Combiner Box

adam1984adam1984 New UserPosts: 72Solar Expert ✭✭✭
For a few panels that will be acting as battery chargers (360w total), running DC power (no inverter) , with a charge controller that is rated well above the panel, why would I need a combiner box?


  • westbranchwestbranch Not So New User Posts: 3,852Solar Expert ✭✭✭
    Re: Combiner Box

    Is this a Code compliant application?
    CL 647 DOA? 24V 900Ah C&D AGM
    CL 4592  175A E-Panel WBjr, 2 x 4s 140W to 24V 900Ah C&D AGM + 4s 140W 
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  • adam1984adam1984 New User Posts: 72Solar Expert ✭✭✭
    Re: Combiner Box

    it is not. Its for a boat.
  • BB.BB. Just some guy Posts: 23,740Super Moderators admin
    Re: Combiner Box

    Basically, the limit to placing solar cells in parallel (and following the National Electric Code) is to help protect "reasonable" failures (shorts, cut wires, failed internals, etc.) of any wiring and attached equipment.

    Protecting means limiting fire/shock--not protecting the equipment to work another day (for example, I used NEC and UL/CE to design computer equipment--I may have a computer internal part fail, but if the fire was limited to the enclosure and nobody gets shocked, such as a fireman, then my system was "fine"--it may never "compute" again--but the surroundings did not burn down and nobody was shocked/killed).

    So, regarding solar panels... They are "electrical sources" that can fail internally (cell or diode short, rock through face, object up through underside, etc.). The panels are rated as "intrinsically safe" if any of these failures will not cause a fire or other hazardous conditions (shock is a bit of a problem--cannot "turn off" a solar panel's generation--that is why NEC and Fire are so "fixated" on solar array safety--or not--cannot turn of a master switch and all of the voltage/current "stops" if the sun is up).

    Now, we take the one panel (say it is 20 volts and 10 amps or 200 watts) and put four 200 watt panels in parallel. If we now "short" some part of that one panel or its external wiring connections--we not only have the 200 watts/10 amps/20 volts available to "glow" a piece of wire/conductor, we also have those three (or more) other panels dumping there energy into that short circuit...

    So, for each "solar array", we need to evaluate the ability of other panels/energy sources to feed into a short in "that one panel/wiring failure". And we look for Series Protection Fuse Rating.

    Turns out for various reasons the SPFR is pretty much a little more than 2x the solar panels Isc rating.

    One lone panel cannot over current itself. And two panels in parallel cannot over current either of themselves.

    However, 3 or more panels in parallel can over current a single short anywhere in the panel array, then they can exceed the SPF Rated current into the shorted array/distribution wiring...

    So, you need a place to join the series protection fuses to the main bus back to the charge controller/Grid Tied inverter.

    The normal rule is 2 parallel strings or less don't need Series Protection Fuses (or breakers). And for 3 or more parallel strings, you will need a SRP for each parallel string (3 string, 3 fuses/breakers; 5 paralleled strings needs 5 fuses/breakers, etc.).

    Notice, it does not matter if each "string" is one panel, 5 panels, or 10+ panels in series... The fusing / combiner box is only addressing the parallel current control.


    PS: By the way, we are ignoring the Solar Charge Controller which is attached to a big old battery bank with 1,000's of amps available to feed a short circuit if both the Charge Controller and the Solar Array shorted. NEC usually requires protection against one failed component--either the controller or the array failing--not both failing at the same time--controller short and panel short. And some controllers like PWM can failed short and pass current backwards. Other controllers like MPPT type Grid Tied controllers cannot. Those require a high frequency AC current to back drive across the isolation transformers/torrid transformers--So DC from the panels cannot go to the AC line out and AC line cannot feed back to the DC Solar panels--both because of the switching frequency and the "double isolation" or equivalent isolation design of the GT inverter.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Just some guy Posts: 23,740Super Moderators admin
    Re: Combiner Box

    There is "code compliant" and good practices... For Boats which are too small to require all of those licenses/skippers/engineers with Coast Guard tickets--It is still a good idea to follow NEC type codes as they usually are good practices anyway.

    There are some things that are done in Boats/Cars/Etc. that would not pass NEC requirements (things people can touch like 12 volt battery terminals and not using fuses/breakers from a battery bank to the starter relay/motor).

    They sort of assume the battery will go "dead" before you could have a starter motor fault start a fire... But people are installing larger battery banks for "house loads" too now (instead of running a generator to supply loads while not under power)--and should really be looking at fusing/breakers on starter motor circuits these days.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • adam1984adam1984 New User Posts: 72Solar Expert ✭✭✭
    Re: Combiner Box

    BB- makes complete sense, it would be 6 panels in parallel. strings of 1 panel each. So i guess the answer would be a fuse box would come in handy to not burn down the boat and as a safety measure in general. Although code compliance has nothing to do with it, as you said, there is always good safe practices, which obviously should be done for a reason.
  • dwhdwh Solar Stumblebum Posts: 1,332Solar Expert
    Re: Combiner Box

    You're going to have to combine the panel outputs anyway, so you might as well use a combiner box. For one or two panels, I might just tie them together but for half a dozen I'd go with a proper combiner.
  • halfcrazyhalfcrazy New User Posts: 710Solar Expert
    Re: Combiner Box

    and if you only need 3 strings Midnite Solar's MNPV3 would be a very cost effective solution and you will be Listed,Safe and Code Compliant.
  • PhotowhitPhotowhit Old and in the Way Posts: 3,048Solar Expert ✭✭✭
    Re: Combiner Box

    If your shooting for saftey, but not particularly for code compiance. You might look into using a fused power distribution block.

    Might be a space saving as well, important on boats.
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