Fuse ideas for 125w x 4 setup

Nila
Nila Solar Expert Posts: 173 ✭✭
HI All

Im trying to setup 125w x 4 panels , 4 parallel strings

ISC is around 7.9 so I think I need a fuse rated above this.. 10A fuses are common.

Can i simply use a automobile fuse box which is famous here that is used in cars,etc.?

It has nice fuse holders.. so I can easy send each panel's + side into that and use this as only a fuse holder.

Also it has some default interconnections on the top row of fuses so it makes even the combining part very easy.


Standard combiner boxes available locally are like 100$,

I would like to know what is the disadvantage in using some cheap stuff like this?


Is it okay todo small copper plate/tin coat bar for combining all 4 strings into one and then take a larger cable as the PV input into the charge controller..?

How do i find how think the plate should be for carrying almost about 35A?


but wanted to know if this is 100% required or if i can use the interconnected fuses on this same fuse box to combine? I believe this would melt those small plates lol.


Reason Im trying to do this is because the combiner box is not available in my city and costs higher else where, and all it does is adds 4 fuses and a bar.. so I thought why not DIY

So whilst doing it i wanted to play safe and wanted to ask the experts here ;)

Comments

  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    Here is an attached pic

    http://s22.postimage.org/gr7crhsm9/20130318_205917.jpg
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Fuse ideas for 125w x 4 setup
    paimpozhil wrote: »
    HI All

    Im trying to setup 125w x 4 panels , 4 parallel strings

    ISC is around 7.9 so I think I need a fuse rated above this.. 10A fuses are common.

    Two things to worry about:

    1. The fuse and holder need to able to handle the current needed and give you a good solid place to attach the wires. Should not be a problem to determine just by looking at it.
    2. Auto fuse holders and the fuses that go in them are designed to interrupt 12 volts DC. Maybe at the upper limit 40 volts DC. If your panel string has a Voc which is higher than that, the fuses may just arc and melt the fuse holder until it drips burning plastic. Not a good idea.
    Other common fuses are rated for AC up to 120 or 240 volts, but far less for DC. There are fuse types which are specific for higher DC voltages. Depending on you panel and string voltage you may need to use them, along with a matching fuse holder.
    SMA SB 3000, old BP panels.
  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    Panels wouldnt go above the 40Voc lol.. here climates are hot and the panel voc is 22.1 :) so that is safe?

    My fuses work on the cars so they are 99% DC and i did ask the store about it before buying one to play with.
  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    If the fuse is lets say 10A, and I use about 4x7A panels to parallel

    should the fuse holder be atleast 4x7 = 28A or they can be just about same as fuse?
  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    Several recent post with links on where to get what you might be looking for. Also I believe naws this sites store has some cheep 3 pole boxes. I don't think it would be a problim to put two panels in paralel and then you would only need two fuse/breakers for four panels. You might need to combine the pos. leads from the two panels to one wire prior to the fuse breaker conection. Probly a 20 amp fuse/breaker.
    This should make it a little cheeper. If my advice sucks I am counting on someone pointing it out to us.
    Good luck
    gww

    My I type slow, four answers while I was typing.
  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    Thanks for the answer. :),

    So that makes it even easier I can use a T MC4 and combine 2 strings into 1 before going to a fuse with 20A.

    Now question is about the bus bar.. how think the bar should be minimum to carry the max amperage lets say 32A ?
    Im thinking about utilizing interconnected sections of this same fuse holder If thats okay but i m happy to build a bus bar and setup outputs myself if not
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Fuse ideas for 125w x 4 setup
    paimpozhil wrote: »
    Thanks for the answer. :),

    So that makes it even easier I can use a T MC4 and combine 2 strings into 1 before going to a fuse with 20A.

    Take a close look at the panel specs first. They may list a maximum series fuse size. If that is 10 amp, for example, then that is the biggest you can use, even after putting the two panels in parallel.
    Otherwise you could get 14 amps from one pair and 7 amps from the other single, all going through one damaged panel.
    SMA SB 3000, old BP panels.
  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    Inetdog
    If he had six panels with three pairs. Would it work conected in this fasion as it would definatly over load the fuse/breaker and protect the panel?

    Looking at inetdogs answer it looks as my advice may be wrong. It was a question I had ask earlier and thought I had got a positive responce on but I am guessing I should not have passed it on.
    I was trying to help and had confidence that enough members read this to correct me. For that I am thankfull.
    gww
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Fuse ideas for 125w x 4 setup
    gww1 wrote: »
    Inetdog
    If he had six panels with three pairs. Would it work conected in this fasion as it would definatly over load the fuse/breaker and protect the panel?

    Looking at inetdogs answer it looks as my advice may be wrong. It was a question I had ask earlier and thought I had got a positive responce on but I am guessing I should not have passed it on.
    I was trying to help and had confidence that enough members read this to correct me. For that I am thankfull.
    gww

    Fusing of panels or panel strings in parallel is one of those subjects where your first thought is usually not correct, and the reason for that it the fuse is not there to protect wiring or other equipment from the panels. The panels are current limited and the wire size is already large enough to carry the full short circuit current (Isc) of the panels.
    When you realize that the fuses are there to protect the panels from each other or from a backward connection of the battery through a failed CC, the picture changes.

    Keep watching, and feel free to join in the discussions, but make sure for the sake of newbies that you make it clear, as you did, that you know you may be wrong.
    Some of us make a habit of being wrong. Or at least disagreeing with each other, and that is fine.

