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48v off grid system: battery cable advice needed please

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  • #46
    Originally posted by Mike90250 View Post
    I am a firm believer of a IR thermometer in every battery shed tool box.
    OK now you are talking to the hand. Great tool to have no off-grider should be without. Off grid is dangerous with all that current, and most are cluleless what high current means and does.

    MSEE, PE

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    • #47
      Hammick back to your Fuse Question. Now that you have learned a little about the nasty things that come from high current, time to rethink fuse size. If your Schneider Inverter manual states it needs a 105 amp fuse, there is no good reason to use anything larger. Doing so is just asking for trouble.

      Example a 110 amp fuse has higher resistance than say a 200 amp fuse. A 110 amp fuse will limit fault current more than a 200 amp. A 110 amp fuse will operate faster than a 200 amp fuse under a fault. Although your cable sizes are sized to operate at higher than designed current that is a function of Low Voltage to limit voltage drop. There is nothing good about using anything higher than required by the Inverter. Using more than required just means more damage potential.
      MSEE, PE

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      • #48
        Originally posted by Sunking View Post
        Hammick back to your Fuse Question. Now that you have learned a little about the nasty things that come from high current, time to rethink fuse size. If your Schneider Inverter manual states it needs a 105 amp fuse, there is no good reason to use anything larger. Doing so is just asking for trouble.

        Example a 110 amp fuse has higher resistance than say a 200 amp fuse. A 110 amp fuse will limit fault current more than a 200 amp. A 110 amp fuse will operate faster than a 200 amp fuse under a fault. Although your cable sizes are sized to operate at higher than designed current that is a function of Low Voltage to limit voltage drop. There is nothing good about using anything higher than required by the Inverter. Using more than required just means more damage potential.

        Damn Sunking just when I thought I had it figured out. Schneider requires in the USA that DC switchgear be used. Their DC switchgear only comes with a 250a breaker regardless of which inverter is used. I am using the Midnight Solar E-panel which is AC/DC combined. The choices from MS are 175a or 250a breakers. I chose the 250a because that's what Schneider uses. The specs in the manual for my SW4048 manual say max DC is 105a. PDF of the manual attached. 4048 Owners manual.pdf Specs are at page 108

        So I don't know what to do. I'm pretty sure I am going to do a test runnning my three big loads at the same time (well pump, fridge and microwave) and see what my Bogart meter is telling me and then choose my fuse size. It measures DC amps off a shunt.

        Right now everything is shut down except for the charge controller. CC is protected by 60a breakers in and out. I installed the battery cables myself and there is no way possible I am going to get a battery cable short from any type of failure. But hey, I'm no professional.

        My biggest concern is blowing a fuse when I'm not there and my batteries start self discharging. Probably unrealistic since my CC can only put out 60a max and with my panels probably no more than 35a.

        I'll buy the 125a Blue Seas fuses if you are certain that is what I need. Otherwise I'm going to run the test with my Bogart when I'm up there in March.

        Thanks much.
        Attached Files
        Last edited by hammick; 02-12-2016, 11:42 PM.
        Conext XW5548
        Conext MPPT60-150

        Comment


        • #49
          OK I looked at the manual. Again Schneider and MS are only looking out for their equipment. The manual gives me enough info to determine Fuse selection on the Battery Post for which MS and Schneider have no say or concern about. That is not their problem or concern.

          There are two ways to determine what the Fuse Size should be at the BATTERIES. Keep in mind the batteries are the Source of Power, not the Inverter. You need to understand that.

          1. From the manual clearly states the maximum continuous DC current is 105 AMPS. In electrical engineering if we know the maximum CONTINOIUS CURRENT selecting a Fuse Size is straight forward simple 5th grade math. You DO NOT SELECT a 105 AMP FUSE because fuses are not rated for continuous power. If you ran 105 amps continuous on a 105 amps thermal fuse it will operate after some period of time. that means it will heat up enough to melt the thermal link inside the cartridge. So both in electrical code and engineering design practices we make the fuse 125% larger than the maximum continuous current. So the perfect fit for 105 amps is 1.25 x 105 amps = 131 amps. Now you are not going to find a 131 amp fuse. But you can find 125 and 150. Guess which one you want?

          You had better say 150 amps.

          2. Second method is to use Inverter KVA rating and efficiency. From the manual max output power is 4400 watts, and I will use 90% efficiency and rate 125%

          4400 watts / .9 / 48 volts x 1.25 = 127 amps.

          So look what happens. Both ways give us roughly the exact same answer. 150 amp fuse. In Electrical Engineering we measure with a micrometer, mark the spot with chalk, and cut it with an axe.

          Next check point is we must determine what the minimum size wire is required to handle 150 amps continuously. This is not quite so straight forward because it depends on what Insulation your wire is using or Temperature rating the insulation is, and if it is ran in a raceway and how many current carrying conductors are in the Raceway. You are not running your cable in a Raceway, you are running it in Free Air and only a single pair or 2 current carrying conductors. That means we use NEC Table 310.15 (B)(17). I assume you will use at least a 75 degree insulation and that means the minimum size wire you can use with a 150 amp fuse is #2 AWG. You are using 4/0 and it is good up to 360 amps.

          You want 150 Amp Fuse on the battery.

          Stop worrying

          150 AMPS
          Trust me I know what I am talking about and done this professionally for 35 years. No other pro on this forum is going to dispute the 150 amp fuse.

          Last edited by Sunking; 02-13-2016, 12:30 AM.
          MSEE, PE

          Comment


          • #50
            Originally posted by hammick View Post
            My biggest concern is blowing a fuse when I'm not there and my batteries start self discharging. Probably unrealistic since my CC can only put out 60a max and with my panels probably no more than 35a.
            Use a 150 amp fuse. I upped from 125 because earlier I though you said that was Schneider said to use which I found strange because there is no such thing as a 105 amp fuse to my knowledge. But the spec is 105 amps max cont current. In all honesty 125 would work, but design principles dictate 150 for worry free operation.

            FWIW if you were pulling a permit and have to pass Inspection and only had fusses or breakers at the input of the Inverter or output of your EBOX, you would fail Inspection. The Inspector would demand OCPD on the Battery Term Post. Those downstream OCPD do not protect the battery cables. Keep in mind OCPD only protects what is DOWN STREAM.

            Try this if you got balls. Take a 2 foot long 26 AWG wire, the stuff you use for telephone and data communications. Get a heavy pair of leather gloves and eye protection. Touch one end of the wire on the negative post of the battery, and the other on the positive post. Do so with the wire above you and out to the side and see what happens.

            I will tell you what will NOT happen.


            1. No fuse will blow.
            2. You won' have clean underwear after the test.
            Last edited by Sunking; 02-13-2016, 12:45 AM.
            MSEE, PE

            Comment


            • #51
              OK. 150a fuses on both + and - terminals. Thank you for figuring this out. I'll get them ordered and installed in early March.
              Conext XW5548
              Conext MPPT60-150

              Comment


              • #52
                Originally posted by Sunking View Post
                Try this if you got balls. Take a 2 foot long 26 AWG wire, the stuff you use for telephone and data communications. Get a heavy pair of leather gloves and eye protection. Touch one end of the wire on the negative post of the battery, and the other on the positive post. Do so with the wire above you and out to the side and see what happens.

                I will tell you what will NOT happen.


                1. No fuse will blow.
                2. You won' have clean underwear after the test.
                I think I will pass on that one.
                Conext XW5548
                Conext MPPT60-150

                Comment


                • #53
                  Originally posted by hammick View Post

                  I think I will pass on that one.
                  It was a joke to make a point.
                  MSEE, PE

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