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Wire Size For Interconnect Cables in 24V Battery Bank

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  • Wire Size For Interconnect Cables in 24V Battery Bank

    I have a Samlex 24V 2000W high-efficiency pure sine inverter to run, via ~3ft cables, on a 24V battery bank consisting of four 6V Trojan T105-RE batteries wired in series. That gives a possible maximum continuous (not surge) power draw of about 2000W, which is about 85A at 24V.

    Now, according to my research, all documentation I've read says that to pass a continuous 85A current I will require #2 wire. Yet the manual to my inverter states that I should use #4 wire, which is SMALLER! What do you guys think?

    Also, what size wire should I use for the interconnect cables on my battery bank? For an 85A draw on the battery bank (wired in series), will it pull approximately 22A from each of the four batteries (in that case I could use #6 or #8 wire?) ... Is that how it works, or do I still need the bigger wire for the interconnects?

  • #2
    Originally posted by joeyp View Post
    I have a Samlex 24V 2000W high-efficiency pure sine inverter to run on a 24V battery bank consisting of four 6V Trojan T105-RE batteries wired in series. That gives a possible maximum continuous (not surge) power draw of about 2000W, which is about 85A at 24V.

    Now, according to my research, all documentation I've read says that to pass a continuous 85A current I will require #2 wire. Yet the manual to my inverter states that I should use #4 wire, which is SMALLER? What do you guys think?

    Also, what size wire should I use for the interconnect cables on my battery bank? For an 85A draw on the battery bank (wired in series), will it pull approximately 22A from each of the four batteries (in that case I could use #6 or #8 wire?) ... Is that how it works, or do I still need the bigger wire for the interconnects?
    According to the NEC, for most wire insulation types in free air rather than in conduit, you can use #4 copper for 105A or more. And even in conduit you can use some insulation types for 85A or more.
    However, if the wire is longer than a few feet, you may want to use the larger gauge wire such as #2 to reduce the voltage drop in the wire.

    As for the interconnect wiring, what you use will depend in part on just how you wire the batteries.
    For example, in a two battery diagonal terminal connection, interconnect wire of half the ampacity of the main leads will be large enough, but why go to smaller wire, requiring different terminal lugs, for such a short distance?

    If you are putting more than 2 batteries in parallel, then you may (will?) be making a design mistake. And the wire sizing will be more complicated.
    One factor to consider is that if something happens (like a failed cell or a broken battery terminal connector) all of the current may be passing through the smaller wire. And that would not be good.
    All of the wires should be large enough that the single fuse at the main termination point protects them individually as well as in parallel.
    If you fuse each battery independently, then you would be safe using smaller wire for some of the interconnects.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      Originally posted by joeyp View Post
      Now, according to my research, all documentation I've read says that to pass a continuous 85A current I will require #2 wire. Yet the manual to my inverter states that I should use #4 wire, which is SMALLER! What do you guys think?

      Also, what size wire should I use for the interconnect cables on my battery bank? For an 85A draw on the battery bank (wired in series), will it pull approximately 22A from each of the four batteries (in that case I could use #6 or #8 wire?) ... Is that how it works, or do I still need the bigger wire for the interconnects?
      NEC tables have many tables, but the one you missed is for FREE AIR. It also depends on insulation type and operating temperature. At 85 amps if you use a 75 C cable insulation you are required to use #6 AWG copper which is good up to 95 amps.

      But there is a catch. NEC only deals with minimum safety requirements and does not take operations into considerations. NEC is not design guide, it is minimum safety requirements only. What the NEC does not tell you is voltage drop, and for low voltage systems voltage drop is critical. So all the NEC is saying is you have to use a minimum 6 AWG. You can use any cable size you want as long as it is not smaller than # 6 AWG copper with 75 degree insulation rating.

      You other error is all 4 batteries wired in series current is equal and that current is 85 amps, not 22 amps. So you use a minimum 6 AWG to interconnect the batteries in series with.

