midnite Solar MNEPV 150 VDC Solar Array Breakers: $11.50
    • Total: $11.50

    So, I would use breakers because fuses are a lot more expensive.

    The other factor with these parts is that the breakers are rated for 150 Volts DC and the fuses are rated for 600 Volts DC, so it also depends on your PV panel configuration.

    Keith
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,870 ✭✭✭✭✭
    Re: Combiner boxes; Fuses or Breakers ?

    You get what you pay for! You must look at the specs to do this right. The OP probably does not need the attributes of a good fuse. Those being the ability to interrupt in a really bad fault condition and the ability to not breakdown under high voltage.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
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  • GreenerPowerGreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Combiner boxes; Fuses or Breakers ?

    It depends on your PV voltage in each string. If it's 12V or 24V nominal, you can use inexpensive fuse since those are normally rated for 32VDC. Above that, I found these inexpensive fuses that are rated 125VDC - http://www.littelfuse.com/data/en/Data_Sheets/314_324.pdf and they are not expensive (http://www.futureelectronics.com/en/technologies/electromechanical/circuit-protection/electronic-fuses/Pages/3080355-0324010.HXP.aspx ). Wish our host carry these.

    Fuses for breakers depending on PV configuration. Personally I use both, combining 2 or 3 strings with fuse in each string and run wires to the combiner breakers box.
    GP
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Combiner boxes; Fuses or Breakers ?

    If this is a ground mount array. Then go with breakers, as the double as a array disconnect. Use fuses, then you will also need an array disconnect also.
  • bethbeth Solar Expert Posts: 32
    Re: Combiner boxes; Fuses or Breakers ?

    Thanks All!

    In regard to what Ken said:

    "If this is a ground mount array. Then go with breakers, as the double as a array disconnect. Use fuses, then you will also need an array disconnect also. "

    So in that case why wouldn't you need a breaker betwen the combiner box and controller?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Combiner boxes; Fuses or Breakers ?
    beth wrote: »
    Thanks All!

    In regard to what Ken said:

    "If this is a ground mount array. Then go with breakers, as the double as a array disconnect. Use fuses, then you will also need an array disconnect also. "

    So in that case why wouldn't you need a breaker betwen the combiner box and controller?

    Location, location, location. :D

    If your array is up on the roof and your combiner box is up on the roof (feeding one common down-lead) it's inconvenient to get at to shut off the power feeding in to the charge controller. Hence the use of a disconnect. For that type of install, my preference would be fuses up top, switch down below.

    If it's ground mount and the array/combiner is easily accessed, then the use of circuit breakers at the combiner give you both over-current protection and disconnect of power to the controller in one package. Each parallel string is on its own breaker which will trip if current goes too high. Likewise, shut them all off and the feed to the controller is zero.

    And if we could draw pictures easily this could be explained neatly in two frames. :D
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Combiner boxes; Fuses or Breakers ?

    For ground mount. It is common to combine the panels/strings at the array.
  • BB.BB. Super Moderators, Administrators Posts: 29,854 admin
    Re: Combiner boxes; Fuses or Breakers ?

    That question will need to refer back to the installation manual of the charge controller and what they require to meet their UL/NRTL Listing requirements (if the device is Listed).

    More or less, fuses/breaker/etc. are there to protect the wiring from over current (shorts, overload, equipment failure) situations.

    So, looking at where current is "sourced" we see that the solar panels are an obvious choice. So, the wiring needs to be sized/protected up to the rating of the power source. Solar panels are rated for Isc (Current short circuit) and can exceed the rated value if a whole bunch of sun or possibly reflections from water/snow, etc... So, normally, we use the 1.25xIsc as a safety factor to allow for any possible amount of current supplied by the solar panel.

    I have avoided NEC for solar systems, but they also require (as I understand) a derating of the PV input of the charge controller by 1/1.25 or 0.8x (60 amp input derated to 48 Amps)... Depending on controller and design--I think this is an overkill--but that is for another day.

    For solar panels, they only require a CB/fuse to protect against one panel/wiring being shorted and the power from two or more other solar panels back-feeding power into the shorted circuit and causing a fire.

    At this point, the charge controller is assumed to not be able to supply current from the battery bank to the solar panels (if Listed, this should include any possible "single fault" in the charge controller--but this is an assumption on my part--internal UL Listing Files/Documents are proprietary information and not available to the public--only to the manufacturer and UL/NRTL's).

    The next question is a disconnect required to allow servicing of components. It certainly is nice to have--but there is no requirement for a DC panel to solar charge controller for a disconnect (that I know of). A circuit breaker or even a fuse block could be used as a service disconnect in this case (I believe).

    Lots of hand waving--In the end, read the installation instructions and that will tell you pretty much the minimum requirements. If something does not look right (mistakes happen), ask here or elsewhere for help.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Combiner boxes; Fuses or Breakers ?
    BB. wrote: »
    (60 amp input derated to 48 volts)...
    -Bill
    Should be amps not volts.<= Fixed
  • BB.BB. Super Moderators, Administrators Posts: 29,854 admin
    Re: Combiner boxes; Fuses or Breakers ?

    Thank you Ken... Fixed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Combiner boxes; Fuses or Breakers ?
    BB. wrote: »
    The next question is a disconnect required to allow servicing of components. It certainly is nice to have--but there is no requirement for a DC panel to solar charge controller for a disconnect (that I know of). A circuit breaker or even a fuse block could be used as a service disconnect in this case (I believe).
    -Bill

    From the 2005 NEC.
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Combiner boxes; Fuses or Breakers ?

    As for the disconnects, NEC states switches or circuit breakers, nothing about fuses. I did use fuses in the combiner box but also have a fused switch just prior to entry in the house.

    From NEC 2005:
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Combiner boxes; Fuses or Breakers ?

    Technically a fuse can't be considered a 'disconnect' because it doesn't smoothly break the connection the way a spring-loaded switch or circuit breaker does; there's too much risk of a partial disconnection resulting in arcing.

    (The above run-on sentence is a free bonus! :p)
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