    To shamelessly steal a phrase from another forum: "Some of my friends are idiots, <pause> and I consider all of you my friends."
    SMA SB 3000, old BP panels.
  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    Thank you for stepping in before i did something actually :)

    So i will use 1 10A fuse per string, Now only thing I need is to know the size of the bus bar?

    is 30A region small enough to consider ANYTHING wrks? like a small plate of copper or I should be careful

    I tried to look for information from web and didnt succeed yet . They are all for like 100A plus , etc
    inetdog wrote: »
    Fusing of panels or panel strings in parallel is one of those subjects where your first thought is usually not correct, and the reason for that it the fuse is not there to protect wiring or other equipment from the panels. The panels are current limited and the wire size is already large enough to carry the full short circuit current (Isc) of the panels.
    When you realize that the fuses are there to protect the panels from each other or from a backward connection of the battery through a failed CC, the picture changes.

    Keep watching, and feel free to join in the discussions, but make sure for the sake of newbies that you make it clear, as you did, that you know you may be wrong.
    Some of us make a habit of being wrong. Or at least disagreeing with each other, and that is fine.

    To shamelessly steal a phrase from another forum: "Some of my friends are idiots, <pause> and I consider all of you my friends."
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Fuse ideas for 125w x 4 setup
    paimpozhil wrote: »
    Thank you for stepping in before i did something actually :)

    So i will use 1 10A fuse per string, Now only thing I need is to know the size of the bus bar?
    Fuse size should be Isc X 1.25 (continuous use) X 1.25 (excess insolation), and then the next standard size up.

    7.9A X 1.25 X 1.25 = 12.34A. Fuse size is 15A. The busbar rating needs to be at least 4 X 15A / 1.2 = 50A (120% rule).

    You cannot use automotive fuses or holders; they will not stand up to the heat.
  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    Panel spec says max series fuse rating is 10A,

    Are you sure this would still be 12.34 min?

    ggunn wrote: »
    Fuse size should be Isc X 1.25 (continuous use) X 1.25 (excess insolation), and then the next standard size up.

    7.9A X 1.25 X 1.25 = 12.34A. Fuse size is 15A. The busbar rating needs to be at least 4 X 15A / 1.2 = 50A (120% rule).

    You cannot use automotive fuses or holders; they will not stand up to the heat.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Fuse ideas for 125w x 4 setup
    paimpozhil wrote: »
    Panel spec says max series fuse rating is 10A,

    Are you sure this would still be 12.34 min?
    I am surprised to see a module with 7.9A Isc that specifies a 10A max fuse size, but if you exceed whatever it says and you have a problem, you may have voided the warranty.

    But see NEC 690.8(A)(1) which says that the maximum current is 1.25 times Isc, and 690.8(B)(1) which says that overcurrent devices are sized at minimum 1.25 times the maximum current. I see in 690.9(C) that fuse sizes go up by 1A increments from 1A to 15A, so by code 13A fuses would be required.
  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    Haha I do not care about NEC or whatever,

    As our govt provides no code or support .

    Panel manufacturer gives us some warrenty that i wouldn't void :)

    Would something like this work
    http://www.ebay.com/itm/Lot-2-Legrand-Fuse-Block-Holder-10x38-400V-32A-058-08-/120749324725?pt=LH_DefaultDomain_0&hash=item1c1d3879b5
    http://www.ebay.com/itm/NEW-Legrand-CEI-269-2-NFC-63210-Fuse-10A-10-Amp-500-V-/180950768047?pt=LH_DefaultDomain_0&hash=item2a218199af
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Fuse ideas for 125w x 4 setup
    paimpozhil wrote: »
    Thanks for the answer. :),

    So that makes it even easier I can use a T MC4 and combine 2 strings into 1 before going to a fuse with 20A.
    Not a good idea. The fuses are there to protect the strings and string conductors from backfeeding each other in the event of a string fault, not to protect the inverter from the strings. If you combine 4 strings into two without fuses and then fuse the pairs at 20A, it's like combining 4 into 1 with no fuses. If a string faults, then it has three other strings' Isc available as fault current and no fuses will blow. MC4 T's are to be used if and only if there are two strings total.
  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    Re: Fuse ideas for 125w x 4 setup

    As I try to help more then I hurt, below is where I come up with my advice. The answers given are to the questions I had ask.
    Midnite Solar Breakers, Bus Bars, Ground Bars

    3. If the answer to question two is no then I assume if I bought the 12 breaker one I would be regulated to 6 fuses per aray not 4 breakers to on array and 8 breakers to the other.
    If you can get a 4 bus and 8 breaker bus that will fit the box, I don't see why not.

    4. If you only need two fuse pv series if they are wired 3 or more in parallel due to two arrays being able to over load one (something that can't happen with just two) Could a guy put two series on each breaker instead of just one?
    Probably--But I do not believe it is "legal" to put two wires into one breaker--You may have to use a wire nut to bring two wires to one, then connect to breaker

    I am glad I posted my advice. I am glad I was corrected before the person I gave it to got hurt. And I am glad to learn more.
    gww

    PS In theory though, "discounting warranty" If a guy had 6 60watt evergreen panels at 3.6 amps each and wired series of two each and used three breakers it seems the panels would be protected. I realize that the bigger panels have a limit to the componants due to the higher amp rating but the smaller panels are probly made with the same componants that the larger ones are. I don't ask this to get anyone to do it but I have 2000 watts of 60watt homemade panels and felt this would be good enough protection to help gaurd against my own little fire. It seems like it would work with a nine amp fuse.