      Bottom line if your cable distance is in fact only 3 feet one-way, #6 AWG copper with 90 degree insulation is good to go.
      MSEE, PE

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      • #4
        Originally posted by inetdog View Post
        According to the NEC, for most wire insulation types in free air rather than in conduit, you can use #4 copper for 105A or more.
        Incorrect answer. In Free air it is 125 and 140 depending on insulation type being either 75 or 90 degree. However that is a moot point because it is what the terminal temperature rating is listed as, and everything except high temperature devices like ovens are 75 degrees C. So at 75 is 125 amps. Using 75 degree column #6 AWG is the minimum size required which is good for 95 amps.
        MSEE, PE

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        • #5
          Originally posted by Sunking View Post
          Incorrect answer. In Free air it is 125 and 140 depending on insulation type being either 75 or 90 degree. However that is a moot point because it is what the terminal temperature rating is listed as, and everything except high temperature devices like ovens are 75 degrees C. So at 75 is 125 amps. Using 75 degree column #6 AWG is the minimum size required which is good for 95 amps.
          Thanks! I'll use AWG 4 for the inverter and the interconnects.

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          • #6
            Sunking, can you tell me what a battery shunt is, and why I need one? The wiring diagram in my CC manual shows one just off the DC negative bus bar, coming from the negative side of the battery bank.

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            • #7
              Originally posted by joeyp View Post
              Sunking, can you tell me what a battery shunt is, and why I need one? The wiring diagram in my CC manual shows one just off the DC negative bus bar, coming from the negative side of the battery bank.
              Not so sure you need one but a Shunt is nothing more than a high power low resistance precision resistor to allow you to measure Current. So if you want to monitor or measure current you need a shunt. Otherwise you do not need one unless you have some monitoring equipment that needs to know. Most work on a 50 mv scale. So lets say you buy a 100 amp Meter Shunt what you are getting is a precision .0005 Ohm, 5 watt resistor.

              One word of advice and warning here. With the size of cables and amount of current you are working with I strongly advise you not to attempt to terminate any cables. DIY's do not have access to the tooling or training to terminate properly. Not many electricians even have to proper tooling. Instead go to a cable shop and have them fabricated to your dimensions and specifications. You are literally playing with fire and one little mistake will cost you dearly. Even with manufactured cables you are taking a lot of risk if the connections are not properly torqued, inspected and maintained.

              Try finding a Golf Cart dealer as they should have the tooling.

              Last comment minimum wire size is dictated by the Over Current Protection Device supplying power to a conductor, not the actual load current. I assume you will be using a 100 amp Fuse on both battery post to supply the inverter. Minimum size required in your application using 75 degree cable insulation is 6 AWG copper, so #4 AWG exceeds the minimum requirement and good to go if you can terminate it correctly.
              MSEE, PE

              Comment


              • #8
                Originally posted by Sunking View Post
                Not so sure you need one but a Shunt is nothing more than a high power low resistance precision resistor to allow you to measure Current. So if you want to monitor or measure current you need a shunt. Otherwise you do not need one unless you have some monitoring equipment that needs to know. Most work on a 50 mv scale. So lets say you buy a 100 amp Meter Shunt what you are getting is a precision .0005 Ohm, 5 watt resistor.

                One word of advice and warning here. With the size of cables and amount of current you are working with I strongly advise you not to attempt to terminate any cables. DIY's do not have access to the tooling or training to terminate properly. Not many electricians even have to proper tooling. Instead go to a cable shop and have them fabricated to your dimensions and specifications. You are literally playing with fire and one little mistake will cost you dearly. Even with manufactured cables you are taking a lot of risk if the connections are not properly torqued, inspected and maintained.

                Try finding a Golf Cart dealer as they should have the tooling.

                Last comment minimum wire size is dictated by the Over Current Protection Device supplying power to a conductor, not the actual load current. I assume you will be using a 100 amp Fuse on both battery post to supply the inverter. Minimum size required in your application using 75 degree cable insulation is 6 AWG copper, so #4 AWG exceeds the minimum requirement and good to go if you can terminate it correctly.
                I had decided to get the AWG 4 cables from here:

                http://www.wireandsupply.com/4_awg_S...at-4g-lead.htm

                But now, I'm wondering if I should find a Golf Cart shop as you suggested!...?

