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so many fuses - did I pick the right ones?

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  • so many fuses - did I pick the right ones?

    I'm trying to finalize details for a 400W 12v 230Ah RV system. 4 100W/12v panels - 2 sets in parallel of 2 panels in series (ie. there are 2 wires with 24V/11a/200W each ready to plug into the controller)

    A little overwhelmed by fuse choices. Primarily not knowing the difference in different shapes. Also fast/slow blow etc.
    I've come up with a first pass below, and would appreciate any feedback, especially w/r to slow/fast blow or if the form factor I've picked is wrong for some reason.

    Fuse 1: Main battery fuse (needs to fit in battery box close to battery):
    MIDI fuse in a terminal-connected block (40A). Small, cheap, terminal connections. Example:
    http://www.littelfuse.com/products/f...useholder.aspx

    Fuse 2: Charge converter -> Battery.
    Interchangeability and low price. Another 40A MIDI.

    Fuse(s) 3: Panels to charge controller (2 - 8awg wires, merged, then into controller):
    I happen to have this style fused distribution block laying around. Silly, but any reason it won't work if I make sure the fuse used it rated for 24V+? 10A fuse on each pair.
    http://cdn3.volusion.com/afotr.kmxvb...SCOEADB2-2.jpg
    Alternately, I considered a 10A inline MC4 fuses on each leg on the roof.

  • #2
    I am not sure about your choice of a 40A fuse for your main battery fuse.
    If you are feeding a 400W inverter, and it has a surge capability, there may be times when a normal load draws more than 40A, and possibly enough to blow the fuse.

    An AC output of 400W powered from a 10V input at the inverter terminals under load would be >40A, and a surge to 600W would take you to >60A, which could blow the fuse.

    Your 230AH battery would be safe to deliver C/4 at least for a reasonable time, so a 60A fuse would not be out of line. The main reason that the fuse is there is to protect against short circuit current or a current so large that it would overheat the connecting wiring. With a 12V system you will be using a large enough wire to keep the voltage drop down that you are not going to be at any great risk of a low level overload damaging the wires or the terminals.

    I do not recall your earlier description of the CC, but I will assume that it is MPPT but with a relatively low maximum voltage. Your 100W, nominal 12V panels should be putting out an Isc around 5.5A, so the 10A fuse there would be fine. But the important thing about the panel fuse is that it be no larger than the maximum series fuse value from the panel nameplate.
    When you have only two strings, there really is no need to fuse them separately, and the fuse holder you have chosen will not be able to disconnect a string circuit under load for testing purposes. A single 15-20A fuse after the combiner would be all the protection you really need.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      With 6awg lines (min) to the batteries, your 60A fuse recommendation is probably a reasonable one. So I'm going with ANL fuse holders with that size. They are larger physical size fuses, but have more standard terminal sizes which makes wire interchangeability easier and I think will still fit in the battery box.

      I'm not too concerned about the inverter for now. I have a cheapo 800W one someone gave me and I have it fused down real low just using it for a laptop and charging electronics for now. I shouldn't need more than a 300W decent one when the time comes, but putting that $$ off for now.

      I have seen varying stories on not needing fuses for only 2 panel strings in parallel. What you say (not necessary) makes sense to me because it seems if one short circuited, it would only see the current from the other - which is no more than it's normal current. But I was just being cautious because I read so many varying things on the interwebs!

      Your post makes me wonder though - why is there really a need to fuse between the combined parallel panels and the charge controller? Is that just for protection of the panels if something shorts and blows power back towards the panels or something? I can't see the panels damaging the CC - unless lightning or something? I did go with a decent MPPT controller that can take 150V if that matters.

      Thanks for the response.

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      • #4
        Originally posted by chilly View Post
        I have seen varying stories on not needing fuses for only 2 panel strings in parallel. What you say (not necessary) makes sense to me because it seems if one short circuited, it would only see the current from the other - which is no more than it's normal current. But I was just being cautious because I read so many varying things on the interwebs!
        Don't believe a word of it.

        Your post makes me wonder though - why is there really a need to fuse between the combined parallel panels and the charge controller? Is that just for protection of the panels if something shorts and blows power back towards the panels or something?
        The answer lies in Electrical Codes and Installation practices. All you need to know is:

        690.9 Overcurrent Protection


        (A) Circuits and Equipment. Photovoltaic source circuit,
        photovoltaic output circuit, inverter output circuit, and storage
        battery circuit conductors and equipment shall be protected
        in accordance with the requirements of Article 240.
        Circuits connected to more than one electrical source shall
        have overcurrent devices located so as to provide overcurrent
        protection from all sources.


        Exception: An overcurrent device shall not be required for
        PV modules or PV source circuit conductors sized in accordance
        with 690.8(B) where one of the following applies:


        (a) There are no external sources such as parallel-connected
        source circuits, batteries, or backfeed from inverters.


        (b) The short-circuit currents from all sources do not exceed
        the ampacity of the conductors or the maximum
        overcurrent protective device size specified on the PV
        module nameplate.
        Last edited by inetdog; 08-27-2014, 11:04 PM. Reason: fixed quote tag
        MSEE, PE

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        • #5
          OK, sounds like a fuse is in order.

          I just figured out that the trailer, as-is, has a 30A maxi blade fuse right at the battery. Then about 4' away there is a 40A automotive type self-reset circuit breaker (redundant). The fuse looks like the prev owner hacked it on - standard radio shack 12awg maxi fuse holder crimped inline with the 6awg battery cable. But that would be a long dangerous run around some frame rails to the circuit breaker without being fused - so not really sure how it came from the factory. Anyway, I"ll put a good fuse near the battery and ditch the breaker (tiny terminals, flimsy looking).

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