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What higher voltage fuse in between CC & Panels?

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  • What higher voltage fuse in between CC & Panels?

    I just use bolt on 60A midi fuses rated at 32VDC max on 6ga wire for camping panels. Looking to get higher voltage panels at 54Voc as my panels are up to 20M away from CC (victron 100/50). Panels are in parallel. what fuse do I get? All the higher voltage stuff looks complicated & I'm overwhelmed. Please give an example, something bolt on. Thanks

  • #2
    No fuses required unless you have 3 or more parallel strings. Solar panels are current sources, not voltage. You can short out all day long and nothing happens. Take note of the panel spec Isc aka current short circuit, is the most current th epanel is capable of delivering. Something like 6 to 10 amps. You are using 6 AWG wire which has a fusing current of 500 amps. and can handle 60 amps all day long.
    MSEE, PE

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    • #3
      Sunking,

      could you give us a basic explanation of why we fuse 3 parallel strings and not 2 parallel strings?

      I have seen mention of the requirement but don't understand the "why" of it.

      Thanks.

      S.

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      • #4
        It has to do with the power a panel can dissipate.

        A 60W 4A panel can, obviously manage 60W ( 4A ) ( volts has no function in this exercise )
        If you have 2 in parallel, and one shorts internally, the functioning 60W panel can dump 60w into the other panel all day long, and nothing bad happens -usually.
        But if you have 3 panels in parallel, and one shorts internally, it will now be presented 120W and that starts to heat things up, maybe it will flame, maybe just char, but if it had a 5A fuse, that protects the bad panel from a possible 8A from the 2 working panels.
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
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        • #5
          Originally posted by Sunking View Post
          No fuses required unless you have 3 or more parallel strings. Solar panels are current sources, not voltage. You can short out all day long and nothing happens. Take note of the panel spec Isc aka current short circuit, is the most current th epanel is capable of delivering. Something like 6 to 10 amps. You are using 6 AWG wire which has a fusing current of 500 amps. and can handle 60 amps all day long.
          Makes sense to me with the Panels. Are we suppose to protect the CC by having a fuse on the input side? Just asking as my CC was expensive & no easy way to access a fuse on it.

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          • #6
            While fusing is not required with less than 3 strings it still is very convenient to have a breaker at the input and output of the CC for system maintenance,

            Array - controller breaker box..JPG
            2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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            • #7
              Thank you .

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              • #8
                Originally posted by PNW_Steve View Post
                Sunking,

                could you give us a basic explanation of why we fuse 3 parallel strings and not 2 parallel strings?
                Easy Peasy. Solar Panels are Current Sources, not Voltage Sources like a battery with unlimited current. So look at any Solar panel Isc spec aka Short Circuit Current. Just about any panel made is less than 10 amps. That means it is impossible for the panel to generate whatever Isc rating. You can short a panel all day long and not hurt anything, in fact that is how you test a panel.

                Most panels have 12 or 14 AWG wire which can handle 20 amps all day long with no thermal damage. 1 string up to 10 amps, 2 strings up to 20 amps and no problem. Add the third string up to 30 amps and you got a problem.

                Short a voltage source out like a battery out, and you got a real big problem like wire vaporizing spewing molten copper everywhere.

                There is no problem using Breakers/Fuses on just 1 or 2 strings if you want them, but if given the option of saving the expense, you might elect to not use them because they are not required. As for me on my system, I just have a Switch on th eInput of my Charge Controller that Shorts the Panel out to disable them. That way there is no Voltage to get bit by.
                Last edited by Sunking; 06-06-2018, 04:43 PM.
                MSEE, PE

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                • #9
                  Originally posted by littleharbor View Post
                  While fusing is not required with less than 3 strings it still is very convenient to have a breaker at the input and output of the CC for system maintenance,
                  Are you using AC breakers as DC switches? Even at half the AC rating, I'd be VERY wary of ever opening one under load.

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                  • #10
                    Those are Square D QO breakers. They are AC/DC rated. I don't feel like digging up all the literature but here's a start.

                    https://www.schneider-electric.us/en...akers#tabs-top

                    https://www.schneider-electric.us/en...rcuit-breakers
                    Last edited by littleharbor; 06-09-2018, 10:42 PM.
                    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                    • #11
                      Originally posted by gbynum View Post

                      Are you using AC breakers as DC switches? Even at half the AC rating, I'd be VERY wary of ever opening one under load.
                      No no. AC and DC despite what politics say is not allowed. Laws of physics do not tolerate mixing.

                      MSEE, PE

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                      • #12
                        Originally posted by littleharbor View Post
                        While fusing is not required with less than 3 strings it still is very convenient to have a breaker at the input and output of the CC for system maintenance,

                        Array - controller breaker box..JPG
                        My setup is just a portable battery pack. Looks like the CC is probably safe as its rated for the solars input. Over as mine is 50a and my panels are only 500watts total.
                        I just put an on off switch between cc and panels. Will come in handy at times.

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                        • #13
                          Originally posted by Jman View Post

                          My setup is just a portable battery pack. Looks like the CC is probably safe as its rated for the solars input. Over as mine is 50a and my panels are only 500watts total.
                          I just put an on off switch between cc and panels. Will come in handy at times.
                          That is fine but thought you needed to know it is not necessary and the expense can be avoided. More you know the better the decisions.

                          MSEE, PE

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                          • #14
                            Originally posted by littleharbor View Post
                            Those are Square D QO breakers. They are AC/DC rated. I don't feel like digging up all the literature but here's a start.
                            Thank you! I didn't realize they had a 48VDC rating on single breakers, 125VDC on a common trip pair. That's awesome! So 1 60 or 72 cell panel can be on a single, probably 3 in series on a common trip pair. I now want to investigate the other competitive manufacturers. The AIC rating might be a little low for batteries, but 4kA is probably limited ok by cabling.

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                            • #15
                              The only other Mfg. I know are Midnite Solar and Outback
                              2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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