                Comment


                • #9
                  Originally posted by joeyp View Post
                  I had decided to get the AWG 4 cables from here:

                  http://www.wireandsupply.com/4_awg_S...at-4g-lead.htm

                  But now, I'm wondering if I should find a Golf Cart shop as you suggested!...?
                  Those look to be manufactured. Just make sure the terminal lug hole size matches the terminal bolt size and surface area.
                  MSEE, PE

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                  • #10
                    Originally posted by Sunking View Post
                    Those look to be manufactured. Just make sure the terminal lug hole size matches the terminal bolt size and surface area.
                    The T105-RE batteries have 3/8" connectors. The inverter has 5/16" connectors. I'll match everything, five AWG #4 wires total for the interconnects (1') and inverter connections (3').

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                    • #11
                      Originally posted by joeyp View Post
                      But now, I'm wondering if I should find a Golf Cart shop as you suggested!...?
                      If you ever really need custom cables and cannot find a golf cart shop nearby, you can also look for a welding equipment shop. Particularly if you want to use fine stranded cable for flexibility.
                      SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                      • #12
                        Joey I think you will be OK now that you know what to look for. I mentioned a golf cart shop because even a small town with a golf course has one and the tooling to terminate. Even many a electrician that does commercial work Aircraft AP mechanics will have it also. Point I am driving at is it is important to the operation and safety.

                        I may pretend not to care about folks, but I really do care and do not want to see anyone get hurt. I just get frustrated because I see so many people come here who have no idea what they are getting into. I cannot save or educate them all to the dangers they are lurking in. I bet you a dollar you had no idea how dangerous a 24 volt 2000 watt inverter can be?
                        MSEE, PE

                        Comment


                        • #13
                          Originally posted by Sunking View Post
                          Joey I think you will be OK now that you know what to look for. I mentioned a golf cart shop because even a small town with a golf course has one and the tooling to terminate. Even many a electrician that does commercial work Aircraft AP mechanics will have it also. Point I am driving at is it is important to the operation and safety.

                          I may pretend not to care about folks, but I really do care and do not want to see anyone get hurt. I just get frustrated because I see so many people come here who have no idea what they are getting into. I cannot save or educate them all to the dangers they are lurking in. I bet you a dollar you had no idea how dangerous a 24 volt 2000 watt inverter can be?
                          No I didn't. And let's not forget that I started out planning with a 12V 2000W inverter. I am taking this very slow. I will use extra caution. I know that it is important to understand exactly what wire sizes are needed, proper grounding, what size circuit breakers, what size fuses, in what order cables need to be hooked up, etc...

                          I'm not done with questions!

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                          • #14
                            Joey I compliment you for asking the right questions up front before you go out and buy anything. At 2000 watt inverter you should really be looking at a 48 volt system. General rule of thumb is you inverter wattage should not be larger than your panel wattage. You have crossed that line by a large magnitude. It can be done but with consequences. So do not get your shorts in a knot when I critique. It is not personal. It only becomes personal when you do not listen and weigh the advice given and when the day comes it blows up on you.
                            MSEE, PE

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                            • #15
                              Joey, a C8 discharge rate on that battery bank at 24V is only 700 watts. If you try to suck much more than that out of them you'll get voltage sag at the inverter (regardless of cable size) and eventually the inverter will shut down due to low input voltage. You can probably run 1000 watt loads for short periods of time (like maybe a circular saw to cut a couple of boards) but don't expect to be able to run a 1500W space heater.
                              When I was putting mine together, I compiled a binder with over 200 pages of electrical code and studies. I found the Sandusky labs PV power systems report to be very good reading and I incorperated all the labeling from it into my system. You're going to have to get into the code for PV disconnect requirements, fusing, breakers, labeling etc. It's no small task to figure it out without any help.
                              1150W, Midnite Classic 200, Cotek PSW, 8 T-605s

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