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  • Fusing question

    I have four 300W solar panels. 1200W 24V.

    What DC breaker should I use between the solar panels and the charge controller?
    Basically I'm asking about Voltage and Amps. That is a topic I don't understand.

    I want a breaker that has no delay before it trips. So if there's a surge, I want the breaker to trip immediately.

    TIA.

  • #2
    How will you wire your panels, series, parallel or a combination ?

    What is your Charge Controller, PWM or MPPT ?

    Can you upload a pic of the data label ( or type it out ) here, for us to see and provide better advice for you.

    What is the coldest weather you will see at your site ( important because cold panels produce higher voltage that can fry a marginal controller)
    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
    gen: http://tinyurl.com/LMR-Lister

    Comment


    • #3
      > How will you wire your panels, series, parallel or a combination ?
      Parallel.

      > What is your Charge Controller, PWM or MPPT ?
      PWM, although that could change. I'm just trying to learn the general principle behind fusing.

      > Can you upload a pic of the data label ( or type it out ) here, for us to see and provide better advice for you.
      http://www.famicareindustry.com/prod...--38757--.html

      > What is the coldest weather you will see at your site ( important because cold panels produce higher voltage that can fry a marginal controller)
      The weather is warm all year round.



      Comment


      • #4
        Unfortunately, no circuit breaker trips instantly. They rely on something inside heating up, and that takes time, perhaps 10ms-20ms for an extreme overload of 100X rating, much longer for a smaller overload.

        I'm trying to understand why you need a circuit breaker at all. You may not need one. That inverter has input and output circuit breakers built in.

        What is the source of the surge that you anticipate?

        You asked about volts and amps. The specifications for a typical 24V solar panel have maximum output voltage of 36V and a maximum open-circuit voltage of 45V. So if you put a circuit breaker between the panels and the inverter, it must be rated for greater than 45V DC. You can't use a common 240V AC circuit breaker, unless it is also rated for >45V DC. Solar supply companies sell special circuit breakers rated for 50V DC and higher. In the US, a common brand is MidNight Solar. They make 150V DC breakers like this one:
        https://www.wholesalesolar.com/85300...solar-mnepv-30

        You may want four small circuit breakers, one per panel. If that's the plan, then you would need a circuit breaker rated for >120% of the panel output. A typical 300W 24V panel can produce 8.25A at 36V, so each panel would need a 10A breaker.

        I got the 8.25A number, the 36V number, and the 45V number from the specification sheet of a 300 watt 24V solar panel that I found on the web. Yours might be a little bit different. Check the specs for your specific panel. Look for maximum open circuit voltage and Current at Maximum Power.

        If you want one circuit breaker for all four panels, it will need to be rated for at least 40A. If you use a larger breaker, then you can accomodate more current without tripping, but when you do get a fault, it will be slower to trip. So use the smallest that will work within the 120% rule.

        I hope this helps. Please keep asking questions.
        7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

        Comment


        • #5
          For Specific breakers, I went with the ones from midnight solar.

          They have a 150 VDC I decided on, which is more than I plan on feeding through it, but with three panels in series, the open circuit voltage could be as high as 63. The next breaker I will buy will likely be a two pole breaker to help prevent arcing.

          MidniteSolar also makes breakers for more voltage than 150.

          I've seen some setups that use Audio Circuit breakers, but I would not want to use those even though they are easy to install.

          I am not an expert, but the correct fuse is supposed to blow quicker than a breaker.

          Comment


          • #6
            Originally posted by fw12 View Post
            I have four 300W solar panels. 1200W 24V.

            What DC breaker should I use between the solar panels and the charge controller?
            Basically I'm asking about Voltage and Amps. That is a topic I don't understand.

            I want a breaker that has no delay before it trips. So if there's a surge, I want the breaker to trip immediately.

            TIA.
            IMHO the best fuse is the fuse you don't need. Solar Panels are current limiting devices. A Solar Panel with a Short Circuit current of 10A will never exceed ~10A. I would just wire your four panels in 2 strings of 2 in parallel. No fusing required. You only need fuses if you have more than 2 strings in parallel.

            Comment


            • #7
              Thanks for your response.

              >> I'm trying to understand why you need a circuit breaker at all. You may not need one. That inverter has input and output circuit breakers built in.
              What is the source of the surge that you anticipate?

              I'm thinking of protecting the system, since that is what is recommended. I don't know where the surge could come from. Like I said, I'm just following recommended practice.
              Would breakers work to arrest lightening, given that a lightening is over in a split second, while delivering devastating voltage. Maybe grounding is the answer there.

              >> You asked about volts and amps. The specifications for a typical 24V solar panel have maximum output voltage of 36V and a maximum open-circuit voltage of 45V. So if you put a circuit breaker between the panels and the inverter, it must be rated for greater than 45V DC.

              Boom! That is what I've been trying to understand. So the reasoning is to figure out the open circuit voltage, and get a breaker that can handle voltage a bit higher than that.

