Fused combiner box vs mc4 branch connector

Dan_Dan_ Solar Expert Posts: 61 ✭✭✭✭
What are your thoughts on these branch connectors http://www.amazon.com/gp/aw/d/B00E127Z1U?vs=1
With the connector you could skip a combiner box and then have a circuit breaker to protect charge controller. Obviously then the panels and their wiring would have no protection
In what scenario though would a pv circuit breaker in a combiner box get tripped? Isn't the short circuit amperage the highest to expect?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Fused combiner box vs mc4 branch connector
    Dan_ wrote: »
    What are your thoughts on these branch connectors http://www.amazon.com/gp/aw/d/B00E127Z1U?vs=1
    With the connector you could skip a combiner box and then have a circuit breaker to protect charge controller. Obviously then the panels and their wiring would have no protection
    In what scenario though would a pv circuit breaker in a combiner box get tripped? Isn't the short circuit amperage the highest to expect?

    I would not use them at all, nor any variation on such a theme above two panel connections.

    The circuit breaker will not "protect the charge controller". It isn't how it works. PV is a current source and self-limiting; it simply cannot produce so much current that the charge controller is in danger. The controller is the load and ultimately determines how much current the PV produces. Over-current protection between an array and a controller is pretty much useless.

    However, multiple parallel PV connections above two can cause a problem is one of the panels should short: 2X (or more) the Isc will be put to the shorted panel by the other panels, and that can start a fire. Hence the combiner boxes with their built-in per PV string fuses/breakers.
  • Dan_Dan_ Solar Expert Posts: 61 ✭✭✭✭
    Re: Fused combiner box vs mc4 branch connector

    I see. Thank you. So three 10a panels paralleled could have 30a on one panel if short circuit, yes? Is it the internal wiring of the panel or the external wiring that is susceptible or both?
    Iirc outback says to have input protection on the cc. Although if you had array capable of 25A and a 60A cc there isn't much point (besides providing a means of disconnect)
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Fused combiner box vs mc4 branch connector
    Dan_ wrote: »
    I see. Thank you. So three 10a panels paralleled could have 30a on one panel if short circuit, yes? Is it the internal wiring of the panel or the external wiring that is susceptible or both?

    Both, but mainly the external wiring. The risk of a serious disaster increases with every string you add in parallel.
    Iirc outback says to have input protection on the cc. Although if you had array capable of 25A and a 60A cc there isn't much point (besides providing a means of disconnect.

    I think outback setups usually have overcurrent protection between the charge controller and the batteries and inverter. If they recommend overcurrent protection between the array and charge controller then it's for the reason stated above. If it's a general recommendation, it's probably for simplicity's sake.
  • Dan_Dan_ Solar Expert Posts: 61 ✭✭✭✭
    Re: Fused combiner box vs mc4 branch connector
    jaggedben wrote: »
    Both, but mainly the external wiring. The risk of a serious disaster increases with every string you add in parallel.



    I think outback setups usually have overcurrent protection between the charge controller and the batteries and inverter. If they recommend overcurrent protection between the array and charge controller then it's for the reason stated above. If it's a general recommendation, it's probably for simplicity's sake.
    So theoretically in my scenario above with 30A going to a shorted panel you could size the wiring such that it would never be an issue (ie #10 wire should handle 30A)
  • 2twisty2twisty Solar Expert Posts: 199 ✭✭✭
    Re: Fused combiner box vs mc4 branch connector

    Would it make more sense to use an inline fuse? I would imagine that an overcurrent condition would be a rarity that would not justify the expense of a breaker. The only advantage I can see over a fuse is that you have the ability to switch off one or more of the paralleled strings. I have 2 parallel strings through an MC4 combiner into a breaker just before the CC. The breaker is there so that I can turn off the solar input, mostly.

    Those MC4 combiners would really make quite a rats nest if you have more than 2-3 parallel strings, though. When I add my third string soon, I may replace those MC4 branch connectors with a pair of buss bars so that I avoid the rats nest.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Fused combiner box vs mc4 branch connector

    Three 10 Amp panels in parallel, one shorts, it receives 20 Amps of current (the combined Isc of the two remaining good ones) not 30 Amps.

    Panels usually don't need to be 'shut off' so sometimes fuses are a cheaper choice. You want the panel off? Cover it: no light = no output.

    You can't use just bus bars; the connections need to be weather-tight and that means a box and once you put bus bars in a box you have a combiner box and all you need add is the fuses or breakers to make it correct.

    These branch connectors ... check the wire size and current rating. Not all panels are the same. Some are 5 Amps, some are 8 Amps. If the conductors are capable of 30 Amps and you put four 5 Amp panels on them that's okay (except for not having the per-string protection). Put four 8 Amp panels on them and you've exceeded their current rating and are asking for trouble.
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Fused combiner box vs mc4 branch connector
    2twisty wrote: »
    Would it make more sense to use an inline fuse? I would imagine that an overcurrent condition would be a rarity that would not justify the expense of a breaker.

    Mate if your place burns down, in hindsight, will 11 bucks for a breaker seem much at all? Do it right.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • Alaska ManAlaska Man Solar Expert Posts: 252 ✭✭
    Re: Fused combiner box vs mc4 branch connector
    zoneblue wrote: »
    Mate if your place burns down, in hindsight, will 11 bucks for a breaker seem much at all? Do it right.

    That's what I was thinking, even if you need 2 breakers at $15 each so what? When one considers the alternative of fire or equipment damage; I'll spend the thirty bucks.
  • Dan_Dan_ Solar Expert Posts: 61 ✭✭✭✭
    Re: Fused combiner box vs mc4 branch connector

    Appreciate the replies folks. I ask personally less because of cost but more just to understand the reason why they're there so I'm more likely to understand the whole and put together a coherent system
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Fused combiner box vs mc4 branch connector
    Dan_ wrote: »
    So theoretically in my scenario above with 30A going to a shorted panel you could size the wiring such that it would never be an issue (ie #10 wire should handle 30A)

    Yes, but it's not just the wiring... the shorted panel itself need protection.

    one other reason for a combiner box: it's a great place to put a lightning arrestor.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Dan_Dan_ Solar Expert Posts: 61 ✭✭✭✭
    Re: Fused combiner box vs mc4 branch connector

    Do panel manufacturers list the maximum current a panel can sustain before going up in smoke?
  • PhotowhitPhotowhit Solar Expert Posts: 4,996 ✭✭✭✭
    Re: Fused combiner box vs mc4 branch connector

    There is normally a fuse/breaker rating.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Fused combiner box vs mc4 branch connector
    Photowhit wrote: »
    There is normally a fuse/breaker rating.

    Which for decent panels anyway is always written on the nameplate sticker.

    Breakers (decent ones with 100000+ amps interupt capacity ) can be relied upon to break the circuit when called upon, (unlike dodgy automotive etc) fuses which might melt, arc over, weld, and do other silly things. Properly rated and installed fuses, are ok, however a blown one requires a downtime delay. And the price difference isnt much if any. Then the breaker can be used as a switch. For all those reasons i would never spec fuses for RE. Yes breakers require a suitable enclosure, but cheap ones are available online,right here, and said enclosure helps make a tidy, secure and safe system.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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