              >> You may want four small circuit breakers, one per panel. If that's the plan, then you would need a circuit breaker rated for >120% of the panel output. A typical 300W 24V panel can produce 8.25A at 36V, so each panel would need a 10A breaker.

              >> If you want one circuit breaker for all four panels, it will need to be rated for at least 40A. If you use a larger breaker, then you can accomodate more current without tripping, but when you do get a fault, it will be slower to trip. So use the smallest that will work within the 120% rule.

              Very nice. Now I know not only what to do, but why.
              Thank you so much.
              Last edited by fw12; 10-09-2020, 12:16 AM.

              Comment


              • #8
                Originally posted by chrisski View Post
                I am not an expert, but the correct fuse is supposed to blow quicker than a breaker.
                I'll look into fuses then if they blow quicker. Even with the inconvenience of replacing any burnt fuse. Cost-wise, they are very cheap.

                Comment


                • #9
                  Originally posted by nwdiver View Post

                  IMHO the best fuse is the fuse you don't need. Solar Panels are current limiting devices. A Solar Panel with a Short Circuit current of 10A will never exceed ~10A. I would just wire your four panels in 2 strings of 2 in parallel. No fusing required. You only need fuses if you have more than 2 strings in parallel.
                  That makes sense. I was watching this video https://www.youtube.com/watch?v=gmi-vrGOFyk and thought I should go with their recommendation to have a breaker between the panels and the controller.

                  To protect against lightening, is grounding the only way?

                  Comment


                  • #10
                    Originally posted by fw12 View Post


                    To protect against lightening, is grounding the only way?
                    Yes. A breaker or fuse will do ~nothing to protect from lightning. Even a good ground can only do so much. I compare it to a seat belt and lightning is like a head on collision at 60mph. Better than nothing... but often no where near enough....

                    Yeah; The purpose of fuses on the PV side are to protect strings from each other. One string can't 'over-current' itself. One string can't 'over-current' another string. The only way you can exceed the string rating is for 2 strings to back-feed a 3rd string. Hence the need for fusing if you have 3 or more strings in parallel. Your charge controller should be sized to be able to handle the short-circuit output of the array it's connected to. If it's not then string level fuses won't help...
                    Last edited by nwdiver; 10-08-2020, 09:32 PM.

                    Comment


                    • #11
                      Originally posted by bob-n View Post
                      Unfortunately, no circuit breaker trips instantly. They rely on something inside heating up, and that takes time, perhaps 10ms-20ms for an extreme overload of 100X rating, much longer for a smaller overload.
                      I like what this guy has to say about that,
                      https://youtu.be/HVaawubUMo4?t=120

                      Comment


                      • #12
                        Originally posted by fw12 View Post

                        I want a breaker that has no delay before it trips. So if there's a surge, I want the breaker to trip immediately.
                        Then you want a magneto-hydraulic breaker. they use the surge to create a magnetic field to trip the breaker, which is more reliable that thermal which can take a long time for a light overload. (many of the DC breakers from Midnight are this type)

                        What you should have is a spec from the inverter mfg, as to the speed of disconnect, you can get fast blow fuses, slow blow fuses, time delay, anything. Same with breakers

                        While fuses are cheap, they become expensive as you get to higher DC voltages, and the fuse holders are seldom rated for circuit interruption, whereas a breaker can.

                        If your system is still in the design/planning, consider moving to 24V instead of 12V. The wire cables can be smaller, you can run your panels 2 Series, 2 parallel and not need a fuse on the PV side of the charge controller (but having a breaker or switch on each PV string, makes it super easy to compare panel power from one to the other, and give you confidence that both panel strings are working identically.)

                        You linked the inverter data, but the PV panel label is what I was hoping for.

                        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
                        gen: http://tinyurl.com/LMR-Lister

                        Comment


                        • #13
                          You are throwing money away. Can I have some of it. I won't loose it like you are going to do.. I respect and welcome money..

                          You are going about this all wrong. You have grid tied high voltage panels with a PWM controller. With that controller you are turning your 300 watt panels into 200 watt panels if operating at 24 volt battery. If you are a fool running 12 volts, then you turn you have turned your 300 watt panels into 100 watt panel. Great job there Joe throwing money and energy away.

                          Next using a PWM controller forces you to wire your panels in parallel robbing you blind in material cost and things you do not need or have any use for like Breakers and Combiners. Tell you what, buy all that stuff from me and we will both be tickled happy happy. You will be happy loosing a lot of money, and I will be happy to take your money away from you. Like they say a fool and their money will soon part ways. You are in the express lane.

                          Use a MPPT Controller that will allow you to wire all your panels in series, or 2S2P. No breakers or expensive combiners and wiring required or needed. Say goodbye to your money and power. Your choice.
                          Last edited by Sunking; 10-10-2020, 11:57 AM.
                          MSEE, PE

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